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Merge pull request #282 from chriseth/docs

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# Makefile for Sphinx documentation
#
# You can set these variables from the command line.
SPHINXOPTS =
SPHINXBUILD = sphinx-build
PAPER =
BUILDDIR = _build
# User-friendly check for sphinx-build
ifeq ($(shell which $(SPHINXBUILD) >/dev/null 2>&1; echo $$?), 1)
$(error The '$(SPHINXBUILD)' command was not found. Make sure you have Sphinx installed, then set the SPHINXBUILD environment variable to point to the full path of the '$(SPHINXBUILD)' executable. Alternatively you can add the directory with the executable to your PATH. If you don't have Sphinx installed, grab it from http://sphinx-doc.org/)
endif
# Internal variables.
PAPEROPT_a4 = -D latex_paper_size=a4
PAPEROPT_letter = -D latex_paper_size=letter
ALLSPHINXOPTS = -d $(BUILDDIR)/doctrees $(PAPEROPT_$(PAPER)) $(SPHINXOPTS) .
# the i18n builder cannot share the environment and doctrees with the others
I18NSPHINXOPTS = $(PAPEROPT_$(PAPER)) $(SPHINXOPTS) .
.PHONY: help clean html dirhtml singlehtml pickle json htmlhelp qthelp devhelp epub latex latexpdf text man changes linkcheck doctest gettext
help:
@echo "Please use \`make <target>' where <target> is one of"
@echo " html to make standalone HTML files"
@echo " dirhtml to make HTML files named index.html in directories"
@echo " singlehtml to make a single large HTML file"
@echo " pickle to make pickle files"
@echo " json to make JSON files"
@echo " htmlhelp to make HTML files and a HTML help project"
@echo " qthelp to make HTML files and a qthelp project"
@echo " devhelp to make HTML files and a Devhelp project"
@echo " epub to make an epub"
@echo " latex to make LaTeX files, you can set PAPER=a4 or PAPER=letter"
@echo " latexpdf to make LaTeX files and run them through pdflatex"
@echo " latexpdfja to make LaTeX files and run them through platex/dvipdfmx"
@echo " text to make text files"
@echo " man to make manual pages"
@echo " texinfo to make Texinfo files"
@echo " info to make Texinfo files and run them through makeinfo"
@echo " gettext to make PO message catalogs"
@echo " changes to make an overview of all changed/added/deprecated items"
@echo " xml to make Docutils-native XML files"
@echo " pseudoxml to make pseudoxml-XML files for display purposes"
@echo " linkcheck to check all external links for integrity"
@echo " doctest to run all doctests embedded in the documentation (if enabled)"
clean:
rm -rf $(BUILDDIR)/*
html:
$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
@echo
@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."
dirhtml:
$(SPHINXBUILD) -b dirhtml $(ALLSPHINXOPTS) $(BUILDDIR)/dirhtml
@echo
@echo "Build finished. The HTML pages are in $(BUILDDIR)/dirhtml."
singlehtml:
$(SPHINXBUILD) -b singlehtml $(ALLSPHINXOPTS) $(BUILDDIR)/singlehtml
@echo
@echo "Build finished. The HTML page is in $(BUILDDIR)/singlehtml."
pickle:
$(SPHINXBUILD) -b pickle $(ALLSPHINXOPTS) $(BUILDDIR)/pickle
@echo
@echo "Build finished; now you can process the pickle files."
json:
$(SPHINXBUILD) -b json $(ALLSPHINXOPTS) $(BUILDDIR)/json
@echo
@echo "Build finished; now you can process the JSON files."
htmlhelp:
$(SPHINXBUILD) -b htmlhelp $(ALLSPHINXOPTS) $(BUILDDIR)/htmlhelp
@echo
@echo "Build finished; now you can run HTML Help Workshop with the" \
".hhp project file in $(BUILDDIR)/htmlhelp."
qthelp:
$(SPHINXBUILD) -b qthelp $(ALLSPHINXOPTS) $(BUILDDIR)/qthelp
@echo
@echo "Build finished; now you can run "qcollectiongenerator" with the" \
".qhcp project file in $(BUILDDIR)/qthelp, like this:"
@echo "# qcollectiongenerator $(BUILDDIR)/qthelp/Solidity.qhcp"
@echo "To view the help file:"
@echo "# assistant -collectionFile $(BUILDDIR)/qthelp/Solidity.qhc"
devhelp:
$(SPHINXBUILD) -b devhelp $(ALLSPHINXOPTS) $(BUILDDIR)/devhelp
@echo
@echo "Build finished."
@echo "To view the help file:"
@echo "# mkdir -p $$HOME/.local/share/devhelp/Solidity"
@echo "# ln -s $(BUILDDIR)/devhelp $$HOME/.local/share/devhelp/Solidity"
@echo "# devhelp"
epub:
$(SPHINXBUILD) -b epub $(ALLSPHINXOPTS) $(BUILDDIR)/epub
@echo
@echo "Build finished. The epub file is in $(BUILDDIR)/epub."
latex:
$(SPHINXBUILD) -b latex $(ALLSPHINXOPTS) $(BUILDDIR)/latex
@echo
@echo "Build finished; the LaTeX files are in $(BUILDDIR)/latex."
@echo "Run \`make' in that directory to run these through (pdf)latex" \
"(use \`make latexpdf' here to do that automatically)."
latexpdf:
$(SPHINXBUILD) -b latex $(ALLSPHINXOPTS) $(BUILDDIR)/latex
@echo "Running LaTeX files through pdflatex..."
$(MAKE) -C $(BUILDDIR)/latex all-pdf
@echo "pdflatex finished; the PDF files are in $(BUILDDIR)/latex."
latexpdfja:
$(SPHINXBUILD) -b latex $(ALLSPHINXOPTS) $(BUILDDIR)/latex
@echo "Running LaTeX files through platex and dvipdfmx..."
$(MAKE) -C $(BUILDDIR)/latex all-pdf-ja
@echo "pdflatex finished; the PDF files are in $(BUILDDIR)/latex."
text:
$(SPHINXBUILD) -b text $(ALLSPHINXOPTS) $(BUILDDIR)/text
@echo
@echo "Build finished. The text files are in $(BUILDDIR)/text."
man:
$(SPHINXBUILD) -b man $(ALLSPHINXOPTS) $(BUILDDIR)/man
@echo
@echo "Build finished. The manual pages are in $(BUILDDIR)/man."
texinfo:
$(SPHINXBUILD) -b texinfo $(ALLSPHINXOPTS) $(BUILDDIR)/texinfo
@echo
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@echo "Run \`make' in that directory to run these through makeinfo" \
"(use \`make info' here to do that automatically)."
info:
$(SPHINXBUILD) -b texinfo $(ALLSPHINXOPTS) $(BUILDDIR)/texinfo
@echo "Running Texinfo files through makeinfo..."
make -C $(BUILDDIR)/texinfo info
@echo "makeinfo finished; the Info files are in $(BUILDDIR)/texinfo."
gettext:
$(SPHINXBUILD) -b gettext $(I18NSPHINXOPTS) $(BUILDDIR)/locale
@echo
@echo "Build finished. The message catalogs are in $(BUILDDIR)/locale."
changes:
$(SPHINXBUILD) -b changes $(ALLSPHINXOPTS) $(BUILDDIR)/changes
@echo
@echo "The overview file is in $(BUILDDIR)/changes."
linkcheck:
$(SPHINXBUILD) -b linkcheck $(ALLSPHINXOPTS) $(BUILDDIR)/linkcheck
@echo
@echo "Link check complete; look for any errors in the above output " \
"or in $(BUILDDIR)/linkcheck/output.txt."
doctest:
$(SPHINXBUILD) -b doctest $(ALLSPHINXOPTS) $(BUILDDIR)/doctest
@echo "Testing of doctests in the sources finished, look at the " \
"results in $(BUILDDIR)/doctest/output.txt."
xml:
$(SPHINXBUILD) -b xml $(ALLSPHINXOPTS) $(BUILDDIR)/xml
@echo
@echo "Build finished. The XML files are in $(BUILDDIR)/xml."
pseudoxml:
$(SPHINXBUILD) -b pseudoxml $(ALLSPHINXOPTS) $(BUILDDIR)/pseudoxml
@echo
@echo "Build finished. The pseudo-XML files are in $(BUILDDIR)/pseudoxml."

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###############
Common Patterns
###############
.. index:: access;restricting
******************
Restricting Access
******************
Restricting access is a common pattern for contracts.
Note that you can never restrict any human or computer
from reading the content of your transactions or
your contract's state. You can make it a bit harder
by using encryption, but if your contract is supposed
to read the data, so will everyone else.
You can restrict read access to your contract's state
by **other contracts**. That is actually the default
unless you declare make your state variables `public`.
Furthermore, you can restrict who can make modifications
to your contract's state or call your contract's
functions and this is what this page is about.
.. index:: function;modifier
The use of **function modifiers** makes these
restrictions highly readable.
.. {% include open_link gist="fe4ef267cbdeac151b98" %}
::
contract AccessRestriction {
// These will be assigned at the construction
// phase, where `msg.sender` is the account
// creating this contract.
address public owner = msg.sender;
uint public creationTime = now;
// Modifiers can be used to change
// the body of a function.
// If this modifier is used, it will
// prepend a check that only passes
// if the function is called from
// a certain address.
modifier onlyBy(address _account)
{
if (msg.sender != _account)
throw;
// Do not forget the "_"! It will
// be replaced by the actual function
// body when the modifier is invoked.
_
}
/// Make `_newOwner` the new owner of this
/// contract.
function changeOwner(address _newOwner)
onlyBy(owner)
{
owner = _newOwner;
}
modifier onlyAfter(uint _time) {
if (now < _time) throw;
_
}
/// Erase ownership information.
/// May only be called 6 weeks after
/// the contract has been created.
function disown()
onlyBy(owner)
onlyAfter(creationTime + 6 weeks)
{
delete owner;
}
// This modifier requires a certain
// fee being associated with a function call.
// If the caller sent too much, he or she is
// refunded, but only after the function body.
// This is dangerous, because if the function
// uses `return` explicitly, this will not be
// done!
modifier costs(uint _amount) {
if (msg.value < _amount)
throw;
_
if (msg.value > _amount)
msg.sender.send(_amount - msg.value);
}
function forceOwnerChange(address _newOwner)
costs(200 ether)
{
owner = _newOwner;
// just some example condition
if (uint(owner) & 0 == 1)
// in this case, overpaid fees will not
// be refunded
return;
// otherwise, refund overpaid fees
}
}
A more specialised way in which access to function
calls can be restricted will be discussed
in the next example.
.. index:: state machine
*************
State Machine
*************
Contracts often act as a state machine, which means
that they have certain **stages** in which they behave
differently or in which different functions can
be called. A function call often ends a stage
and transitions the contract into the next stage
(especially if the contract models **interaction**).
It is also common that some stages are automatically
reached at a certain point in **time**.
An example for this is a blind auction contract which
starts in the stage "accepting blinded bids", then
transitions to "revealing bids" which is ended by
"determine auction autcome".
.. index:: function;modifier
Function modifiers can be used in this situation
to model the states and guard against
incorrect usage of the contract.
Example
=======
In the following example,
the modifier `atStage` ensures that the function can
only be called at a certain stage.
Automatic timed transitions
are handled by the modifier `timeTransitions`, which
should be used for all functions.
.. note::
**Modifier Order Matters**.
If atStage is combined
with timedTransitions, make sure that you mention
it after the latter, so that the new stage is
taken into account.
Finally, the modifier `transitionNext` can be used
to automatically go to the next stage when the
function finishes.
.. note::
**Modifier May be Skipped**.
Since modifiers are applied by simply replacing
code and not by using a function call,
the code in the transitionNext modifier
can be skipped if the function itself uses
return. If you want to do that, make sure
to call nextStage manually from those functions.
.. {% include open_link gist="0a221eaceb6d708bf271" %}
::
contract StateMachine {
enum Stages {
AcceptingBlindedBids,
RevealBids,
AnotherStage,
AreWeDoneYet,
Finished
}
// This is the current stage.
Stages public stage = Stages.AcceptingBlindedBids;
uint public creationTime = now;
modifier atStage(Stages _stage) {
if (stage != _stage) throw;
_
}
function nextStage() internal {
stage = Stages(uint(stage) + 1);
}
// Perform timed transitions. Be sure to mention
// this modifier first, otherwise the guards
// will not take the new stage into account.
modifier timedTransitions() {
if (stage == Stages.AcceptingBlindedBids &&
now >= creationTime + 10 days)
nextStage();
if (stage == Stages.RevealBids &&
now >= creationTime + 12 days)
nextStage();
// The other stages transition by transaction
}
// Order of the modifiers matters here!
function bid()
timedTransitions
atStage(Stages.AcceptingBlindedBids)
{
// We will not implement that here
}
function reveal()
timedTransitions
atStage(Stages.RevealBids)
{
}
// This modifier goes to the next stage
// after the function is done.
// If you use `return` in the function,
// `nextStage` will not be called
// automatically.
modifier transitionNext()
{
_
nextStage();
}
function g()
timedTransitions
atStage(Stages.AnotherStage)
transitionNext
{
// If you want to use `return` here,
// you have to call `nextStage()` manually.
}
function h()
timedTransitions
atStage(Stages.AreWeDoneYet)
transitionNext
{
}
function i()
timedTransitions
atStage(Stages.Finished)
{
}
}

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#!/usr/bin/env python3
# -*- coding: utf-8 -*-
#
# Solidity documentation build configuration file, created by
# sphinx-quickstart on Mon Dec 7 12:32:57 2015.
#
# This file is execfile()d with the current directory set to its
# containing dir.
#
# Note that not all possible configuration values are present in this
# autogenerated file.
#
# All configuration values have a default; values that are commented out
# serve to show the default.
import sys
import os
# If extensions (or modules to document with autodoc) are in another directory,
# add these directories to sys.path here. If the directory is relative to the
# documentation root, use os.path.abspath to make it absolute, like shown here.
#sys.path.insert(0, os.path.abspath('.'))
# -- General configuration ------------------------------------------------
# If your documentation needs a minimal Sphinx version, state it here.
#needs_sphinx = '1.0'
# Add any Sphinx extension module names here, as strings. They can be
# extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
# ones.
extensions = []
# Add any paths that contain templates here, relative to this directory.
templates_path = ['_templates']
# The suffix of source filenames.
source_suffix = '.rst'
# The encoding of source files.
#source_encoding = 'utf-8-sig'
# The master toctree document.
master_doc = 'index'
# General information about the project.
project = 'Solidity'
copyright = '2015, Ethereum'
# The version info for the project you're documenting, acts as replacement for
# |version| and |release|, also used in various other places throughout the
# built documents.
#
# The short X.Y version.
version = '0.2.0'
# The full version, including alpha/beta/rc tags.
release = '0.2.0'
# The language for content autogenerated by Sphinx. Refer to documentation
# for a list of supported languages.
#language = None
# There are two options for replacing |today|: either, you set today to some
# non-false value, then it is used:
#today = ''
# Else, today_fmt is used as the format for a strftime call.
#today_fmt = '%B %d, %Y'
# List of patterns, relative to source directory, that match files and
# directories to ignore when looking for source files.
exclude_patterns = ['_build']
# The reST default role (used for this markup: `text`) to use for all
# documents.
#default_role = None
# If true, '()' will be appended to :func: etc. cross-reference text.
#add_function_parentheses = True
# If true, the current module name will be prepended to all description
# unit titles (such as .. function::).
#add_module_names = True
# If true, sectionauthor and moduleauthor directives will be shown in the
# output. They are ignored by default.
#show_authors = False
# The name of the Pygments (syntax highlighting) style to use.
pygments_style = 'sphinx'
highlight_language = 'javascript'
# A list of ignored prefixes for module index sorting.
#modindex_common_prefix = []
# If true, keep warnings as "system message" paragraphs in the built documents.
#keep_warnings = False
# -- Options for HTML output ----------------------------------------------
# The theme to use for HTML and HTML Help pages. See the documentation for
# a list of builtin themes.
html_theme = 'default'
# Theme options are theme-specific and customize the look and feel of a theme
# further. For a list of options available for each theme, see the
# documentation.
#html_theme_options = {}
# Add any paths that contain custom themes here, relative to this directory.
#html_theme_path = []
# The name for this set of Sphinx documents. If None, it defaults to
# "<project> v<release> documentation".
#html_title = None
# A shorter title for the navigation bar. Default is the same as html_title.
#html_short_title = None
# The name of an image file (relative to this directory) to place at the top
# of the sidebar.
#html_logo = None
# The name of an image file (within the static path) to use as favicon of the
# docs. This file should be a Windows icon file (.ico) being 16x16 or 32x32
# pixels large.
#html_favicon = None
# Add any paths that contain custom static files (such as style sheets) here,
# relative to this directory. They are copied after the builtin static files,
# so a file named "default.css" will overwrite the builtin "default.css".
html_static_path = ['_static']
# Add any extra paths that contain custom files (such as robots.txt or
# .htaccess) here, relative to this directory. These files are copied
# directly to the root of the documentation.
#html_extra_path = []
# If not '', a 'Last updated on:' timestamp is inserted at every page bottom,
# using the given strftime format.
#html_last_updated_fmt = '%b %d, %Y'
# If true, SmartyPants will be used to convert quotes and dashes to
# typographically correct entities.
#html_use_smartypants = True
# Custom sidebar templates, maps document names to template names.
#html_sidebars = {}
# Additional templates that should be rendered to pages, maps page names to
# template names.
#html_additional_pages = {}
# If false, no module index is generated.
#html_domain_indices = True
# If false, no index is generated.
#html_use_index = True
# If true, the index is split into individual pages for each letter.
#html_split_index = False
# If true, links to the reST sources are added to the pages.
#html_show_sourcelink = True
# If true, "Created using Sphinx" is shown in the HTML footer. Default is True.
#html_show_sphinx = True
# If true, "(C) Copyright ..." is shown in the HTML footer. Default is True.
#html_show_copyright = True
# If true, an OpenSearch description file will be output, and all pages will
# contain a <link> tag referring to it. The value of this option must be the
# base URL from which the finished HTML is served.
#html_use_opensearch = ''
# This is the file name suffix for HTML files (e.g. ".xhtml").
#html_file_suffix = None
# Output file base name for HTML help builder.
htmlhelp_basename = 'Soliditydoc'
# -- Options for LaTeX output ---------------------------------------------
latex_elements = {
# The paper size ('letterpaper' or 'a4paper').
#'papersize': 'letterpaper',
# The font size ('10pt', '11pt' or '12pt').
#'pointsize': '10pt',
# Additional stuff for the LaTeX preamble.
#'preamble': '',
}
# Grouping the document tree into LaTeX files. List of tuples
# (source start file, target name, title,
# author, documentclass [howto, manual, or own class]).
latex_documents = [
]
# The name of an image file (relative to this directory) to place at the top of
# the title page.
#latex_logo = None
# For "manual" documents, if this is true, then toplevel headings are parts,
# not chapters.
#latex_use_parts = False
# If true, show page references after internal links.
#latex_show_pagerefs = False
# If true, show URL addresses after external links.
#latex_show_urls = False
# Documents to append as an appendix to all manuals.
#latex_appendices = []
# If false, no module index is generated.
#latex_domain_indices = True
# -- Options for manual page output ---------------------------------------
# One entry per manual page. List of tuples
# (source start file, name, description, authors, manual section).
man_pages = [
]
# If true, show URL addresses after external links.
#man_show_urls = False
# -- Options for Texinfo output -------------------------------------------
# Grouping the document tree into Texinfo files. List of tuples
# (source start file, target name, title, author,
# dir menu entry, description, category)
texinfo_documents = [
]
# Documents to append as an appendix to all manuals.
#texinfo_appendices = []
# If false, no module index is generated.
#texinfo_domain_indices = True
# How to display URL addresses: 'footnote', 'no', or 'inline'.
#texinfo_show_urls = 'footnote'
# If true, do not generate a @detailmenu in the "Top" node's menu.
#texinfo_no_detailmenu = False

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.. index:: ! contract
##########
Contracts
##########
Contracts in Solidity what classes are in object oriented languages.
They persistent data in state variables and functions that can modify these variables.
Calling a function on a different contract (instance) will perform an EVM
function call and thus switch the context such that state variables are
inaccessible.
.. index:: ! contract;creation
******************
Creating Contracts
******************
Contracts can be created "from outside" or from Solidity contracts.
When a contract is created, its constructor (a function with the same
name as the contract) is executed once.
From `web3.js`, i.e. the JavaScript
API, this is done as follows::
// The json abi array generated by the compiler
var abiArray = [
{
"inputs":[
{"name":"x","type":"uint256"},
{"name":"y","type":"uint256"}
],
"type":"constructor"
},
{
"constant":true,
"inputs":[],
"name":"x",
"outputs":[{"name":"","type":"bytes32"}],
"type":"function"
}
];
var MyContract = web3.eth.contract(abiArray);
// deploy new contract
var contractInstance = MyContract.new(
10,
{from: myAccount, gas: 1000000}
);
.. index:: constructor;arguments
Internally, constructor arguments are passed after the code of
the contract itself, but you do not have to care about this
if you use `web3.js`.
If a contract wants to create another contract, the source code
(and the binary) of the created contract has to be known to the creator.
This means that cyclic creation dependencies are impossible.
::
contract OwnedToken {
// TokenCreator is a contract type that is defined below.
// It is fine to reference it as long as it is not used
// to create a new contract.
TokenCreator creator;
address owner;
bytes32 name;
// This is the constructor which registers the
// creator and the assigned name.
function OwnedToken(bytes32 _name) {
owner = msg.sender;
// We do an explicit type conversion from `address`
// to `TokenCreator` and assume that the type of
// the calling contract is TokenCreator, there is
// no real way to check that.
creator = TokenCreator(msg.sender);
name = _name;
}
function changeName(bytes32 newName) {
// Only the creator can alter the name --
// the comparison is possible since contracts
// are implicitly convertible to addresses.
if (msg.sender == creator) name = newName;
}
function transfer(address newOwner) {
// Only the current owner can transfer the token.
if (msg.sender != owner) return;
// We also want to ask the creator if the transfer
// is fine. Note that this calls a function of the
// contract defined below. If the call fails (e.g.
// due to out-of-gas), the execution here stops
// immediately.
if (creator.isTokenTransferOK(owner, newOwner))
owner = newOwner;
}
}
contract TokenCreator {
function createToken(bytes32 name)
returns (OwnedToken tokenAddress)
{
// Create a new Token contract and return its address.
// From the JavaScript side, the return type is simply
// "address", as this is the closest type available in
// the ABI.
return new OwnedToken(name);
}
function changeName(OwnedToken tokenAddress, bytes32 name) {
// Again, the external type of "tokenAddress" is
// simply "address".
tokenAddress.changeName(name);
}
function isTokenTransferOK(
address currentOwner,
address newOwner
) returns (bool ok) {
// Check some arbitrary condition.
address tokenAddress = msg.sender;
return (sha3(newOwner) & 0xff) == (bytes20(tokenAddress) & 0xff);
}
}
.. index:: ! visibility, external, public, private, internal
************************
Visibility and Accessors
************************
Since Solidity knows two kinds of function calls (internal
ones that do not create an actual EVM call (also called
a "message call") and external
ones that do), there are four types of visibilities for
functions and state variables.
Functions can be specified as being `external`,
`public`, `internal` or `private`, where the default is
`public`. For state variables, `external` is not possible
and the default is `internal`.
`external`:
External functions are part of the contract
interface, which means they can be called from other contracts and
via transactions. An external function `f` cannot be called
internally (i.e. `f()` does not work, but `this.f()` works).
External functions are sometimes more efficient when
they receive large arrays of data.
`public`:
Public functions are part of the contract
interface and can be either called internally or via
messages. For public state variables, an automatic accessor
function (see below) is generated.
`internal`:
Those functions and state variables can only be
accessed internally (i.e. from within the current contract
or contracts deriving from it), without using `this`.
`private`:
Private functions and state variables are only
visible for the contract they are defined in and not in
derived contracts.
The visibility specifier is given after the type for
state variables and between parameter list and
return parameter list for functions.
::
contract c {
function f(uint a) private returns (uint b) { return a + 1; }
function setData(uint a) internal { data = a; }
uint public data;
}
Other contracts can call `c.data()` to retrieve the value of
data in state storage, but are not able to call `f`.
Contracts derived from `c` can call `setData` to alter the
value of `data` (but only in their own state).
.. index:: ! accessor;function, ! function;accessor
Accessor Functions
==================
The compiler automatically creates accessor functions for
all public state variables. The contract given below will
have a function called `data` that does not take any
arguments and returns a uint, the value of the state
variable `data`. The initialization of state variables can
be done at declaration.
The accessor functions have external visibility. If the
symbol is accessed internally (i.e. without `this.`),
it is a state variable and if it is accessed externally
(i.e. with `this.`), it is a function.
::
contract test {
uint public data = 42;
}
The next example is a bit more complex:
::
contract complex {
struct Data { uint a; bytes3 b; mapping(uint => uint) map; }
mapping(uint => mapping(bool => Data[])) public data;
}
It will generate a function of the following form::
function data(uint arg1, bool arg2, uint arg3) returns (uint a, bytes3 b)
{
a = data[arg1][arg2][arg3].a;
b = data[arg1][arg2][arg3].b;
}
Note that the mapping in the struct is omitted because there
is no good way to provide the key for the mapping.
.. index:: ! function;modifier
******************
Function Modifiers
******************
Modifiers can be used to easily change the behaviour of functions, for example
to automatically check a condition prior to executing the function. They are
inheritable properties of contracts and may be overridden by derived contracts.
::
contract owned {
function owned() { owner = msg.sender; }
address owner;
// This contract only defines a modifier but does not use
// it - it will be used in derived contracts.
// The function body is inserted where the special symbol
// "_" in the definition of a modifier appears.
// This means that if the owner calls this function, the
// function is executed and otherwise, an exception is
// thrown.
modifier onlyowner { if (msg.sender != owner) throw; _ }
}
contract mortal is owned {
// This contract inherits the "onlyowner"-modifier from
// "owned" and applies it to the "close"-function, which
// causes that calls to "close" only have an effect if
// they are made by the stored owner.
function close() onlyowner {
selfdestruct(owner);
}
}
contract priced {
// Modifiers can receive arguments:
modifier costs(uint price) { if (msg.value >= price) _ }
}
contract Register is priced, owned {
mapping (address => bool) registeredAddresses;
uint price;
function Register(uint initialPrice) { price = initialPrice; }
function register() costs(price) {
registeredAddresses[msg.sender] = true;
}
function changePrice(uint _price) onlyowner {
price = _price;
}
}
Multiple modifiers can be applied to a function by specifying them in a
whitespace-separated list and will be evaluated in order. Explicit returns from
a modifier or function body immediately leave the whole function, while control
flow reaching the end of a function or modifier body continues after the "_" in
the preceding modifier. Arbitrary expressions are allowed for modifier
arguments and in this context, all symbols visible from the function are
visible in the modifier. Symbols introduced in the modifier are not visible in
the function (as they might change by overriding).
.. index:: ! constant
**********
Constants
**********
State variables can be declared as constant (this is not yet implemented
for array and struct types and not possible for mapping types).
::
contract C {
uint constant x = 32**22 + 8;
string constant text = "abc";
}
This has the effect that the compiler does not reserve a storage slot
for these variables and every occurrence is replaced by their constant value.
The value expression can only contain integer arithmetics.
.. index:: ! fallback function, function;fallback
.. _fallback-function:
*****************
Fallback Function
*****************
A contract can have exactly one unnamed function. This function cannot have
arguments and is executed on a call to the contract if none of the other
functions matches the given function identifier (or if no data was supplied at
all).
Furthermore, this function is executed whenever the contract receives plain
Ether (witout data). In such a context, there is very little gas available to
the function call, so it is important to make fallback functions as cheap as
possible.
::
contract Test {
function() { x = 1; }
uint x;
}
// This contract rejects any Ether sent to it. It is good
// practise to include such a function for every contract
// in order not to loose Ether.
contract Rejector {
function() { throw; }
}
contract Caller {
function callTest(address testAddress) {
Test(testAddress).call(0xabcdef01); // hash does not exist
// results in Test(testAddress).x becoming == 1.
Rejector r = Rejector(0x123);
r.send(2 ether);
// results in r.balance == 0
}
}
.. index:: ! event
******
Events
******
Events allow the convenient usage of the EVM logging facilities,
which in turn can be used to "call" JavaScript callbacks in the user interface
of a dapp, which listen for these events.
Events are
inheritable members of contracts. When they are called, they cause the
arguments to be stored in the transaction's log - a special data structure
in the blockchain. These logs are associated with the address of
the contract and will be incorporated into the blockchain
and stay there as long as a block is accessible (forever as of
Frontier and Homestead, but this might change with Serenity). Log and
event data is not accessible from within contracts (not even from
the contract that created a log).
SPV proofs for logs are possible, so if an external entity supplies
a contract with such a proof, it can check that the log actually
exists inside the blockchain (but be aware of the fact that
ultimately, also the block headers have to be supplied because
the contract can only see the last 256 block hashes).
Up to three parameters can
receive the attribute `indexed` which will cause the respective arguments
to be searched for: It is possible to filter for specific values of
indexed arguments in the user interface.
If arrays (including `string` and `bytes`) are used as indexed arguments, the
sha3-hash of it is stored as topic instead.
The hash of the signature of the event is one of the topics except if you
declared the event with `anonymous` specifier. This means that it is
not possible to filter for specific anonymous events by name.
All non-indexed arguments will be stored in the data part of the log.
::
contract ClientReceipt {
event Deposit(
address indexed _from,
bytes32 indexed _id,
uint _value
);
function deposit(bytes32 _id) {
// Any call to this function (even deeply nested) can
// be detected from the JavaScript API by filtering
// for `Deposit` to be called.
Deposit(msg.sender, _id, msg.value);
}
}
The use in the JavaScript API would be as follows:
::
var abi = /* abi as generated by the compiler */;
var ClientReceipt = web3.eth.contract(abi);
var clientReceipt = ClientReceipt.at(0x123 /* address */);
var event = clientReceipt.Deposit();
// watch for changes
event.watch(function(error, result){
// result will contain various information
// including the argumets given to the Deposit
// call.
if (!error)
console.log(result);
});
// Or pass a callback to start watching immediately
var event = clientReceipt.Deposit(function(error, result) {
if (!error)
console.log(result);
});
.. index:: ! log
Low-Level Interface to Logs
===========================
It is also possible to access the low-level interface to the logging
mechanism via the functions `log0`, `log1`, `log2`, `log3` and `log4`.
`logi` takes `i + 1` parameter of type `bytes32`, where the first
argument will be used for the data part of the log and the others
as topics. The event call above can be performed in the same way as
::
log3(
msg.value,
0x50cb9fe53daa9737b786ab3646f04d0150dc50ef4e75f59509d83667ad5adb20,
msg.sender,
_id
);
where the long hexadecimal number is equal to
`sha3("Deposit(address,hash256,uint256)")`, the signature of the event.
Additional Resources for Understanding Events
==============================================
- `Javascript documentation <https://github.com/ethereum/wiki/wiki/JavaScript-API#contract-events>`_
- `Example usage of events <https://github.com/debris/smart-exchange/blob/master/lib/contracts/SmartExchange.sol>`_
- `How to access them in js <https://github.com/debris/smart-exchange/blob/master/lib/exchange_transactions.js>`_
.. index:: ! inheritance, ! base class, ! contract;base, ! deriving
***********
Inheritance
***********
Solidity supports multiple inheritance by copying code including polymorphism.
All function calls are virtual, which means that the most derived function
is called, except when the contract is explicitly given.
Even if a contract inherits from multiple other contracts, only a single
contract is created on the blockchain, the code from the base contracts
is always copied into the final contract.
The general inheritance system is very similar to
`Python's <https://docs.python.org/3/tutorial/classes.html#inheritance>`_,
especially concerning multiple inheritance.
Details are given in the following example.
::
contract owned {
function owned() { owner = msg.sender; }
address owner;
}
// Use "is" to derive from another contract. Derived
// contracts can access all non-private members including
// internal functions and state variables. These cannot be
// accessed externally via `this`, though.
contract mortal is owned {
function kill() {
if (msg.sender == owner) selfdestruct(owner);
}
}
// These abstract contracts are only provided to make the
// interface known to the compiler. Note the function
// without body. If a contract does not implement all
// functions it can only be used as an interface.
contract Config {
function lookup(uint id) returns (address adr);
}
contract NameReg {
function register(bytes32 name);
function unregister();
}
// Multiple inheritance is possible. Note that "owned" is
// also a base class of "mortal", yet there is only a single
// instance of "owned" (as for virtual inheritance in C++).
contract named is owned, mortal {
function named(bytes32 name) {
Config config = Config(0xd5f9d8d94886e70b06e474c3fb14fd43e2f23970);
NameReg(config.lookup(1)).register(name);
}
// Functions can be overridden, both local and
// message-based function calls take these overrides
// into account.
function kill() {
if (msg.sender == owner) {
Config config = Config(0xd5f9d8d94886e70b06e474c3fb14fd43e2f23970);
NameReg(config.lookup(1)).unregister();
// It is still possible to call a specific
// overridden function.
mortal.kill();
}
}
}
// If a constructor takes an argument, it needs to be
// provided in the header (or modifier-invocation-style at
// the constructor of the derived contract (see below)).
contract PriceFeed is owned, mortal, named("GoldFeed") {
function updateInfo(uint newInfo) {
if (msg.sender == owner) info = newInfo;
}
function get() constant returns(uint r) { return info; }
uint info;
}
Note that above, we call `mortal.kill()` to "forward" the
destruction request. The way this is done is problematic, as
seen in the following example::
contract mortal is owned {
function kill() {
if (msg.sender == owner) selfdestruct(owner);
}
}
contract Base1 is mortal {
function kill() { /* do cleanup 1 */ mortal.kill(); }
}
contract Base2 is mortal {
function kill() { /* do cleanup 2 */ mortal.kill(); }
}
contract Final is Base1, Base2 {
}
A call to `Final.kill()` will call `Base2.kill` as the most
derived override, but this function will bypass
`Base1.kill`, basically because it does not even know about
`Base1`. The way around this is to use `super`::
contract mortal is owned {
function kill() {
if (msg.sender == owner) selfdestruct(owner);
}
}
contract Base1 is mortal {
function kill() { /* do cleanup 1 */ super.kill(); }
}
contract Base2 is mortal {
function kill() { /* do cleanup 2 */ super.kill(); }
}
contract Final is Base2, Base1 {
}
If `Base1` calls a function of `super`, it does not simply
call this function on one of its base contracts, it rather
calls this function on the next base contract in the final
inheritance graph, so it will call `Base2.kill()` (note that
the final inheritance sequence is -- starting with the most
derived contract: Final, Base1, Base2, mortal, owned).
The actual function that is called when using super is
not known in the context of the class where it is used,
although its type is known. This is similar for ordinary
virtual method lookup.
.. index:: ! base;constructor
Arguments for Base Constructors
===============================
Derived contracts need to provide all arguments needed for
the base constructors. This can be done at two places::
contract Base {
uint x;
function Base(uint _x) { x = _x; }
}
contract Derived is Base(7) {
function Derived(uint _y) Base(_y * _y) {
}
}
Either directly in the inheritance list (`is Base(7)`) or in
the way a modifier would be invoked as part of the header of
the derived constructor (`Base(_y * _y)`). The first way to
do it is more convenient if the constructor argument is a
constant and defines the behaviour of the contract or
describes it. The second way has to be used if the
constructor arguments of the base depend on those of the
derived contract. If, as in this silly example, both places
are used, the modifier-style argument takes precedence.
.. index:: ! inheritance;multiple, ! linearization, ! C3 linearization
Multiple Inheritance and Linearization
======================================
Languages that allow multiple inheritance have to deal with
several problems, one of them being the `Diamond Problem <https://en.wikipedia.org/wiki/Multiple_inheritance#The_diamond_problem>`_.
Solidity follows the path of Python and uses "`C3 Linearization <https://en.wikipedia.org/wiki/C3_linearization>`_"
to force a specific order in the DAG of base classes. This
results in the desirable property of monotonicity but
disallows some inheritance graphs. Especially, the order in
which the base classes are given in the `is` directive is
important. In the following code, Solidity will give the
error "Linearization of inheritance graph impossible".
::
contract X {}
contract A is X {}
contract C is A, X {}
The reason for this is that `C` requests `X` to override `A`
(by specifying `A, X` in this order), but `A` itself
requests to override `X`, which is a contradiction that
cannot be resolved.
A simple rule to remember is to specify the base classes in
the order from "most base-like" to "most derived".
.. index:: ! contract;abstract, ! abstract contract
******************
Abstract Contracts
******************
Contract functions can lack an implementation as in the following example (note that the function declaration header is terminated by `;`)::
contract feline {
function utterance() returns (bytes32);
}
Such contracts cannot be compiled (even if they contain implemented functions alongside non-implemented functions), but they can be used as base contracts::
contract Cat is feline {
function utterance() returns (bytes32) { return "miaow"; }
}
If a contract inherits from an abstract contract and does not implement all non-implemented functions by overriding, it will itself be abstract.
.. index:: ! library, callcode
.. _libraries:
************
Libraries
************
Libraries are similar to contracts, but their purpose is that they are deployed
only once at a specific address and their code is reused using the `CALLCODE`
feature of the EVM. This means that if library functions are called, their code
is executed in the context of the calling contract, i.e. `this` points to the
calling contract and especially the storage from the calling contract can be
accessed. As a library is an isolated piece of source code, it can only access
state variables of the calling contract if they are explicitly supplied (it
would have to way to name them, otherwise).
.. index:: using for, set
The following example illustrates how to use libraries (but
be sure to check out :ref:`using for <using-for>` for a
more advanced example to implement a set).
::
library Set {
// We define a new struct datatype that will be used to
// hold its data in the calling contract.
struct Data { mapping(uint => bool) flags; }
// Note that the first parameter is of type "storage
// reference" and thus only its storage address and not
// its contents is passed as part of the call. This is a
// special feature of library functions. It is idiomatic
// to call the first parameter 'self', if the function can
// be seen as a method of that object.
function insert(Data storage self, uint value)
returns (bool)
{
if (self.flags[value])
return false; // already there
self.flags[value] = true;
return true;
}
function remove(Data storage self, uint value)
returns (bool)
{
if (!self.flags[value])
return false; // not there
self.flags[value] = false;
return true;
}
function contains(Data storage self, uint value)
returns (bool)
{
return self.flags[value];
}
}
contract C {
Set.Data knownValues;
function register(uint value) {
// The library functions can be called without a
// specific instance of the library, since the
// "instance" will be the current contract.
if (!Set.insert(knownValues, value))
throw;
}
// In this contract, we can also directly access knownValues.flags, if we want.
}
Of course, you do not have to follow this way to use
libraries - they can also be used without defining struct
data types, functions also work without any storage
reference parameters, can have multiple storage reference
parameters and in any position.
The calls to `Set.contains`, `Set.insert` and `Set.remove`
are all compiled as calls (`CALLCODE`s) to an external
contract/library. If you use libraries, take care that an
actual external function call is performed, so `msg.sender`
does not point to the original sender anymore but to the the
calling contract and also `msg.value` contains the funds
sent during the call to the library function.
As the compiler cannot know where the library will be
deployed at, these addresses have to be filled into the
final bytecode by a linker (see [Using the Commandline
Compiler](#using-the-commandline-compiler) on how to use the
commandline compiler for linking). If the addresses are not
given as arguments to the compiler, the compiled hex code
will contain placeholders of the form `__Set______` (where
`Set` is the name of the library). The address can be filled
manually by replacing all those 40 symbols by the hex
encoding of the address of the library contract.
Restrictions for libraries in comparison to contracts:
- no state variables
- cannot inherit nor be inherited
(these might be lifted at a later point)
Common pitfalls for libraries
=============================
.. index:: msg;sender
The value of `msg.sender`
-------------------------
The value for `msg.sender` will be that of the contract which is calling the library function.
For example, if A calls contract B which internally calls library C, then within the function call of library C, `msg.sender` will be the address of contract B.
The reason for this is that the expression `LibraryName.functionName()`
performs an external function call using `CALLCODE`, which maps to a real EVM
call just like `otherContract.functionName()` or `this.functionName()`. This
call extends the call depth by one (limited to 1024), stores the caller (the
current contract) as `msg.sender`, and then executes the library contract's
code against the current contracts storage. This execution occurs in a
completely new memory context meaning that memory types will be copied and
cannot be passed by reference.
Transferring Ether
-------------------------
It is *in principle* possible to transfer ether using
`LibraryName.functionName.value(x)()`, but as `CALLCODE` is used, the Ether
will just end up at the current contract.
.. index:: ! using for, library
.. _using-for:
*********
Using For
*********
The directive `using A for B;` can be used to attach library
functions (from the library `A`) to any type (`B`).
These functions will receive the object they are called on
as their first parameter (like the `self` variable in
Python).
The effect of `using A for *;` is that the functions from
the library `A` are attached to any type.
In both situations, all functions, even those where the
type of the first parameter does not match the type of
the object, are attached. The type is checked at the
point the function is called and function overload
resolution is performed.
The `using A for B;` directive is active for the current
scope, which is limited to a contract for now but will
be lifted to the global scope later, so that by including
a module, its data types including library functions are
available without having to add further code.
Let us rewrite the set example from the
:ref:`libraries` in this way::
// This is the same code as before, just without comments
library Set {
struct Data { mapping(uint => bool) flags; }
function insert(Data storage self, uint value)
returns (bool)
{
if (self.flags[value])
return false; // already there
self.flags[value] = true;
return true;
}
function remove(Data storage self, uint value)
returns (bool)
{
if (!self.flags[value])
return false; // not there
self.flags[value] = false;
return true;
}
function contains(Data storage self, uint value)
returns (bool)
{
return self.flags[value];
}
}
contract C {
using Set for Set.Data; // this is the crucial change
Set.Data knownValues;
function register(uint value) {
// Here, all variables of type Set.Data have
// corresponding member functions.
// The following function call is identical to
// Set.insert(knownValues, value)
if (!knownValues.insert(value))
throw;
}
}
It is also possible to extend elementary types in that way::
library Search {
function indexOf(uint[] storage self, uint value) {
for (uint i = 0; i < self.length; i++)
if (self[i] == value) return i;
return uint(-1);
}
}
contract C {
using Search for uint[];
uint[] data;
function append(uint value) {
data.push(value);
}
function replace(uint _old, uint _new) {
// This performs the library function call
uint index = data.find(_old);
if (index == -1)
data.push(_new);
else
data[index] = _new;
}
}
Note that all library calls are actual EVM function calls. This means that
if you pass memory or value types, a copy will be performed, even of the
`self` variable. The only situation where no copy will be performed
is when storage reference variables are used.

167
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##################################
Expressions and Control Structures
##################################
.. index:: if, else, while, for, break, continue, return, switch, goto
Control Structures
===================
Most of the control structures from C/JavaScript are available in Solidity
except for `switch` and `goto`. So
there is: `if`, `else`, `while`, `for`, `break`, `continue`, `return`, with
the usual semantics known from C / JavaScript.
Parentheses can *not* be omitted for conditionals, but curly brances can be omitted
around single-statement bodies.
Note that there is no type conversion from non-boolean to boolean types as
there is in C and JavaScript, so `if (1) { ... }` is _not_ valid Solidity.
.. index:: ! function;call, function;internal, function;external
Function Calls
==============
Internal Function Calls
-----------------------
Functions of the current contract can be called directly ("internally"), also recursively, as seen in
this nonsensical example::
contract c {
function g(uint a) returns (uint ret) { return f(); }
function f() returns (uint ret) { return g(7) + f(); }
}
These function calls are translated into simple jumps inside the EVM. This has
the effect that the current memory is not cleared, i.e. passing memory references
to internally-called functions is very efficient. Only functions of the same
contract can be called internally.
External Function Calls
-----------------------
The expression `this.g(8);` is also a valid function call, but this time, the function
will be called "externally", via a message call and not directly via jumps.
Functions of other contracts have to be called externally. For an external call,
all function arguments have to be copied to memory.
When calling functions
of other contracts, the amount of Wei sent with the call and the gas can be specified::
contract InfoFeed {
function info() returns (uint ret) { return 42; }
}
contract Consumer {
InfoFeed feed;
function setFeed(address addr) { feed = InfoFeed(addr); }
function callFeed() { feed.info.value(10).gas(800)(); }
}
Note that the expression `InfoFeed(addr)` performs an explicit type conversion stating
that "we know that the type of the contract at the given address is `InfoFeed`" and
this does not execute a constructor. We could also have used `function setFeed(InfoFeed _feed) { feed = _feed; }` directly. Be careful about the fact that `feed.info.value(10).gas(800)`
only (locally) sets the value and amount of gas sent with the function call and only the
parentheses at the end perform the actual call.
Named Calls and Anonymous Function Parameters
---------------------------------------------
Function call arguments can also be given by name, in any order, and the names
of unused parameters (especially return parameters) can be omitted.
::
contract c {
function f(uint key, uint value) { ... }
function g() {
// named arguments
f({value: 2, key: 3});
}
// omitted parameters
function func(uint k, uint) returns(uint) {
return k;
}
}
Order of Evaluation of Expressions
==================================
The evaluation order of expressions is not specified (more formally, the order
in which the children of one node in the expression tree are evaluated is not
specified, but they are of course evaluated before the node itself). It is only
guaranteed that statements are executed in order and short-circuiting for
boolean expressions is done.
.. index:: ! assignment
Assignment
==========
.. index:: ! assignment;destructuring
Destructuring Assignments and Returning Multiple Values
-------------------------------------------------------
Solidity internally allows tuple types, i.e. a list of objects of potentially different types whose size is a constant at compile-time. Those tuples can be used to return multiple values at the same time and also assign them to multiple variables (or LValues in general) at the same time::
contract C {
uint[] data;
function f() returns (uint, bool, uint) {
return (7, true, 2);
}
function g() {
// Declares and assigns the variables. Specifying the type explicitly is not possible.
var (x, b, y) = f();
// Assigns to a pre-existing variable.
(x, y) = (2, 7);
// Common trick to swap values -- does not work for non-value storage types.
(x, y) = (y, x);
// Components can be left out (also for variable declarations).
// If the tuple ends in an empty component,
// the rest of the values are discarded.
(data.length,) = f(); // Sets the length to 7
// The same can be done on the left side.
(,data[3]) = f(); // Sets data[3] to 2
// Components can only be left out at the left-hand-side of assignments, with
// one exception:
(x,) = (1,);
// (1,) is the only way to specify a 1-component tuple, because (1) is
// equivalent to 1.
}
}
Complications for Arrays and Structs
------------------------------------
The semantics of assignment are a bit more complicated for non-value types like arrays and structs.
Assigning *to* a state variable always creates an independent copy. On the other hand, assigning to a local variable creates an independent copy only for elementary types, i.e. static types that fit into 32 bytes. If structs or arrays (including `bytes` and `string`) are assigned from a state variable to a local variable, the local variable holds a reference to the original state variable. A second assignment to the local variable does not modify the state but only changes the reference. Assignments to members (or elements) of the local variable *do* change the state.
.. index:: ! exception, ! throw
Exceptions
==========
There are some cases where exceptions are thrown automatically (see below). You can use the `throw` instruction to throw an exception manually. The effect of an exception is that the currently executing call is stopped and reverted (i.e. all changes to the state and balances are undone) and the exception is also "bubbled up" through Solidity function calls (exceptions are `send` and the low-level functions `call` and `callcode`, those return `false` in case of an exception).
Catching exceptions is not yet possible.
In the following example, we show how `throw` can be used to easily revert an Ether transfer and also how to check the return value of `send`::
contract Sharer {
function sendHalf(address addr) returns (uint balance) {
if (!addr.send(msg.value/2))
throw; // also reverts the transfer to Sharer
return this.balance;
}
}
Currently, there are three situations, where exceptions happen automatically in Solidity:
1. If you access an array beyond its length (i.e. `x[i]` where `i >= x.length`)
2. If a function called via a message call does not finish properly (i.e. it runs out of gas or throws an exception itself).
3. If a non-existent function on a library is called or Ether is sent to a library.
Internally, Solidity performs an "invalid jump" when an exception is thrown and thus causes the EVM to revert all changes made to the state. The reason for this is that there is no safe way to continue execution, because an expected effect did not occur. Because we want to retain the atomicity of transactions, the safest thing to do is to revert all changes and make the whole transaction (or at least call) without effect.

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###########################
Frequently Asked Questions
###########################
This list was originally compiled by [fivedogit](mailto:fivedogit@gmail.com).
***************
Basic Questions
***************
What is Solidity?
=============================
Solidity is the DEV-created (i.e. Ethereum Foundation-created),
Javascript-inspired language that can be used to create smart contracts
on the Ethereum blockchain. There are other
languages you can use as well (LLL, Serpent, etc). The main points in
favour of Solidity is that it is statically typed and offers many
advanced features like inheritance, libraries, complex
user-defined types and a bytecode optimizer.
Solidity contracts can be compiled a few different ways (see below) and the
resulting output can be cut/pasted into a geth console to deploy them to the
Ethereum blockchain.
There are some `contract examples <https://github.com/fivedogit/solidity-baby-steps/tree/master/contracts/>`_ by fivedogit and
there should be a `test contract <https://github.com/ethereum/solidity/blob/develop/test/libsolidity/SolidityEndToEndTest.cpp>`_ for every single feature of Solidity.
How do I compile contracts?
=============================
Probably the fastest way is the `online compiler <https://chriseth.github.io/browser-solidity/>`_.
You can also use the `solc` binary which comes with cpp-ethereum to compile
contracts or an emerging option is to use Mix, the IDE.
Create and publish the most basic contract possible
===================================================
A quite simple contract is the `greeter <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/05_greeter.sol>`_
Is it possible to do something on a specific block number? (e.g. publish a contract or execute a transaction)
=============================================================================================================
Transactions are not guaranteed to happen on the next block or any future
specific block, since it is up to the miners to include transactions and not up
to the submitter of the transaction. This applies to function calls/transactions and contract
creation transactions.
If you want to schedule future calls of your contract, you can use the
`alarm clock <http://www.ethereum-alarm-clock.com/>`_.
What is the transaction "payload"?
==================================
This is just the bytecode "data" sent along with the request.
Is there a decompiler available?
================================
There is no decompiler to Solidity. This is in principle possible
to some degree, but for example variable names will be lost and
great effort will be necessary to make it look similar to
the original source code.
Bytecode can be decompiled to opcodes, a service that is provided by
several blockchain explorers.
Contracts on the blockchain should have their original source
code published if they are to be used by third parties.
Does selfdestruct() free up space in the blockchain?
====================================================
It removes the contract bytecode and storage from the current block
into the future, but since the blockchain stores every single block (i.e.
all history), this will not actually free up space on full/achive nodes.
Create a contract that can be killed and return funds
=====================================================
First, a word of warning: Killing contracts sounds like a good idea, because "cleaning up"
is always good, but as seen above, it does not really clean up. Furthermore,
if Ether is sent to removed contracts, the Ether will be forever lost.
If you want to deactivate your contracts, rather **disable** them by changing some
internal state which causes all functions to throw. This will make it impossible
to use the contract and ether sent to the contract will be returned automatically.
Now to answering the question: Inside a constructor, `msg.sender` is the
creator. Save it. Then `selfdestruct(creator);` to kill and return funds.
`example <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/05_greeter.sol>`_
Note that if you `import "mortal"` at the top of your contracts and declare
`contract SomeContract is mortal { ...` and compile with a compiler that already
has it (which includes `browser-solidity <https://chriseth.github.io/browser-solidity/>`_), then
`kill()` is taken care of for you. Once a contract is "mortal", then you can
`contractname.kill.sendTransaction({from:eth.coinbase})`, just the same as my
examples.
Store Ether in a contract
=========================
The trick is to create the contract with `{from:someaddress, value: web3.toWei(3,"ether")...}`
See `endowment_retriever.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/30_endowment_retriever.sol>`_.
Use a non-constant function (req sendTransaction) to increment a variable in a contract
=======================================================================================
See `value_incrementer.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/20_value_incrementer.sol>`_.
Get contract address in Solidity
================================
Short answer: The global variable `this` is the contract address.
See `basic_info_getter <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/15_basic_info_getter.sol>`_.
Long answer: `this` is a variable representing the current contract.
Its type is the type of the contract. Since any contract type basically inherits from the
`address` type, `this` is always convertible to `address` and in this case contains
its own address.
What is the difference between a function marked constant and one that is not?
==============================================================================
`constant` functions can perform some action and return a value, but cannot
change state (this is not yet enforced by the compiler). In other words, a
constant function cannot save or update any variables within the contract or wider
blockchain. These functions are called using `c.someFunction(...)` from
geth or any other web3.js environment.
"non-constant" functions (those lacking the `constant` specifier) must be called
with `c.someMethod.sendTransaction({from:eth.accounts[x], gas: 1000000});`
That is, because they can change state, they have to have a gas
payment sent along to get the work done.
Get a contract to return its funds to you (not using selfdestruct(...)).
========================================================================
This example demonstrates how to send funds from a contract to an address.
See `endowment_retriever <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/30_endowment_retriever.sol>`_.
What is a mapping and how do we use them?
=========================================
A mapping is very similar to a K->V hashmap.
If you have a state variable of type `mapping (string -> uint) x;`, then you can
access the value by `x["somekeystring"]`.
How can I get the length of a mapping?
======================================
Mappings are a rather low-level data structure. It does not store the keys
and it is not possible to know which or how many values are "set". Actually,
all values to all possible keys are set by default, they are just
initialised with the zero value.
In this sense, the attribute `length` for a mapping does not really apply.
If you want to have a "sized mapping", you can use the iterable mapping
(see below) or just a dynamically-sized array of structs.
Are mappings iterable?
======================
Mappings themselves are not iterable, but you can use a higher-level
datastructure on top of it, for example the `iterable mapping <https://github.com/ethereum/dapp-bin/blob/master/library/iterable_mapping.sol>`_.
Can you return an array or a string from a solidity function call?
==================================================================
Yes. See `array_receiver_and_returner.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/60_array_receiver_and_returner.sol>`_.
What is problematic, though, is returning any variably-sized data (e.g. a
variably-sized array like `uint[]`) from a fuction **called from within Solidity**.
This is a limitation of the EVM and will be solved with the next protocol update.
Returning variably-sized data as part of an external transaction or call is fine.
How do you represent double/float in Solidity?
==============================================
This is not yet possible.
Is it possible to in-line initialize an array like so: string32[] myarray = ["a", "b"];
=======================================================================================
This is not yet possible.
What are events and why do we need them?
========================================
Let us suppose that you need a contract to alert the outside world when
something happens. The contract can fire an event, which can be listened to
from web3 (inside geth or a web application). The main advantage of events
is that they are stored in a special way on the blockchain so that it
is very easy to search for them.
What are the different function visibilities?
=============================================
The visibility specifiers do not only change the visibility but also
the way functions can be called. In general, functions in the
same contract can also be called internally (which is cheaper
and allows for memory types to be passed by reference). This
is done if you just use `f(1,2)`. If you use `this.f(1,2)`
or `otherContract.f(1,2)`, the function is called externally.
Internal function calls have the advantage that you can use
all Solidity types as parameters, but you have to stick to the
simpler ABI types for external calls.
* external: all, only externally
* public: all (this is the default), externally and internally
* internal: only this contract and contracts deriving from it, only internally
* private: only this contract, only internally
Do contract constructors have to be publicly visible?
=====================================================
You can use the visibility specifiers, but they do not yet have any effect.
The constructor is removed from the contract code once it is deployed,
Can a contract have multiple constructors?
==========================================
No, a contract can have only one constructor.
More specifically, it can only have one function whose name matches
that of the constructor.
Having multiple constructors with different number of arguments
or argument types, as it is possible in other languages
is not allowed in Solidity.
Is a constructor required?
==========================
No. If there is no constructor, a generic one without arguments and no actions will be used.
Are timestamps (now, block.timestamp) reliable?
===============================================
This depends on what you mean by "reliable".
In general, they are supplied by miners and are therefore vulnerable.
Unless someone really messes up the blockchain or the clock on
your computer, you can make the following assumptions:
You publish a transaction at a time X, this transaction contains same
code that calls `now` and is included in a block whose timestamp is Y
and this block is included into the canonical chain (published) at a time Z.
The value of `now` will be identical to Y and X <= Y <= Z.
Never use `now` or `block.hash` as a source of randomness, unless you know
what you are doing!
Can a contract function return a struct?
========================================
Yes, but only in "internal" function calls.
If I return an enum, I only get integer values in web3.js. How to get the named values?
=======================================================================================
Enums are not supported by the ABI, they are just supported by Solidity.
You have to do the mapping yourself for now, we might provide some help
later.
What is the deal with "function () { ... }" inside Solidity contracts? How can a function not have a name?
==========================================================================================================
This function is called "fallback function" and it
is called when someone just sent Ether to the contract without
providing any data or if someone messed up the types so that they tried to
call a function that does not exist.
The default behaviour (if no fallback function is explicitly given) in
these situations is to just accept the call and do nothing.
This is desireable in many cases, but should only be used if there is
a way to pull out Ether from a contract.
If the contract is not meant to receive Ether with simple transfers, you
should implement the fallback function as
`function() { throw; }`
this will cause all transactions to this contract that do not call an
existing function to be reverted, so that all Ether is sent back.
Another use of the fallback function is to e.g. register that your
contract received ether by using an event.
*Attention*: If you implement the fallback function take care that it uses as
little gas as possible, because `send()` will only supply a limited amount.
Is it possible to pass arguments to the fallback function?
==========================================================
The fallback function cannot take parameters.
Under special circumstances, you can send data. If you take care
that none of the other functions is invoked, you can access the data
by `msg.data`.
Can state variables be initialized in-line?
===========================================
Yes, this is possible for most types (even for structs), but not for arrays.
Examples::
contract C {
struct S { uint a; uint b; }
S public x = S(1, 2);
string name = "Ada";
}
contract D {
C c = new C();
}
What is the "modifier" keyword?
===============================
Modifiers are a way to prepend or append code to a function in order
to add guards, initialisation or cleanup functionality in a concise way.
For examples, see the `features.sol <https://github.com/ethereum/dapp-bin/blob/master/library/features.sol>`_.
How do structs work?
====================
See `struct_and_for_loop_tester.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/65_struct_and_for_loop_tester.sol>`_.
How do for loops work?
======================
Very similar to JavaScript. There is one point to watch out for, though:
If you use `for (var i = 0; i < a.length; i ++) { a[i] = i; }`, then
the type of `i` will be inferred only from `0`, whose type is `uint8`.
This means that if `a` has more than `255` elements, your loop will
not terminate because `i` can only hold values up to `255`.
Better use `for (uint i = 0; i < a.length...`
See `struct_and_for_loop_tester.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/65_struct_and_for_loop_tester.sol>`_.
What character set does Solidity use?
=====================================
Solidity is character set agnostic concerning strings in the source code, although
utf-8 is recommended. Identifiers (variables, functions, ...) can only use
ASCII.
What are some examples of basic string manipulation (substring, indexOf, charAt, etc)?
======================================================================================
There are some string utility functions at `stringUtils.sol <https://github.com/ethereum/dapp-bin/blob/master/library/stringUtils.sol>`_
which will be extended in the future.
For now, if you want to modify a string (even when you only want to know its length),
you should always convert it to a `bytes` first::
contract C {
string s;
function append(byte c) {
bytes(s).push(c);
}
function set(uint i, byte c) {
bytes(s)[i] = c;
}
}
Can I concatenate two strings?
==============================
You have to do it manually for now.
Why is the low-level function .call() less favorable than instantiating a contract with a variable (ContractB b;) and executing its functions (b.doSomething();)?
=================================================================================================================================================================
If you use actual functions, the compiler will tell you if the types
or your arguments do not match, if the function does not exist
or is not visible and it will do the packing of the
arguments for you.
See `ping.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/45_ping.sol>`_ and
`pong.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/45_pong.sol>`_.
Is unused gas automatically refunded?
=====================================
Yes and it is immediate, i.e. done as part of the transaction.
When returning a value of say "uint" type, is it possible to return an "undefined" or "null"-like value?
========================================================================================================
This is not possible, because all types use up the full value range.
You have the option to `throw` on error, which will also revert the whole
transaction, which might be a good idea if you ran into an unexpected
situation.
If you do not want to throw, you can return a pair::
contract C {
uint[] counters;
function getCounter(uint index)
returns (uint counter, bool error) {
if (index >= counters.length) return (0, true);
else return (counters[index], false);
}
function checkCounter(uint index) {
var (counter, error) = getCounter(index);
if (error) { ... }
else { ... }
}
}
Are comments included with deployed contracts and do they increase deployment gas?
==================================================================================
No, everything that is not needed for execution is removed during compilation.
This includes, among others, comments, variable names and type names.
What happens if you send ether along with a function call to a contract?
========================================================================
It gets added to the total balance of the contract, just like when you send ether when creating a contract.
Is it possible to get a tx receipt for a transaction executed contract-to-contract?
===================================================================================
No, a function call from one contract to another does not create its own transaction,
you have to look in the overall transaction. This is also the reason why several
block explorer do not show Ether sent between contracts correctly.
What is the memory keyword? What does it do?
============================================
The Ethereum Virtual Machine has three areas where it can store items.
The first is "storage", where all the contract state variables reside.
Every contract has its own storage and it is persistent between function calls
and quite expensive to use.
The second is "memory", this is used to hold temporary values. It
is erased between (external) function calls and is cheaper to use.
The third one is the stack, which is used to hold small local variables.
It is almost free to use, but can only hold a limited amount of values.
For almost all types, you cannot specify where they should be stored, because
they are copied everytime they are used.
The types where the so-called storage location is important are structs
and arrays. If you e.g. pass such variables in function calls, their
data is not copied if it can stay in memory or stay in storage.
This means that you can modify their content in the called function
and these modifications will still be visible in the caller.
There are defaults for the storage location depending on which type
of variable it concerns:
* state variables are always in storage
* function arguments are always in memory
* local variables always reference storage
Example::
contract C {
uint[] data1;
uint[] data2;
function appendOne() {
append(data1);
}
function appendTwo() {
append(data2);
}
function append(uint[] storage d) {
d.push(1);
}
}
The function `append` can work both on `data1` and `data2` and its modifications will be
stored permanently. If you remove the `storage` keyword, the default
is to use `memory` for function arguments. This has the effect that
at the point where `append(data1)` or `append(data2)` is called, an
independent copy of the state variable is created in memory and
`append` operates on this copy (which does not support `.push` - but that
is another issue). The modifications to this independent copy do not
carry back to `data1` or `data2`.
A common mistake is to declare a local variable and assume that it will
be created in memory, although it will be created in storage::
/// THIS CONTRACT CONTAINS AN ERROR
contract C {
uint someVariable;
uint[] data;
function f() {
uint[] x;
x.push(2);
data = x;
}
}
The type of the local variable `x` is `uint[] storage`, but since
storage is not dynamically allocated, it has to be assigned from
a state variable before it can be used. So no space in storage will be
allocated for `x`, but instead it functions only as an alias for
a pre-existing variable in storage.
What will happen is that the compiler interprets `x` as a storage
pointer and will make it point to the storage slot `0` by default.
This has the effect that `someVariable` (which resides at storage
slot `0`) is modified by `x.push(2)`.
The correct way to do this is the following::
contract C {
uint someVariable;
uint[] data;
function f() {
uint[] x = data;
x.push(2);
}
}
Can a regular (i.e. non-contract) ethereum account be closed permanently like a contract can?
=============================================================================================
No. Non-contract accounts "exist" as long as the private key is known by
someone or can be generated in some way.
******************
Advanced Questions
******************
How do you get a random number in a contract? (Implement a self-returning gambling contract.)
=============================================================================================
Getting randomness right is often the crucial part in a crypto project and
most failures result from bad random number generators.
If you do not want it to be safe, you build something similar to the `coin flipper <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/35_coin_flipper.sol>`_
but otherwise, rather use a contract that supplies randomness, like the `RANDAO <https://github.com/randao/randao>`_.
Get return value from non-constant function from another contract
=================================================================
The key point is that the calling contract needs to know about the function it intends to call.
See `ping.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/45_ping.sol>`_
and `pong.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/45_pong.sol>`_.
Get contract to do something when it is first mined
===================================================
Use the constructor. Anything inside it will be executed when the contract is first mined.
See `replicator.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/50_replicator.sol>`_.
Can a contract create another contract?
=======================================
Yes, see `replicator.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/50_replicator.sol>`_.
Note that the full code of the created contract has to be included in the creator contract.
This also means that cyclic creations are not possible (because the contract would have
to contain its own code) - at least not in a general way.
How do you create 2-dimensional arrays?
=======================================
See `2D_array.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/55_2D_array.sol>`_.
Note that filling a 10x10 square of `uint8` + contract creation took more than `800,000`
gas at the time of this writing. 17x17 took `2,000,000` gas. With the limit at
3.14 million... well, there’s a pretty low ceiling for what you can create right
now.
Note that merely "creating" the array is free, the costs are in filling it.
Note2: Optimizing storage access can pull the gas costs down considerably, because
32 `uint8` values can be stored in a single slot. The problem is that these optimizations
currently do not work across loops and also have a problem with bounds checking.
You might get much better results in the future, though.
What does p.recipient.call.value(p.amount)(p.data) do?
======================================================
Every external function call in Solidity can be modified in two ways:
1. You can add Ether together with the call
2. You can limit the amount of gas available to the call
This is done by "calling a function on the function":
`f.gas(2).value(20)()` calls the modified function `f` and thereby sending 20
Wei and limiting the gas to 2 (so this function call will most likely go out of
gas and return your 20 Wei).
In the above example, the low-level function `call` is used to invoke another
contract with `p.data` as payload and `p.amount` Wei is sent with that call.
Can a contract function accept a two-dimensional array?
=======================================================
This is not yet implemented for external calls and dynamic arrays -
you can only use one level of dynamic arrays.
What is the relationship between bytes32 and string? Why is it that ‘bytes32 somevar = "stringliteral";’ works and what does the saved 32-byte hex value mean?
==============================================================================================================================================================
The type `bytes32` can hold 32 (raw) bytes. In the assignment `bytes32 samevar = "stringliteral";`,
the string literal is interpreted in its raw byte form and if you inspect `somevar` and
see a 32-byte hex value, this is just `"stringliteral"` in hex.
The type `bytes` is similar, only that it can change its length.
Finally, `string` is basically identical to `bytes` only that it is assumed
to hold the utf-8 encoding of a real string. Since `string` stores the
data in utf-8 encoding it is quite expensive to compute the number of
characters in the string (the encoding of some characters takes more
than a single byte). Because of that, `string s; s.length` is not yet
supported and not even index access `s[2]`. But if you want to access
the low-level byte encoding of the string, you can use
`bytes(s).length` and `bytes(s)[2]` which will result in the number
of bytes in the utf-8 encoding of the string (not the number of
characters) and the second byte (not character) of the utf-8 encoded
string, respectively.
Can a contract pass an array (static size) or string or bytes (dynamic size) to another contract?
=================================================================================================
Sure. Take care that if you cross the memory / storage boundary,
independent copies will be created::
contract C {
uint[20] x;
function f() {
g(x);
h(x);
}
function g(uint[20] y) {
y[2] = 3;
}
function h(uint[20] storage y) {
y[3] = 4;
}
The call to `g(x)` will not have an effect on `x` because it needs
to create an independent copy of the storage value in memory
(the default storage location is memory). On the other hand,
`h(x)` successfully modifies `x` because only a reference
and not a copy is passed.
Sometimes, when I try to change the length of an array with ex: "arrayname.length = 7;" I get a compiler error "Value must be an lvalue". Why?
==============================================================================================================================================
You can resize a dynamic array in storage (i.e. an array declared at the
contract level) with `arrayname.length = <some new length>;`. If you get the
"lvalue" error, you are probably doing one of two things wrong.
1. You might be trying to resize an array in "memory", or
2. You might be trying to resize a non-dynamic array.
::
int8[] memory memArr; // Case 1
memArr.length++; // illegal
int8[5] storageArr; // Case 2
somearray.length++; // legal
int8[5] storage storageArr2; // Explicit case 2
somearray2.length++; // legal
**Important note:** In Solidity, array dimensions are declared backwards from the way you
might be used to declaring them in C or Java, but they are access as in
C or Java.
For example, `int8[][5] somearray;` are 5 dynamic `int8` arrays.
The reason for this is that `T[5]` is always an array of 5 `T`s,
no matter whether `T` itself is an array or not (this is not the
case in C or Java).
Is it possible to return an array of strings ( string[] ) from a Solidity function?
===================================================================================
Not yet, as this requires two levels of dynamic arrays (`string` is a dynamic array itself).
If you issue a call for an array, it is possible to retrieve the whole array? Or must you write a helper function for that?
===========================================================================================================================
The automatic accessor function for a public state variable of array type only returns
individual elements. If you want to return the complete array, you have to
manually write a function to do that.
What could have happened if an account has storage value/s but no code? Example: http://test.ether.camp/account/5f740b3a43fbb99724ce93a879805f4dc89178b5
=========================================================================================================================================================
The last thing a constructor does is returning the code of the contract.
The gas costs for this depend on the length of the code and it might be
that the supplied gas is not enough. This situation is the only one
where an "out of gas" exception does not revert changes to the state,
i.e. in this case the initialisation of the state variables.
https://github.com/ethereum/wiki/wiki/Subtleties
After a successful CREATE operation's sub-execution, if the operation returns x, 5 * len(x) gas is subtracted from the remaining gas before the contract is created. If the remaining gas is less than 5 * len(x), then no gas is subtracted, the code of the created contract becomes the empty string, but this is not treated as an exceptional condition - no reverts happen.
How do I use .send()?
=====================
If you want to send 20 Ether from a contract to the address `x`, you use `x.send(20 ether);`.
Here, `x` can be a plain address or a contract. If the contract already explicitly defines
a function `send` (and thus overwrites the special function), you can use `address(x).send(20 ether);`.
More Questions?
===============
If you have more questions or your question is not answered here, please talk to us on
`gitter <https://gitter.im/ethereum/solidity>`_ or file an `issue <https://github.com/ethereum/solidity/issues>`_.

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Welcome to Solidity's documentation!
====================================
See also `Russian version (русский перевод) <https://github.com/ethereum/wiki/wiki/%D0%A0%D1%83%D0%BA%D0%BE%D0%B2%D0%BE%D0%B4%D1%81%D1%82%D0%B2%D0%BE-%D0%BF%D0%BE-Solidity>`_.
Solidity is a high-level language whose syntax is similar to that of JavaScript
and it is designed to compile to code for the Ethereum Virtual Machine.
As you will see, it is quite easy to create contracts for voting,
crowdfunding, blind auctions, multi-signature wallets and more.
On the next pages, we will first see a :ref:`simple smart contract <simple-smart-contract>` written
in Solidity followed by the basics about :ref:`blockchains <blockchain-basics>`
and the :ref:`Ethereum Virtual Machine <the-ethereum-virtual-machine>`.
The next section will explain several *features* of Solidity by giving
useful :ref:`example contracts <voting>`
Remember that you can always try out the contracts
`in your browser <https://chriseth.github.io/browser-solidity>`_!
The last and most extensive section will cover all aspects of Solidity in depth.
If you still have questions or ideas for improving Solidity or this documentation,
please feel free to come to out `gitter channel <https://gitter.im/ethereum/solidity/>`_.
Index and Search
==================
* :ref:`genindex`
* :ref:`search`
Contents
========
.. toctree::
:maxdepth: 2
introduction-to-smart-contracts.rst
solidity-by-example.rst
solidity-in-depth.rst
style-guide.rst
common-patterns.rst
frequently-asked-questions.rst

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###############################
Introduction to Smart Contracts
###############################
.. _simple-smart-contract:
***********************
A Simple Smart Contract
***********************
Let us begin with the most basic example. It is fine if you do not understand everything
right now, we will go into more detail later.
Storage
=======
.. Gist: a4532ce30246847b371b
::
contract SimpleStorage {
uint storedData;
function set(uint x) {
storedData = x;
}
function get() constant returns (uint retVal) {
return storedData;
}
}
A contract in the sense of Solidity is a collection of code (its functions) and
data (its *state*) that resides at a specific address on the Ethereum
blockchain. The line `uint storedData;` declares a state variable called `storedData` of
type `uint` (unsigned integer of 256 bits). You can think of it as a single slot
in a database that can be queried and altered by calling functions of the
code that manages the database. In the case of Ethereum, this is always the owning
contract. And in this case, the functions `set` and `get` can be used to modify
or retrieve the value of the variable.
To access a state variable, you do not need the prefix `this.` as is common in
other languages.
This contract does not yet do much apart from (due to the infrastructure
built by Ethereum) allowing anyone to store a single number that is accessible by
anyone in the world without (feasible) a way to prevent you from publishing
this number. Of course, anyone could just call `set` again with a different value
and overwrite your number, but the number will still be stored in the history
of the blockchain. Later, we will see how you can impose access restrictions
so that only you can alter the number.
.. index:: ! subcurrency
Subcurrency Example
===================
The following contract will implement the simplest form of a
cryptocurrency. It is possible to generate coins out of thin air, but
only the person that created the contract will be able to do that (it is trivial
to implement a different issuance scheme).
Furthermore, anyone can send coins to each other without any need for
registering with username and password - all you need is an Ethereum keypair.
.. note::
This is not a nice example for browser-solidity.
If you use `browser-solidity <https://chriseth.github.io/browser-solidity>`_
to try this example, you cannot change the address where you call
functions from. So you will always be the "minter", you can mint coins and send
them somewhere, but you cannot impersonate someone else. This might change in
the future.
.. Gist: ad490694f3e5b3de47ab
::
contract Coin {
// The keyword "public" makes those variables
// readable from outside.
address public minter;
mapping (address => uint) public balances;
// Events allow light clients to react on
// changes efficiently.
event Sent(address from, address to, uint amount);
// This is the constructor whose code is
// run only when the contract is created.
function Coin() {
minter = msg.sender;
}
function mint(address receiver, uint amount) {
if (msg.sender != minter) return;
balances[receiver] += amount;
}
function send(address receiver, uint amount) {
if (balances[msg.sender] < amount) return;
balances[msg.sender] -= amount;
balances[receiver] += amount;
Sent(msg.sender, receiver, amount);
}
}
This contract introduces some new concepts, let us go through them one by one
The line `address public minter;` declares a state variable of type address
that is publicly accessible. The `address` type is a 160 bit value
that does not allow any arithmetic operations. It is suitable for
storing addresses of contracts or keypairs belonging to external
persons. The keyword `public` automatically generates a function that
allows you to access the current value of the state variable.
Without this keyword, other contracts have no way to access the variable
and only the code of this contract can write to it.
The function will look something like this::
function minter() returns (address) { return minter; }
Of course, adding a function exactly like that will not work
because we would have a
function and a state variable with the same name, but hopefully, you
get the idea - the compiler figures that out for you.
.. index:: mapping
The next line, `mapping (address => uint) public balances;` also
creates a public state variable, but it of a more complex datatype.
The type maps addresses to unsigned integers.
Mappings can be seen as hashtables which are
virtually initialized such that every possible key exists and is mapped to a
value whose byte-representation is all zeros. This analogy does not go
too far, though, as it is neither possible to obtain a list of all keys of
a mapping, nor a list of all values. So either keep in mind (or
better, keep a list or use a more advanced data type) what you
added to the mapping or use it in a context where this is not needed,
like this one. The accessor function created by the `public` keyword
is a bit more complex in this case. It roughly looks like the
following::
function balances(address _account) returns (uint balance) {
return balances[_account];
}
As you see, you can use this function to easily query the balance of a
single account.
.. index:: event
The line `event Sent(address from, address to, uint value);` declares
a so-called "event" which is fired in the last line of the function
`send`. User interfaces (as well as server appliances of course) can
listen for those events being fired on the blockchain without much
cost. As soon as it is fired, the listener will also receive the
arguments `from`, `to` and `value`, which makes it easy to track
transactions. In order to listen for this event, you would use ::
Coin.Sent().watch({}, '', function(error, result) {
if (!error) {
console.log("Coin transfer: " + result.args.amount +
" coins were sent from " + result.args.from +
" to " + result.args.to + ".");
console.log("Balances now:\n" +
"Sender: " + Coin.balances.call(result.args.from) +
"Receiver: " + Coin.balances.call(result.args.to));
}
}
Note how the automatically generated function `balances` is called from
the user interface.
.. index:: coin
The special function `Coin` is the
constructor which is run during creation of the contract and
cannot be called afterwards. It permanently stores the address of the person creating the
contract: `msg` (together with `tx` and `block`) is a magic global variable that
contains some properties which allow access to the blockchain. `msg.sender` is
always the address where the current (external) function call came from.
Finally, the functions that will actually end up with the contract and can be called
by users and contracts alike are `mint` and `send`.
If `mint` is called by anyone except the account that created the contract,
nothing will happen. On the other hand, `send` can be used by anyone (who already
has some of these coins) to send coins to anyone else. Note that if you use
this contract to send coins to an address, you will not see anything when you
look at that address on a blockchain explorer, because the fact that you sent
coins and the changed balances are only stored in the data storage of this
particular coin contract. By the use of events it is relatively easy to create
a "blockchain explorer" that tracks transactions and balances of your new coin.
.. _blockchain-basics:
*****************
Blockchain Basics
*****************
Blockchains as a concept are not too hard to understand for programmers. The reason is that
most of the complications (mining, hashing, elliptic-curve cryptography, peer-to-peer networks, ...)
are just there to provide a certain set of features and promises. Once you accept these
features as given, you do not have to worry about the underlying technology - or do you have
to know how Amazon's AWS works internally in order to use it?
.. index:: transaction
Transactions
============
A blockchain is a globally shared, transactional database.
This means that everyone can read entries in the database just by participating in the network.
If you want to change something in the database, you have to create a so-called transaction
which has to be accepted by all others.
The word transaction implies that the change you want to make (assume you want to change
two values at the same time) is either not done at all or completely applied. Furthermore,
while your transaction is applied to the database, no other transaction can alter it.
As an example, imagine a table that lists the balances of all accounts in an
electronic currency. If a transfer from one account to another is requested,
the transactional nature of the database ensures that if the amount is
subtracted from one account, it is always added to the other account. If due
to whatever reason, adding the amount to the target account is not possible,
the source account is also not modified.
Furthermore, a transaction is always cryptographically signed by the sender (creator).
This makes it straightforward to guard access to specific modifications of the
database. In the example of the electronic currency, a simple check ensures that
only the person holding the keys to the account can transfer money from it.
.. index:: ! block
Blocks
======
One major obstacle to overcome is what in bitcoin terms is called "double-spend attack":
What happens if two transactions exist in the network that both want to empty an account,
a so-called conflict?
The abstract answer to this is that you do not have to care. An order of the transactions
will be selected for you, the transactions will be bundled into what is called a "block"
and then they will be executed and distributed among all participating nodes.
If two transactions contradict each other, the one that ends up being second will
be rejected and not become part of the block.
These blocks form a linear sequence in time and that is where the word "blockchain"
derives from. Blocks are added to the chain in rather regular intervals - for
Ethereum this is roughly every 17 seconds.
As part of the "order selection mechanism" (which is called "mining") it may happen that
blocks are reverted from time to time, but only at the "tip" of the chain. The more
blocks are reverted the less likely it is. So it might be that your transactions
are reverted and even removed from the blockchain, but the longer you wait, the less
likely it will be.
.. _the-ethereum-virtual-machine:
.. index:: !evm, ! ethereum virtual machine
****************************
The Ethereum Virtual Machine
****************************
Overview
========
The Ethereum Virtual Machine or EVM is the runtime environment
for smart contracts in Ethereum. It is not only sandboxed but
actually completely isolated, which means that code running
inside the EVM has no access to network, filesystem or other processes.
Smart contracts even have limited access to other smart contracts.
.. index:: ! account, address, storage, balance
Accounts
========
There are two kinds of accounts in Ethereum which share the same
address space: **External accounts** that are controlled by
public-private key pairs (i.e. humans) and **contract accounts** which are
controlled by the code stored together with the account.
The address of an external account is determined from
the public key while the address of a contract is
determined at the time the contract is created
(it is derived from the creator address and the number
of transactions sent from that address, the so-called "nonce").
Apart from the fact whether an account stores code or not,
the EVM treats the two types equally, though.
Every account has a persistent key-value store mapping 256 bit words to 256 bit
words called **storage**.
Furthermore, every account has a **balance** in
Ether (in "Wei" to be exact) which can be modified by sending transactions that
include Ether.
.. index:: ! transaction
Transactions
============
A transaction is a message that is sent from one account to another
account (which might be the same or the special zero-account, see below).
It can include binary data (its payload) and Ether.
If the target account contains code, that code is executed and
the payload is provided as input data.
If the target account is the zero-account (the account with the
address `0`), the transaction creates a **new contract**.
As already mentioned, the address of that contract is not
the zero address but an address derived from the sender and
its number of transaction sent (the "nonce"). The payload
of such a contract creation transaction is taken to be
EVM bytecode and executed. The output of this execution is
permanently stored as the code of the contract.
This means that in order to create a contract, you do not
send the actual code of the contract, but in fact code that
returns that code.
.. index:: ! gas, ! gas price
Gas
===
Upon creation, each transaction is charged with a certain amount of **gas**,
whose purpose is to limit the amount of work that is needed to execute
the transaction and to pay for this execution. While the EVM executes the
transaction, the gas is gradually depleted according to specific rules.
The **gas price** is a value set by the creator of the transaction, who
has to pay `gas_price * gas` up front from the sending account.
If some gas is left after the execution, it is refunded in the same way.
If the gas is used up at any point (i.e. it is negative),
an out-of-gas exception is triggered, which reverts all modifications
made to the state in the current call frame.
.. index:: ! storage, ! memory, ! stack
Storage, Memory and the Stack
=============================
Each account has a persistent memory area which is called **storage**.
Storage is a key-value store that maps 256 bit words to 256 bit words.
It is not possible to enumerate storage from within a contract
and it is comparatively costly to read and even more so, to modify
storage. A contract can neither read nor write to any storage apart
from its own.
The second memory area is called **memory**, of which a contract obtains
a freshly cleared instance for each message call. Memory can be
addressed at byte level, but read and written to in 32 byte (256 bit)
chunks. Memory is more costly the larger it grows (it scales
quadratically).
The EVM is not a register machine but a stack machine, so all
computations are performed on an area called the **stack**. It has a maximum size of
1024 elements and contains words of 256 bits. Access to the stack is
limited to the top end in the following way:
It is possible to copy one of
the topmost 16 elements to the top of the stack or swap the
topmost element with one of the 16 elements below it.
All other operations take the topmost two (or one, or more, depending on
the operation) elements from the stack and push the result onto the stack.
Of course it is possible to move stack elements to storage or memory,
but it is not possible to just access arbitrary elements deeper in the stack
without first removing the top of the stack.
.. index:: ! instruction
Instruction Set
===============
The instruction set of the EVM is kept minimal in order to avoid
incorrect implementations which could cause consensus problems.
All instructions operate on the basic data type, 256 bit words.
The usual arithmetic, bit, logical and comparison operations are present.
Conditional and unconditional jumps are possible. Furthermore,
contracts can access relevant properties of the current block
like its number and timestamp.
.. index:: ! message call, function;call
Message Calls
=============
Contracts can call other contracts or send Ether to non-contract
accounts by the means of message calls. Message calls are similar
to transactions, in that they have a source, a target, data payload,
Ether, gas and return data. In fact, every transaction consists of
a top-level message call which in turn can create further message calls.
A contract can decide how much of its remaining **gas** should be sent
with the inner message call and how much it wants to retain.
If an out-of-gas exception happens in the inner call (or any
other exception), this will be signalled by an error value put onto the stack.
In this case, only the gas sent together with the call is used up.
In Solidity, the calling contract causes a manual exception by default in
such situations, so that exceptions "bubble up" the call stack.
As already said, the called contract (which can be the same as the caller)
will receive a freshly cleared instance of memory and has access to the
call payload - which will be provided in a separate area called the **calldata**.
After it finished execution, it can return data which will be stored at
a location in the caller's memory preallocated by the caller.
Calls are **limited** to a depth of 1024, which means that for more complex
operations, loops should be preferred over recursive calls.
.. index:: callcode, library
Callcode and Libraries
======================
There exists a special variant of a message call, named **callcode**
which is identical to a message call apart from the fact that
the code at the target address is executed in the context of the calling
contract.
This means that a contract can dynamically load code from a different
address at runtime. Storage, current address and balance still
refer to the calling contract, only the code is taken from the called address.
This makes it possible to implement the "library" feature in Solidity:
Reusable library code that can be applied to a contract's storage in
order to e.g. implement a complex data structure.
.. index:: log
Logs
====
It is possible to store data in a specially indexed data structure
that maps all they way up to the block level. This feature called **logs**
is used by Solidity in order to implement **events**.
Contracts cannot access log data after it has been created, but they
can be efficiently accessed from outside the blockchain.
Since some part of the log data is stored in bloom filters, it is
possible to search for this data in an efficient and cryptographically
secure way, so network peers that do not download the whole blockchain
("light clients") can still find these logs.
.. index:: contract creation
Create
======
Contracts can even create other contracts using a special opcode (i.e.
they do not simply call the zero address). The only difference between
these **create calls** and normal message calls is that the payload data is
executed and the result stored as code and the caller / creator
receives the address of the new contract on the stack.
.. index:: selfdestruct
Selfdestruct
============
The only possibility that code is removed from the blockchain is
when a contract at that address performs the `SELFDESTRUCT` operation.
The remaining Ether stored at that address is sent to a designated
target and then the storage and code is removed.
Note that even if a contract's code does not contain the `SELFDESTRUCT`
opcode, it can still perform that operation using callcode.

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********************************
Layout of a Solidity Source File
********************************
Source files can contain an arbitrary number of contract definitions and include directives.
.. index:: source file, ! import
Importing other Source Files
============================
Other source files can be referenced using `import "filename";` and the symbols
defined there will also be available in the current source file.
.. warning::
Import will not work automatically for the commandline-compiler.
Furthermore, this system of importing other files is not completely fleshed out
yet, so please expect changes.
The `browser-based compiler <https://chriseth.github.io/browser-solidity>`_
has quite advanced support for multiple files and can even import files
directly from github, by using e.g.
```import "github.com/ethereum/dapp-bin/library/iterable_mapping.sol";```
If you want to use multiple source files with the (commandline compiler)[../commandline-compiler/] solc,
you have to specify all files you will use as arguments to solc,
the compiler will not yet search your filesystem on its own.
.. index:: ! comment, natspec
Comments
========
Single-line comments (`//`) and multi-line comments (`/*...*/`) are possible.
There are special types of comments called natspec comments
(documentation yet to be written). These are introduced by
triple-slash comments (`///`) or using double asterisks (`/** ... */`).
Right in front of function declarations or statements,
you can use doxygen-style tags inside them to document functions, annotate conditions for formal
verification and provide a **confirmation text** that is shown to users if they want to
invoke a function.

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echo.
echo.Testing of doctests in the sources finished, look at the ^
results in %BUILDDIR%/doctest/output.txt.
goto end
)
if "%1" == "xml" (
%SPHINXBUILD% -b xml %ALLSPHINXOPTS% %BUILDDIR%/xml
if errorlevel 1 exit /b 1
echo.
echo.Build finished. The XML files are in %BUILDDIR%/xml.
goto end
)
if "%1" == "pseudoxml" (
%SPHINXBUILD% -b pseudoxml %ALLSPHINXOPTS% %BUILDDIR%/pseudoxml
if errorlevel 1 exit /b 1
echo.
echo.Build finished. The pseudo-XML files are in %BUILDDIR%/pseudoxml.
goto end
)
:end

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#############
Miscellaneous
#############
.. index:: storage, state variable, mapping
************************************
Layout of State Variables in Storage
************************************
Statically-sized variables (everything except mapping and dynamically-sized array types) are laid out contiguously in storage starting from position `0`. Multiple items that need less than 32 bytes are packed into a single storage slot if possible, according to the following rules:
- The first item in a storage slot is stored lower-order aligned.
- Elementary types use only that many bytes that are necessary to store them.
- If an elementary type does not fit the remaining part of a storage slot, it is moved to the next storage slot.
- Structs and array data always start a new slot and occupy whole slots (but items inside a struct or array are packed tightly according to these rules).
The elements of structs and arrays are stored after each other, just as if they were given explicitly.
Due to their unpredictable size, mapping and dynamically-sized array types use a `sha3`
computation to find the starting position of the value or the array data. These starting positions are always full stack slots.
The mapping or the dynamic array itself
occupies an (unfilled) slot in storage at some position `p` according to the above rule (or by
recursively applying this rule for mappings to mappings or arrays of arrays). For a dynamic array, this slot stores the number of elements in the array (byte arrays and strings are an exception here, see below). For a mapping, the slot is unused (but it is needed so that two equal mappings after each other will use a different hash distribution).
Array data is located at `sha3(p)` and the value corresponding to a mapping key
`k` is located at `sha3(k . p)` where `.` is concatenation. If the value is again a
non-elementary type, the positions are found by adding an offset of `sha3(k . p)`.
`bytes` and `string` store their data in the same slot where also the length is stored if they are short. In particular: If the data is at most `31` bytes long, it is stored in the higher-order bytes (left aligned) and the lowest-order byte stores `length * 2`. If it is longer, the main slot stores `length * 2 + 1` and the data is stored as usual in `sha3(slot)`.
So for the following contract snippet::
contract c {
struct S { uint a; uint b; }
uint x;
mapping(uint => mapping(uint => S)) data;
}
The position of `data[4][9].b` is at `sha3(uint256(9) . sha3(uint256(4) . uint256(1))) + 1`.
*****************
Esoteric Features
*****************
There are some types in Solidity's type system that have no counterpart in the syntax. One of these types are the types of functions. But still, using `var` it is possible to have local variables of these types::
contract FunctionSelector {
function select(bool useB, uint x) returns (uint z) {
var f = a;
if (useB) f = b;
return f(x);
}
function a(uint x) returns (uint z) {
return x * x;
}
function b(uint x) returns (uint z) {
return 2 * x;
}
}
Calling `select(false, x)` will compute `x * x` and `select(true, x)` will compute `2 * x`.
.. index:: optimizer, common subexpression elimination, constant propagation
*************************
Internals - the Optimizer
*************************
The Solidity optimizer operates on assembly, so it can be and also is used by other languages. It splits the sequence of instructions into basic blocks at JUMPs and JUMPDESTs. Inside these blocks, the instructions are analysed and every modification to the stack, to memory or storage is recorded as an expression which consists of an instruction and a list of arguments which are essentially pointers to other expressions. The main idea is now to find expressions that are always equal (on every input) and combine them into an expression class. The optimizer first tries to find each new expression in a list of already known expressions. If this does not work, the expression is simplified according to rules like `constant` + `constant` = `sum_of_constants` or `X` * 1 = `X`. Since this is done recursively, we can also apply the latter rule if the second factor is a more complex expression where we know that it will always evaluate to one. Modifications to storage and memory locations have to erase knowledge about storage and memory locations which are not known to be different: If we first write to location x and then to location y and both are input variables, the second could overwrite the first, so we actually do not know what is stored at x after we wrote to y. On the other hand, if a simplification of the expression x - y evaluates to a non-zero constant, we know that we can keep our knowledge about what is stored at x.
At the end of this process, we know which expressions have to be on the stack in the end and have a list of modifications to memory and storage. This information is stored together with the basic blocks and is used to link them. Furthermore, knowledge about the stack, storage and memory configuration is forwarded to the next block(s). If we know the targets of all JUMP and JUMPI instructions, we can build a complete control flow graph of the program. If there is only one target we do not know (this can happen as in principle, jump targets can be computed from inputs), we have to erase all knowledge about the input state of a block as it can be the target of the unknown JUMP. If a JUMPI is found whose condition evaluates to a constant, it is transformed to an unconditional jump.
As the last step, the code in each block is completely re-generated. A dependency graph is created from the expressions on the stack at the end of the block and every operation that is not part of this graph is essentially dropped. Now code is generated that applies the modifications to memory and storage in the order they were made in the original code (dropping modifications which were found not to be needed) and finally, generates all values that are required to be on the stack in the correct place.
These steps are applied to each basic block and the newly generated code is used as replacement if it is smaller. If a basic block is split at a JUMPI and during the analysis, the condition evaluates to a constant, the JUMPI is replaced depending on the value of the constant, and thus code like
::
var x = 7;
data[7] = 9;
if (data[x] != x + 2)
return 2;
else
return 1;
is simplified to code which can also be compiled from
::
data[7] = 9;
return 1;
even though the instructions contained a jump in the beginning.
.. index:: ! commandline compiler, compiler;commandline, ! solc, ! linker
******************************
Using the Commandline Compiler
******************************
One of the build targets of the Solidity repository is `solc`, the solidity commandline compiler.
Using `solc --help` provides you with an explanation of all options. The compiler can produce various outputs, ranging from simple binaries and assembly over an abstract syntax tree (parse tree) to estimations of gas usage.
If you only want to compile a single file, you run it as `solc --bin sourceFile.sol` and it will print the binary. Before you deploy your contract, activate the optimizer while compiling using `solc --optimize --bin sourceFile.sol`. If you want to get some of the more advanced output variants of `solc`, it is probably better to tell it to output everything to separate files using `solc -o outputDirectory --bin --ast --asm sourceFile.sol`.
Of course, you can also specify several source files and actually that is also required if you use the `import` statement in Solidity: The compiler will (for now) not automatically discover source files for you, so you have to provide it with all source files your project consists of.
If your contracts use [libraries](#libraries), you will notice that the bytecode contains substrings of the form `__LibraryName______`. You can use `solc` as a linker meaning that it will insert the library addresses for you at those points:
Either add `--libraries "Math:0x12345678901234567890 Heap:0xabcdef0123456"` to your command to provide an address for each library or store the string in a file (one library per line) and run `solc` using `--libraries fileName`.
If `solc` is called with the option `--link`, all input files are interpreted to be unlinked binaries (hex-encoded) in the `__LibraryName____`-format given above and are linked in-place (if the input is read from stdin, it is written to stdout). All options except `--libraries` are ignored (including `-o`) in this case.
***************
Tips and Tricks
***************
* Use `delete` on arrays to delete all its elements.
* Use shorter types for struct elements and sort them such that short types are grouped together. This can lower the gas costs as multiple SSTORE operations might be combined into a single (SSTORE costs 5000 or 20000 gas, so this is what you want to optimise). Use the gas price estimator (with optimiser enabled) to check!
* Make your state variables public - the compiler will create [getters](#accessor-functions) for you for free.
* If you end up checking conditions on input or state a lot at the beginning of your functions, try using [modifiers](#function-modifiers)
* If your contract has a function called `send` but you want to use the built-in send-function, use `address(contractVariable).send(amount)`.
* If you want your contracts to receive ether when called via `send`, you have to implement the [fallback function](#fallback-functions).
* Initialise storage structs with a single assignment: `x = MyStruct({a: 1, b: 2});`
********
Pitfalls
********
Unfortunately, there are some subtleties the compiler does not yet warn you about.
- In `for (var i = 0; i < arrayName.length; i++) { ... }`, the type of `i` will be `uint8`, because this is the smallest type that is required to hold the value `0`. If the array has more than 255 elements, the loop will not terminate.
**********
Cheatsheet
**********
.. index:: block, coinbase, difficulty, number, block;number, timestamp, block;timestamp, msg, data, gas, sender, value, now, gas price, origin, sha3, ripemd160, sha256, ecrecover, addmod, mulmod, cryptography, this, super, selfdestruct, balance, send
Global Variables
================
- `block.coinbase` (`address`): current block miner's address
- `block.difficulty` (`uint`): current block difficulty
- `block.gaslimit` (`uint`): current block gaslimit
- `block.number` (`uint`): current block number
- `block.blockhash` (`function(uint) returns (bytes32)`): hash of the given block - only works for 256 most recent blocks
- `block.timestamp` (`uint`): current block timestamp
- `msg.data` (`bytes`): complete calldata
- `msg.gas` (`uint`): remaining gas
- `msg.sender` (`address`): sender of the message (current call)
- `msg.value` (`uint`): number of wei sent with the message
- `now` (`uint`): current block timestamp (alias for `block.timestamp`)
- `tx.gasprice` (`uint`): gas price of the transaction
- `tx.origin` (`address`): sender of the transaction (full call chain)
- `sha3(...) returns (bytes32)`: compute the Ethereum-SHA3 hash of the (tightly packed) arguments
- `sha256(...) returns (bytes32)`: compute the SHA256 hash of the (tightly packed) arguments
- `ripemd160(...) returns (bytes20)`: compute RIPEMD of 256 the (tightly packed) arguments
- `ecrecover(bytes32, uint8, bytes32, bytes32) returns (address)`: recover public key from elliptic curve signature
- `addmod(uint x, uint y, uint k) returns (uint)`: compute `(x + y) % k` where the addition is performed with arbitrary precision and does not wrap around at `2**256`.
- `mulmod(uint x, uint y, uint k) returns (uint)`: compute `(x * y) % k` where the multiplication is performed with arbitrary precision and does not wrap around at `2**256`.
- `this` (current contract's type): the current contract, explicitly convertible to `address`
- `super`: the contract one level higher in the inheritance hierarchy
- `selfdestruct(address)`: destroy the current contract, sending its funds to the given address
- `<address>.balance`: balance of the address in Wei
- `<address>.send(uint256) returns (bool)`: send given amount of Wei to address, returns `false` on failure.
.. index:: visibility, public, private, external, internal
Function Visibility Specifiers
==============================
::
function myFunction() <visibility specifier> returns (bool) {
return true;
}
- `public`: visible externally and internally (creates accessor function for storage/state variables)
- `private`: only visible in the current contract
- `external`: only visible externally (only for functions) - i.e. can only be message-called (via `this.fun`)
- `internal`: only visible internally
.. index:: modifiers, constant, anonymous, indexed
Modifiers
=========
- `constant` for state variables: Disallows assignment (except initialisation), does not occupy storage slot.
- `constant` for functions: Disallows modification of state - this is not enforced yet.
- `anonymous` for events: Does not store event signature as topic.
- `indexed` for event parameters: Stores the parameter as topic.

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###################
Solidity by Example
###################
.. index:: voting, ballot
.. _voting:
******
Voting
******
The following contract is quite complex, but showcases
a lot of Solidity's features. It implements a voting
contract. Of course, the main problems of electronic
voting is how to assign voting rights to the correct
persons and how to prevent manipulation. We will not
solve all problems here, but at least we will show
how delegated voting can be done so that vote counting
is **automatic and completely transparent** at the
same time.
The idea is to create one contract per ballot,
providing a short name for each option.
Then the creator of the contract who serves as
chairperson will give the right to vote to each
address individually.
The persons behind the addresses can then choose
to either vote themselves or to delegate their
vote to a person they trust.
At the end of the voting time, `winningProposal()`
will return the proposal with the largest number
of votes.
.. Gist: 618560d3f740204d46a5
::
/// @title Voting with delegation.
contract Ballot
{
// This declares a new complex type which will
// be used for variables later.
// It will represent a single voter.
struct Voter
{
uint weight; // weight is accumulated by delegation
bool voted; // if true, that person already voted
address delegate; // person delegated to
uint vote; // index of the voted proposal
}
// This is a type for a single proposal.
struct Proposal
{
bytes32 name; // short name (up to 32 bytes)
uint voteCount; // number of accumulated votes
}
address public chairperson;
// This declares a state variable that
// stores a `Voter` struct for each possible address.
mapping(address => Voter) public voters;
// A dynamically-sized array of `Proposal` structs.
Proposal[] public proposals;
/// Create a new ballot to choose one of `proposalNames`.
function Ballot(bytes32[] proposalNames)
{
chairperson = msg.sender;
voters[chairperson].weight = 1;
// For each of the provided proposal names,
// create a new proposal object and add it
// to the end of the array.
for (uint i = 0; i < proposalNames.length; i++)
// `Proposal({...})` creates a temporary
// Proposal object and `proposal.push(...)`
// appends it to the end of `proposals`.
proposals.push(Proposal({
name: proposalNames[i],
voteCount: 0
}));
}
// Give `voter` the right to vote on this ballot.
// May only be called by `chairperson`.
function giveRightToVote(address voter)
{
if (msg.sender != chairperson || voters[voter].voted)
// `throw` terminates and reverts all changes to
// the state and to Ether balances. It is often
// a good idea to use this if functions are
// called incorrectly. But watch out, this
// will also consume all provided gas.
throw;
voters[voter].weight = 1;
}
/// Delegate your vote to the voter `to`.
function delegate(address to)
{
// assigns reference
Voter sender = voters[msg.sender];
if (sender.voted)
throw;
// Forward the delegation as long as
// `to` also delegated.
while (voters[to].delegate != address(0) &&
voters[to].delegate != msg.sender)
to = voters[to].delegate;
// We found a loop in the delegation, not allowed.
if (to == msg.sender)
throw;
// Since `sender` is a reference, this
// modifies `voters[msg.sender].voted`
sender.voted = true;
sender.delegate = to;
Voter delegate = voters[to];
if (delegate.voted)
// If the delegate already voted,
// directly add to the number of votes
proposals[delegate.vote].voteCount += sender.weight;
else
// If the delegate did not vote yet,
// add to her weight.
delegate.weight += sender.weight;
}
/// Give your vote (including votes delegated to you)
/// to proposal `proposals[proposal].name`.
function vote(uint proposal)
{
Voter sender = voters[msg.sender];
if (sender.voted) throw;
sender.voted = true;
sender.vote = proposal;
// If `proposal` is out of the range of the array,
// this will throw automatically and revert all
// changes.
proposals[proposal].voteCount += sender.weight;
}
/// @dev Computes the winning proposal taking all
/// previous votes into account.
function winningProposal() constant
returns (uint winningProposal)
{
uint winningVoteCount = 0;
for (uint p = 0; p < proposals.length; p++)
{
if (proposals[p].voteCount > winningVoteCount)
{
winningVoteCount = proposals[p].voteCount;
winningProposal = p;
}
}
}
}
Possible Improvements
=====================
Currently, many transactions are needed to assign the rights
to vote to all participants. Can you think of a better way?
.. index:: auction;blind, auction;open, blind auction, open auction
*************
Blind Auction
*************
In this section, we will show how easy it is to create a
completely blind auction contract on Ethereum.
We will start with an open auction where everyone
can see the bids that are made and then extend this
contract into a blind auction where it is not
possible to see the actual bid until the bidding
period ends.
Simple Open Auction
===================
The general idea of the following simple auction contract
is that everyone can send their bids during
a bidding period. The bids already include sending
money / ether in order to bind the bidders to their
bid. If the highest bid is raised, the previously
highest bidder gets her money back.
After the end of the bidding period, the
contract has to be called manually for the
beneficiary to receive his money - contracts cannot
activate themselves.
.. {% include open_link gist="48cd2b65ff83bd04f7af" %}
::
contract SimpleAuction {
// Parameters of the auction. Times are either
// absolute unix timestamps (seconds since 1970-01-01)
// ore time periods in seconds.
address public beneficiary;
uint public auctionStart;
uint public biddingTime;
// Current state of the auction.
address public highestBidder;
uint public highestBid;
// Set to true at the end, disallows any change
bool ended;
// Events that will be fired on changes.
event HighestBidIncreased(address bidder, uint amount);
event AuctionEnded(address winner, uint amount);
// The following is a so-called natspec comment,
// recognizable by the three slashes.
// It will be shown when the user is asked to
// confirm a transaction.
/// Create a simple auction with `_biddingTime`
/// seconds bidding time on behalf of the
/// beneficiary address `_beneficiary`.
function SimpleAuction(uint _biddingTime,
address _beneficiary) {
beneficiary = _beneficiary;
auctionStart = now;
biddingTime = _biddingTime;
}
/// Bid on the auction with the value sent
/// together with this transaction.
/// The value will only be refunded if the
/// auction is not won.
function bid() {
// No arguments are necessary, all
// information is already part of
// the transaction.
if (now > auctionStart + biddingTime)
// Revert the call if the bidding
// period is over.
throw;
if (msg.value <= highestBid)
// If the bid is not higher, send the
// money back.
throw;
if (highestBidder != 0)
highestBidder.send(highestBid);
highestBidder = msg.sender;
highestBid = msg.value;
HighestBidIncreased(msg.sender, msg.value);
}
/// End the auction and send the highest bid
/// to the beneficiary.
function auctionEnd() {
if (now <= auctionStart + biddingTime)
throw; // auction did not yet end
if (ended)
throw; // this function has already been called
AuctionEnded(highestBidder, highestBid);
// We send all the money we have, because some
// of the refunds might have failed.
beneficiary.send(this.balance);
ended = true;
}
function () {
// This function gets executed if a
// transaction with invalid data is sent to
// the contract or just ether without data.
// We revert the send so that no-one
// accidentally loses money when using the
// contract.
throw;
}
}
Blind Auction
================
The previous open auction is extended to a blind auction
in the following. The advantage of a blind auction is
that there is no time pressure towards the end of
the bidding period. Creating a blind auction on a
transparent computing platform might sound like a
contradiction, but cryptography comes to the rescue.
During the **bidding period**, a bidder does not
actually send her bid, but only a hashed version of it.
Since it is currently considered practically impossible
to find two (sufficiently long) values whose hash
values are equal, the bidder commits to the bid by that.
After the end of the bidding period, the bidders have
to reveal their bids: They send their values
unencrypted and the contract checks that the hash value
is the same as the one provided during the bidding period.
Another challenge is how to make the auction
**binding and blind** at the same time: The only way to
prevent the bidder from just not sending the money
after he won the auction is to make her send it
together with the bid. Since value transfers cannot
be blinded in Ethereum, anyone can see the value.
The following contract solves this problem by
accepting any value that is at least as large as
the bid. Since this can of course only be checked during
the reveal phase, some bids might be **invalid**, and
this is on purpose (it even provides an explicit
flag to place invalid bids with high value transfers):
Bidders can confuse competition by placing several
high or low invalid bids.
.. {% include open_link gist="70528429c2cd867dd1d6" %}
::
contract BlindAuction
{
struct Bid
{
bytes32 blindedBid;
uint deposit;
}
address public beneficiary;
uint public auctionStart;
uint public biddingEnd;
uint public revealEnd;
bool public ended;
mapping(address => Bid[]) public bids;
address public highestBidder;
uint public highestBid;
event AuctionEnded(address winner, uint highestBid);
/// Modifiers are a convenient way to validate inputs to
/// functions. `onlyBefore` is applied to `bid` below:
/// The new function body is the modifier's body where
/// `_` is replaced by the old function body.
modifier onlyBefore(uint _time) { if (now >= _time) throw; _ }
modifier onlyAfter(uint _time) { if (now <= _time) throw; _ }
function BlindAuction(uint _biddingTime,
uint _revealTime,
address _beneficiary)
{
beneficiary = _beneficiary;
auctionStart = now;
biddingEnd = now + _biddingTime;
revealEnd = biddingEnd + _revealTime;
}
/// Place a blinded bid with `_blindedBid` = sha3(value,
/// fake, secret).
/// The sent ether is only refunded if the bid is correctly
/// revealed in the revealing phase. The bid is valid if the
/// ether sent together with the bid is at least "value" and
/// "fake" is not true. Setting "fake" to true and sending
/// not the exact amount are ways to hide the real bid but
/// still make the required deposit. The same address can
/// place multiple bids.
function bid(bytes32 _blindedBid)
onlyBefore(biddingEnd)
{
bids[msg.sender].push(Bid({
blindedBid: _blindedBid,
deposit: msg.value
}));
}
/// Reveal your blinded bids. You will get a refund for all
/// correctly blinded invalid bids and for all bids except for
/// the totally highest.
function reveal(uint[] _values, bool[] _fake,
bytes32[] _secret)
onlyAfter(biddingEnd)
onlyBefore(revealEnd)
{
uint length = bids[msg.sender].length;
if (_values.length != length || _fake.length != length ||
_secret.length != length)
throw;
uint refund;
for (uint i = 0; i < length; i++)
{
var bid = bids[msg.sender][i];
var (value, fake, secret) =
(_values[i], _fake[i], _secret[i]);
if (bid.blindedBid != sha3(value, fake, secret))
// Bid was not actually revealed.
// Do not refund deposit.
continue;
refund += value;
if (!fake && bid.deposit >= value)
if (placeBid(msg.sender, value))
refund -= value;
// Make it impossible for the sender to re-claim
// the same deposit.
bid.blindedBid = 0;
}
msg.sender.send(refund);
}
// This is an "internal" function which means that it
// can only be called from the contract itself (or from
// derived contracts).
function placeBid(address bidder, uint value) internal
returns (bool success)
{
if (value <= highestBid)
return false;
if (highestBidder != 0)
// Refund the previously highest bidder.
highestBidder.send(highestBid);
highestBid = value;
highestBidder = bidder;
return true;
}
/// End the auction and send the highest bid
/// to the beneficiary.
function auctionEnd()
onlyAfter(revealEnd)
{
if (ended) throw;
AuctionEnded(highestBidder, highestBid);
// We send all the money we have, because some
// of the refunds might have failed.
beneficiary.send(this.balance);
ended = true;
}
function () { throw; }
}
.. index:: purchase, remote purchase, escrow
********************
Safe Remote Purchase
********************
.. {% include open_link gist="b16e8e76a423b7671e99" %}
::
contract Purchase
{
uint public value;
address public seller;
address public buyer;
enum State { Created, Locked, Inactive }
State public state;
function Purchase()
{
seller = msg.sender;
value = msg.value / 2;
if (2 * value != msg.value) throw;
}
modifier require(bool _condition)
{
if (!_condition) throw;
_
}
modifier onlyBuyer()
{
if (msg.sender != buyer) throw;
_
}
modifier onlySeller()
{
if (msg.sender != seller) throw;
_
}
modifier inState(State _state)
{
if (state != _state) throw;
_
}
event aborted();
event purchaseConfirmed();
event itemReceived();
/// Abort the purchase and reclaim the ether.
/// Can only be called by the seller before
/// the contract is locked.
function abort()
onlySeller
inState(State.Created)
{
aborted();
seller.send(this.balance);
state = State.Inactive;
}
/// Confirm the purchase as buyer.
/// Transaction has to include `2 * value` ether.
/// The ether will be locked until confirmReceived
/// is called.
function confirmPurchase()
inState(State.Created)
require(msg.value == 2 * value)
{
purchaseConfirmed();
buyer = msg.sender;
state = State.Locked;
}
/// Confirm that you (the buyer) received the item.
/// This will release the locked ether.
function confirmReceived()
onlyBuyer
inState(State.Locked)
{
itemReceived();
buyer.send(value); // We ignore the return value on purpose
seller.send(this.balance);
state = State.Inactive;
}
function() { throw; }
}
********************
Micropayment Channel
********************
To be written.

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#################
Solidity in Depth
#################
This section should provide you with all you need to know about Solidity.
If something is missing here, please contact us on
`Gitter <https://gitter.im/ethereum/solidity>`_ or make a pull request on
`Github <https://github.com/ethereum/solidity/pulls>`_.
.. toctree::
:maxdepth: 2
layout-of-source-files.rst
structure-of-a-contract.rst
types.rst
units-and-global-variables.rst
control-structures.rst
contracts.rst
miscellaneous.rst

17
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.. index:: contract, state variable, function, event, struct, enum, function;modifier
***********************
Structure of a Contract
***********************
Contracts in Solidity are similar to classes in object-oriented languages.
Each contract can contain declarations of **state variables**, **functions**,
**function modifiers**, **events**, **structs types** and **enum types**.
Furthermore, contracts can inherit from other contracts.
* State variables are values which are permanently stored in contract storage.
* Functions are the executable units of code within a contract.
* Function modifiers can be used to amend the semantics of functions in a declarative way.
* Events are convenience interfaces with the EVM logging facilities.
* Structs are custom defined types that can group several variables.
* Enums can be used to create custom types with a finite set of values.

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.. index:: style, coding style
#############
Style Guide
#############
************
Introduction
************
This guide is intended to provide coding conventions for writing solidity code.
This guide should be thought of as an evolving document that will change over
time as useful conventions are found and old conventions are rendered obsolete.
Many projects will implement their own style guides. In the event of
conflicts, project specific style guides take precedence.
The structure and many of the recommendations within this style guide were
taken from python's
`pep8 style guide <https://www.python.org/dev/peps/pep-0008/>`_.
The goal of this guide is *not* to be the right way or the best way to write
solidity code. The goal of this guide is *consistency*. A quote from python's
`pep8 <https://www.python.org/dev/peps/pep-0008/#a-foolish-consistency-is-the-hobgoblin-of-little-minds>`_
captures this concept well.
A style guide is about consistency. Consistency with this style guide is important. Consistency within a project is more important. Consistency within one module or function is most important.
But most importantly: know when to be inconsistent -- sometimes the style guide just doesn't apply. When in doubt, use your best judgment. Look at other examples and decide what looks best. And don't hesitate to ask!
***********
Code Layout
***********
Indentation
===========
Use 4 spaces per indentation level.
Tabs or Spaces
==============
Spaces are the preferred indentation method.
Mixing tabs and spaces should be avoided.
Blank Lines
===========
Surround top level declarations in solidity source with two blank lines.
Yes::
contract A {
...
}
contract B {
...
}
contract C {
...
}
No::
contract A {
...
}
contract B {
...
}
contract C {
...
}
Within a contract surround function declarations with a single blank line.
Blank lines may be omitted between groups of related one-liners (such as stub functions for an abstract contract)
Yes::
contract A {
function spam();
function ham();
}
contract B is A {
function spam() {
...
}
function ham() {
...
}
}
No::
contract A {
function spam() {
...
}
function ham() {
...
}
}
Source File Encoding
====================
UTF-8 or ASCII encoding is preferred.
Imports
==========
Import statements should always be placed at the top of the file.
Yes::
import "owned";
contract A {
...
}
contract B is owned {
...
}
No::
contract A {
...
}
import "owned";
contract B is owned {
...
}
Whitespace in Expressions
=========================
Avoid extraneous whitespace in the following situations:
* Immediately inside parenthesis, brackets or braces.
Yes: `spam(ham[1], Coin({name: "ham"}));`
No: `spam( ham[ 1 ], Coin( { name: "ham" } ) );`
* Immediately before a comma, semicolon:
Yes: `function spam(uint i, Coin coin);`
No: `function spam(uint i , Coin coin) ;`
* More than one space around an assignment or other operator to align with
another:
Yes::
x = 1;
y = 2;
long_variable = 3;
No::
x = 1;
y = 2;
long_variable = 3;
Control Structures
==================
The braces denoting the body of a contract, library, functions and structs
should:
* open on the same line as the declaration
* close on their own line at the same indentation level as the beginning of the
declaration.
* The opening brace should be proceeded by a single space.
Yes::
contract Coin {
struct Bank {
address owner;
uint balance;
}
}
No::
contract Coin
{
struct Bank {
address owner;
uint balance;
}
}
The same recommendations apply to the control structures `if`, `else`, `while`,
and `for`.
Additionally there should be a single space between the control structures
`if`, `while`, and `for` and the parenthetic block representing the
conditional, as well as a single space between the conditional parenthetic
block and the opening brace.
Yes::
if (...) {
...
}
for (...) {
...
}
No::
if (...)
{
...
}
while(...){
}
for (...) {
...;}
For control structures who's body contains a single statement, omitting the
braces is ok *if* the statement is contained on a single line.
Yes::
if (x < 10)
x += 1;
No::
if (x < 10)
someArray.push(Coin({
name: 'spam',
value: 42
}));
For `if` blocks which have an `else` or `else if` clause, the `else` should be
placed on it's own line following the previous closing parenthesis. The
parenthesis for the else block should follow the same rules as the other
conditional control structures.
Yes::
if (x < 3) {
x += 1;
}
else {
x -= 1;
}
if (x < 3)
x += 1;
else
x -= 1;
No::
if (x < 3) {
x += 1;
} else {
x -= 1;
}
Function Declaration
====================
For short function declarations, it is recommended for the opening brace of the
function body to be kept on the same line as the function declaration.
The closing brace should be at the same indentation level as the function
declaration.
The opening brace should be preceeded by a single space.
Yes::
function increment(uint x) returns (uint) {
return x + 1;
}
function increment(uint x) public onlyowner returns (uint) {
return x + 1;
}
No::
function increment(uint x) returns (uint)
{
return x + 1;
}
function increment(uint x) returns (uint){
return x + 1;
}
function increment(uint x) returns (uint) {
return x + 1;
}
function increment(uint x) returns (uint) {
return x + 1;}
The visibility modifiers for a function should come before any custom
modifiers.
Yes::
function kill() public onlyowner {
selfdestruct(owner);
}
No::
function kill() onlyowner public {
selfdestruct(owner);
}
For long function declarations, it is recommended to drop each arguent onto
it's own line at the same indentation level as the function body. The closing
parenthesis and opening bracket should be placed on their own line as well at
the same indentation level as the function declaration.
Yes::
function thisFunctionHasLotsOfArguments(
address a,
address b,
address c,
address d,
address e,
address f,
) {
do_something;
}
No::
function thisFunctionHasLotsOfArguments(address a, address b, address c,
address d, address e, address f) {
do_something;
}
function thisFunctionHasLotsOfArguments(address a,
address b,
address c,
address d,
address e,
address f) {
do_something;
}
function thisFunctionHasLotsOfArguments(
address a,
address b,
address c,
address d,
address e,
address f) {
do_something;
}
If a long function declaration has modifiers, then each modifier should be
dropped to it's own line.
Yes::
function thisFunctionNameIsReallyLong(address x, address y, address z)
public
onlyowner
priced
returns (address)
{
do_something;
}
function thisFunctionNameIsReallyLong(
address x,
address y,
address z,
)
public
onlyowner
priced
returns (address)
{
do_something;
}
No::
function thisFunctionNameIsReallyLong(address x, address y, address z)
public
onlyowner
priced
returns (address) {
do_something;
}
function thisFunctionNameIsReallyLong(address x, address y, address z)
public onlyowner priced returns (address)
{
do_something;
}
function thisFunctionNameIsReallyLong(address x, address y, address z)
public
onlyowner
priced
returns (address) {
do_something;
}
For constructor functions on inherited contracts who's bases require arguments,
it is recommended to drop the base constructors onto new lines in the same
manner as modifiers if the function declaration is long or hard to read.
Yes::
contract A is B, C, D {
function A(uint param1, uint param2, uint param3, uint param4, uint param5)
B(param1)
C(param2, param3)
D(param4)
{
// do something with param5
}
}
No::
contract A is B, C, D {
function A(uint param1, uint param2, uint param3, uint param4, uint param5)
B(param1)
C(param2, param3)
D(param4)
{
// do something with param5
}
}
contract A is B, C, D {
function A(uint param1, uint param2, uint param3, uint param4, uint param5)
B(param1)
C(param2, param3)
D(param4) {
// do something with param5
}
}
These guidelines for function declarations are intended to improve readability.
Authors should use their best judgement as this guide does not try to cover all
possible permutations for function declarations.
Mappings
========
TODO
Variable Declarations
=====================
Declarations of array variables should not have a space between the type and
the brackets.
Yes: `uint[] x;`
No: `uint [] x;`
Other Recommendations
=====================
* Surround operators with a single space on either side.
Yes::
x = 3;
x = 100 / 10;
x += 3 + 4;
x |= y && z;
No::
x=3;
x = 100/10;
x += 3+4;
x |= y&&z;
* Operators with a higher priority than others can exclude surrounding
whitespace in order to denote precidence. This is meant to allow for
improved readability for complex statement. You should always use the same
amount of whitespace on either side of an operator:
Yes::
x = 2**3 + 5;
x = 2*y + 3*z;
x = (a+b) * (a-b);
No::
x = 2** 3 + 5;
x = y+z;
x +=1;
Naming Conventions
==================
TODO
General Recommendations
=======================
TODO

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.. index:: type
*****
Types
*****
Solidity is a statically typed language, which means that the type of each
variable (state and local) needs to be specified (or at least known -
see :ref:`type-deduction` below) at
compile-time. Solidity provides several elementary types which can be combined
to complex types.
.. index:: ! value type, ! type;value
Value Types
===========
The following types are also called value types because variables of these
types will always be passed by value, i.e. they are always copied when they
are used as function arguments or in assignments.
.. index:: ! bool, ! true, ! false
Booleans
--------
`bool`: The possible values are constants `true` and `false`.
Operators:
* `!` (logical negation)
* `&&` (logical conjunction, "and")
* `||` (logical disjunction, "or")
* `==` (equality)
* `!=` (inequality)
The operators `||` and `&&` apply the common short-circuiting rules. This means that in the expression `f(x) || g(y)`, if `f(x)` evaluates to `true`, `g(y)` will not be evaluated even if it may have side-effects.
.. index:: ! uint, ! int, ! integer
Integers
--------
`int•` / `uint•`: Signed and unsigned integers of various sizes. Keywords `uint8` to `uint256` in steps of `8` (unsigned of 8 up to 256 bits) and `int8` to `int256`. `uint` and `int` are aliases for `uint256` and `int256`, respectively.
Operators:
* Comparisons: `<=`, `<`, `==`, `!=`, `>=`, `>` (evaluate to `bool`)
* Bit operators: `&`, `|`, `^` (bitwise exclusive or), `~` (bitwise negation)
* Arithmetic operators: `+`, `-`, unary `-`, unary `+`, `*`, `/`, `%` (remainder), `**` (exponentiation)
.. index:: address, balance, send, call, callcode
Address
-------
`address`: Holds a 20 byte value (size of an Ethereum address). Address types also have members(see [Functions on addresses](#functions-on-addresses)) and serve as base for all contracts.
Operators:
* `<=`, `<`, `==`, `!=`, `>=` and `>`
Members of Addresses
^^^^^^^^^^^^^^^^^^^^
* `balance` and `send`
It is possible to query the balance of an address using the property `balance`
and to send Ether (in units of wei) to an address using the `send` function:
::
address x = 0x123;
address myAddress = this;
if (x.balance < 10 && myAddress.balance >= 10) x.send(10);
.. note::
If `x` is a contract address, its code (more specifically: its fallback function, if present) will be executed together with the `send` call (this is a limitation of the EVM and cannot be prevented). If that execution runs out of gas or fails in any way, the Ether transfer will be reverted. In this case, `send` returns `false`.
* `call` and `callcode`
Furthermore, to interface with contracts that do not adhere to the ABI,
the function `call` is provided which takes an arbitrary number of arguments of any type. These arguments are padded to 32 bytes and concatenated. One exception is the case where the first argument is encoded to exactly four bytes. In this case, it is not padded to allow the use of function signatures here.
::
address nameReg = 0x72ba7d8e73fe8eb666ea66babc8116a41bfb10e2;
nameReg.call("register", "MyName");
nameReg.call(bytes4(sha3("fun(uint256)")), a);
`call` returns a boolean indicating whether the invoked function terminated (`true`) or caused an EVM exception (`false`). It is not possible to access the actual data returned (for this we would need to know the encoding and size in advance).
In a similar way, the function `callcode` can be used: The difference is that only the code of the given address is used, all other aspects (storage, balance, ...) are taken from the current contract. The purpose of `callcode` is to use library code which is stored in another contract. The user has to ensure that the layout of storage in both contracts is suitable for callcode to be used.
Both `call` and `callcode` are very low-level functions and should only be used as a *last resort* as they break the type-safety of Solidity.
.. note::
All contracts inherit the members of address, so it is possible to query the balance of the
current contract using `this.balance`.
.. index:: byte array, bytes32
Fixed-size byte arrays
----------------------
`bytes1`, `bytes2`, `bytes3`, ..., `bytes32`. `byte` is an alias for `bytes1`.
Operators:
* Comparisons: `<=`, `<`, `==`, `!=`, `>=`, `>` (evaluate to `bool`)
* Bit operators: `&`, `|`, `^` (bitwise exclusive or), `~` (bitwise negation)
Dynamically-sized byte array
----------------------------
`bytes`:
Dynamically-sized byte array, see :ref:`arrays`. Not a value-type!
`string`:
Dynamically-sized UTF8-encoded string, see :ref:`arrays`. Not a value-type!
.. index:: literal, literal;integer
Integer Literals
-----------------
Integer Literals are arbitrary precision integers until they are used together with a non-literal. In `var x = 1 - 2;`, for example, the value of `1 - 2` is `-1`, which is assigned to `x` and thus `x` receives the type `int8` -- the smallest type that contains `-1`, although the natural types of `1` and `2` are actually `uint8`.
It is even possible to temporarily exceed the maximum of 256 bits as long as only integer literals are used for the computation: `var x = (0xffffffffffffffffffff * 0xffffffffffffffffffff) * 0;` Here, `x` will have the value `0` and thus the type `uint8`.
.. index:: literal, literal;string, string
String Literals
---------------
String Literals are written with double quotes (`"abc"`). As with integer literals, their type can vary, but they are implicitly convertible to `bytes•` if they fit, to `bytes` and to `string`.
.. index:: enum
Enums
=====
Enums are one way to create a user-defined type in Solidity. They are explicitly convertible
to and from all integer types but implicit conversion is not allowed.
::
contract test {
enum ActionChoices { GoLeft, GoRight, GoStraight, SitStill }
ActionChoices choice;
ActionChoices constant defaultChoice = ActionChoices.GoStraight;
function setGoStraight()
{
choice = ActionChoices.GoStraight;
}
// Since enum types are not part of the ABI, the signature of "getChoice"
// will automatically be changed to "getChoice() returns (uint8)"
// for all matters external to Solidity. The integer type used is just
// large enough to hold all enum values, i.e. if you have more values,
// `uint16` will be used and so on.
function getChoice() returns (ActionChoices)
{
return choice;
}
function getDefaultChoice() returns (uint)
{
return uint(defaultChoice);
}
}
.. index:: ! type;reference, ! reference type, storage, memory, location, array, struct
Reference Types
==================
Complex types, i.e. types which do not always fit into 256 bits have to be handled
more carefully than the value-types we have already seen. Since copying
them can be quite expensive, we have to think about whether we want them to be
stored in **memory** (which is not persisting) or **storage** (where the state
variables are held).
Data location
-------------
Every complex type, i.e. *arrays* and *structs*, has an additional
annotation, the "data location", about whether it is stored in memory or in storage. Depending on the
context, there is always a default, but it can be overridden by appending
either `storage` or `memory` to the type. The default for function parameters (including return parameters) is `memory`, the default for local variables is `storage` and the location is forced
to `storage` for state variables (obviously).
There is also a third data location, "calldata", which is a non-modifyable
non-persistent area where function arguments are stored. Function parameters
(not return parameters) of external functions are forced to "calldata" and
it behaves mostly like memory.
Data locations are important because they change how assignments behave:
Assignments between storage and memory and also to a state variable (even from other state variables)
always create an independent copy.
Assignments to local storage variables only assign a reference though, and
this reference always points to the state variable even if the latter is changed
in the meantime.
On the other hand, assignments from a memory stored reference type to another
memory-stored reference type does not create a copy.
::
contract c {
uint[] x; // the data location of x is storage
// the data location of memoryArray is memory
function f(uint[] memoryArray) {
x = memoryArray; // works, copies the whole array to storage
var y = x; // works, assigns a pointer, data location of y is storage
y[7]; // fine, returns the 8th element
y.length = 2; // fine, modifies x through y
delete x; // fine, clears the array, also modifies y
// The following does not work; it would need to create a new temporary /
// unnamed array in storage, but storage is "statically" allocated:
// y = memoryArray;
// This does not work either, since it would "reset" the pointer, but there
// is no sensible location it could point to.
// delete y;
g(x); // calls g, handing over a reference to x
h(x); // calls h and creates an independent, temporary copy in memory
}
function g(uint[] storage storageArray) internal {}
function h(uint[] memoryArray) {}
}
Summary
^^^^^^^
Forced data location:
- parameters (not return) of external functions: calldata
- state variables: storage
Default data location:
- parameters (also return) of functions: memory
- all other local variables: storage
.. index:: ! array
.. _arrays:
Arrays
------
Arrays can have a compile-time fixed size or they can be dynamic.
For storage arrays, the element type can be arbitrary (i.e. also other
arrays, mappings or structs). For memory arrays, it cannot be a mapping and
has to be an ABI type if it is an argument of a publicly-visible function.
An array of fixed size `k` and element type `T` is written as `T[k]`,
an array of dynamic size as `T[]`. As an example, an array of 5 dynamic
arrays of `uint` is `uint[][5]` (note that the notation is reversed when
compared to some other languages). To access the second uint in the
third dynamic array, you use `x[2][1]` (indices are zero-based and
access works in the opposite way of the declaration, i.e. `x[2]`
shaves off one level in the type from the right).
Variables of type `bytes` and `string` are special arrays. A `bytes` is similar to `byte[]`,
but it is packed tightly in calldata. `string` is equal to `bytes` but does not allow
length or index access (for now).
.. note::
If you want to access the byte-representation of a string `s`, use
`bytes(s).length` / `bytes(s)[7] = 'x';`. Keep in mind
that you are accessing the low-level bytes of the utf-8 representation,
and not the individual characters!
.. index:: ! array;length, length, push, !array;push
Members
^^^^^^^
**length**:
Arrays have a `length` member to hold their number of elements.
Dynamic arrays can be resized in storage (not in memory) by changing the
`.length` member. This does not happen automatically when attempting to access elements outside the current length. The size of memory arrays is fixed (but dynamic, i.e. it can depend on runtime parameters) once they are created.
**push**:
Dynamic storage arrays and `bytes` (not `string`) have a member function called `push` that can be used to append an element at the end of the array. The function returns the new length.
.. warning::
It is not yet possible to use arrays of arrays in external functions.
.. warning::
Due to limitations of the EVM, it is not possible to return
dynamic content from external function calls. The function `f` in
`contract C { function f() returns (uint[]) { ... } }` will return
something if called from web3.js, but not if called from Solidity.
The only workaround for now is to use large statically-sized arrays.
::
contract ArrayContract {
uint[2**20] m_aLotOfIntegers;
// Note that the following is not a pair of arrays but an array of pairs.
bool[2][] m_pairsOfFlags;
// newPairs is stored in memory - the default for function arguments
function setAllFlagPairs(bool[2][] newPairs) {
// assignment to a storage array replaces the complete array
m_pairsOfFlags = newPairs;
}
function setFlagPair(uint index, bool flagA, bool flagB) {
// access to a non-existing index will throw an exception
m_pairsOfFlags[index][0] = flagA;
m_pairsOfFlags[index][1] = flagB;
}
function changeFlagArraySize(uint newSize) {
// if the new size is smaller, removed array elements will be cleared
m_pairsOfFlags.length = newSize;
}
function clear() {
// these clear the arrays completely
delete m_pairsOfFlags;
delete m_aLotOfIntegers;
// identical effect here
m_pairsOfFlags.length = 0;
}
bytes m_byteData;
function byteArrays(bytes data) {
// byte arrays ("bytes") are different as they are stored without padding,
// but can be treated identical to "uint8[]"
m_byteData = data;
m_byteData.length += 7;
m_byteData[3] = 8;
delete m_byteData[2];
}
function addFlag(bool[2] flag) returns (uint) {
return m_pairsOfFlags.push(flag);
}
function createMemoryArray(uint size) returns (bytes) {
// Dynamic memory arrays are created using `new`:
uint[2][] memory arrayOfPairs = new uint[2][](size);
// Create a dynamic byte array:
bytes memory b = new bytes(200);
for (uint i = 0; i < b.length; i++)
b[i] = byte(i);
return b;
}
}
.. index:: ! struct, ! type;struct
Structs
-------
Solidity provides a way to define new types in the form of structs, which is
shown in the following example:
::
contract CrowdFunding {
// Defines a new type with two fields.
struct Funder {
address addr;
uint amount;
}
struct Campaign {
address beneficiary;
uint fundingGoal;
uint numFunders;
uint amount;
mapping (uint => Funder) funders;
}
uint numCampaigns;
mapping (uint => Campaign) campaigns;
function newCampaign(address beneficiary, uint goal) returns (uint campaignID) {
campaignID = numCampaigns++; // campaignID is return variable
// Creates new struct and saves in storage. We leave out the mapping type.
campaigns[campaignID] = Campaign(beneficiary, goal, 0, 0);
}
function contribute(uint campaignID) {
Campaign c = campaigns[campaignID];
// Creates a new temporary memory struct, initialised with the given values
// and copies it over to storage.
// Note that you can also use Funder(msg.sender, msg.value) to initialise.
c.funders[c.numFunders++] = Funder({addr: msg.sender, amount: msg.value});
c.amount += msg.value;
}
function checkGoalReached(uint campaignID) returns (bool reached) {
Campaign c = campaigns[campaignID];
if (c.amount < c.fundingGoal)
return false;
c.beneficiary.send(c.amount);
c.amount = 0;
return true;
}
}
The contract does not provide the full functionality of a crowdfunding
contract, but it contains the basic concepts necessary to understand structs.
Struct types can be used inside mappings and arrays and they can itself
contain mappings and arrays.
It is not possible for a struct to contain a member of its own type,
although the struct itself can be the value type of a mapping member.
This restriction is necessary, as the size of the struct has to be finite.
Note how in all the functions, a struct type is assigned to a local variable
(of the default storage data location).
This does not copy the struct but only stores a reference so that assignments to
members of the local variable actually write to the state.
Of course, you can also directly access the members of the struct without
assigning it to a local variable, as in
`campaigns[campaignID].amount = 0`.
.. index:: !mapping
Mappings
========
Mapping types are declared as `mapping _KeyType => _ValueType`, where
`_KeyType` can be almost any type except for a mapping and `_ValueType`
can actually be any type, including mappings.
Mappings can be seen as hashtables which are virtually initialized such that
every possible key exists and is mapped to a value whose byte-representation is
all zeros. The similarity ends here, though: The key data is not actually stored
in a mapping, only its `sha3` hash used to look up the value.
Because of this, mappings do not have a length or a concept of a key or value being "set".
Mappings are only allowed for state variables (or as storage reference types
in internal functions).
.. index:: assignment, ! delete, lvalue
Operators Involving LValues
===========================
If `a` is an LValue (i.e. a variable or something that can be assigned to), the following operators are available as shorthands:
`a += e` is equivalent to `a = a + e`. The operators `-=`, `*=`, `/=`, `%=`, `a |=`, `&=` and `^=` are defined accordingly. `a++` and `a--` are equivalent to `a += 1` / `a -= 1` but the expression itself still has the previous value of `a`. In contrast, `--a` and `++a` have the same effect on `a` but return the value after the change.
delete
------
`delete a` assigns the initial value for the type to `a`. I.e. for integers it is equivalent to `a = 0`, but it can also be used on arrays, where it assigns a dynamic array of length zero or a static array of the same length with all elements reset. For structs, it assigns a struct with all members reset.
`delete` has no effect on whole mappings (as the keys of mappings may be arbitrary and are generally unknown). So if you delete a struct, it will reset all members that are not mappings and also recurse into the members unless they are mappings. However, individual keys and what they map to can be deleted.
It is important to note that `delete a` really behaves like an assignment to `a`, i.e. it stores a new object in `a`.
::
contract DeleteExample {
uint data;
uint[] dataArray;
function f() {
uint x = data;
delete x; // sets x to 0, does not affect data
delete data; // sets data to 0, does not affect x which still holds a copy
uint[] y = dataArray;
delete dataArray; // this sets dataArray.length to zero, but as uint[] is a complex object, also
// y is affected which is an alias to the storage object
// On the other hand: "delete y" is not valid, as assignments to local variables
// referencing storage objects can only be made from existing storage objects.
}
}
.. index:: ! type;conversion, ! cast
Conversions between Elementary Types
====================================
Implicit Conversions
--------------------
If an operator is applied to different types, the compiler tries to
implicitly convert one of the operands to the type of the other (the same is
true for assignments). In general, an implicit conversion between value-types
is possible if it
makes sense semantically and no information is lost: `uint8` is convertible to
`uint16` and `int128` to `int256`, but `int8` is not convertible to `uint256`
(because `uint256` cannot hold e.g. `-1`).
Furthermore, unsigned integers can be converted to bytes of the same or larger
size, but not vice-versa. Any type that can be converted to `uint160` can also
be converted to `address`.
Explicit Conversions
--------------------
If the compiler does not allow implicit conversion but you know what you are
doing, an explicit type conversion is sometimes possible::
int8 y = -3;
uint x = uint(y);
At the end of this code snippet, `x` will have the value `0xfffff..fd` (64 hex
characters), which is -3 in two's complement representation of 256 bits.
If a type is explicitly converted to a smaller type, higher-order bits are
cut off::
uint32 a = 0x12345678;
uint16 b = uint16(a); // b will be 0x5678 now
.. index:: ! type;deduction, ! var
.. _type-deduction:
Type Deduction
==============
For convenience, it is not always necessary to explicitly specify the type of a
variable, the compiler automatically infers it from the type of the first
expression that is assigned to the variable::
uint20 x = 0x123;
var y = x;
Here, the type of `y` will be `uint20`. Using `var` is not possible for function
parameters or return parameters.
.. warning::
The type is only deduced from the first assignment, so
the loop in the following snippet is infinite, as `i` will have the type
`uint8` and any value of this type is smaller than `2000`.
`for (var i = 0; i < 2000; i++) { ... }`

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**************************************
Units and Globally Available Variables
**************************************
.. index:: wei, finney, szabo, ether
Ether Units
===========
A literal number can take a suffix of `wei`, `finney`, `szabo` or `ether` to convert between the subdenominations of Ether, where Ether currency numbers without a postfix are assumed to be "wei", e.g. `2 ether == 2000 finney` evaluates to `true`.
.. index:: time, seconds, minutes, hours, days, weeks, years
Time Units
==========
Suffixes of `seconds`, `minutes`, `hours`, `days`, `weeks` and
`years` after literal numbers can be used to convert between units of time where seconds are the base
unit and units are considered naively in the following way:
* `1 == 1 second`
* `1 minutes == 60 seconds`
* `1 hours == 60 minutes`
* `1 days == 24 hours`
* `1 weeks = 7 days`
* `1 years = 365 days`
Take care if you perform calendar calculations using these units, because
not every year equals 365 days and not even every day has 24 hours
because of `leap seconds <https://en.wikipedia.org/wiki/Leap_second>`_.
Due to the fact that leap seconds cannot be predicted, an exact calendar
library has to be updated by an external oracle.
These suffixes cannot be applied to variables. If you want to
interpret some input variable in e.g. days, you can do it in the following way::
function f(uint start, uint daysAfter) {
if (now >= start + daysAfter * 1 days) { ... }
}
Special Variables and Functions
===============================
There are special variables and functions which always exist in the global
namespace and are mainly used to provide information about the blockchain.
.. index:: block, coinbase, difficulty, number, block;number, timestamp, block;timestamp, msg, data, gas, sender, value, now, gas price, origin
Block and Transaction Properties
------------------------------------
- `block.coinbase` (`address`): current block miner's address
- `block.difficulty` (`uint`): current block difficulty
- `block.gaslimit` (`uint`): current block gaslimit
- `block.number` (`uint`): current block number
- `block.blockhash` (`function(uint) returns (bytes32)`): hash of the given block - only for 256 most recent blocks
- `block.timestamp` (`uint`): current block timestamp
- `msg.data` (`bytes`): complete calldata
- `msg.gas` (`uint`): remaining gas
- `msg.sender` (`address`): sender of the message (current call)
- `msg.sig` (`bytes4`): first four bytes of the calldata (i.e. function identifier)
- `msg.value` (`uint`): number of wei sent with the message
- `now` (`uint`): current block timestamp (alias for `block.timestamp`)
- `tx.gasprice` (`uint`): gas price of the transaction
- `tx.origin` (`address`): sender of the transaction (full call chain)
.. note::
The values of all members of `msg`, including `msg.sender` and
`msg.value` can change for every **external** function call.
This includes calls to library functions.
If you want to implement access restrictions in library functions using
`msg.sender`, you have to manually supply the value of
`sg.sender` as an argument.
.. note::
The block hashes are not available for all blocks for scalability reasons.
You can only access the hashes of the most recent 256 blocks, all other
values will be zero.
.. index:: sha3, ripemd160, sha256, ecrecover, addmod, mulmod, cryptography, this, super, selfdestruct, balance, send
Mathematical and Cryptographic Functions
----------------------------------------
`addmod(uint x, uint y, uint k) returns (uint)`:
compute `(x + y) % k` where the addition is performed with arbitrary precision and does not wrap around at `2**256`.
`mulmod(uint x, uint y, uint k) returns (uint)`:
compute `(x * y) % k` where the multiplication is performed with arbitrary precision and does not wrap around at `2**256`.
`sha3(...) returns (bytes32)`:
compute the Ethereum-SHA-3 hash of the (tightly packed) arguments
`sha256(...) returns (bytes32)`:
compute the SHA-256 hash of the (tightly packed) arguments
`ripemd160(...) returns (bytes20)`:
compute RIPEMD-160 hash of the (tightly packed) arguments
`ecrecover(bytes32, byte, bytes32, bytes32) returns (address)`:
recover public key from elliptic curve signature - arguments are (data, v, r, s)
In the above, "tightly packed" means that the arguments are concatenated without padding.
This means that the following are all identical::
sha3("ab", "c")
sha3("abc")
sha3(0x616263)
sha3(6382179)
sha3(97, 98, 99)
If padding is needed, explicit type conversions can be used: `sha3("\x00\x12")` is the
same as `sha3(uint16(0x12))`.
It might be that you run into Out-of-Gas for `sha256`, `ripemd160` or `ecrecover` on a *private blockchain*. The reason for this is that those are implemented as so-called precompiled contracts and these contracts only really exist after they received the first message (although their contract code is hardcoded). Messages to non-existing contracts are more expensive and thus the execution runs into an Out-of-Gas error. A workaround for this problem is to first send e.g. 1 Wei to each of the contracts before you use them in your actual contracts. This is not an issue on the official or test net.
.. index:: this, selfdestruct
Contract Related
----------------
`this` (current contract's type):
the current contract, explicitly convertible to `address`
`selfdestruct(address)`:
destroy the current contract, sending its funds to the given address
Furthermore, all functions of the current contract are callable directly including the current function.