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docs/frequently-asked-questions.rst
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@ -10,7 +10,7 @@ Basic Questions
*************** ***************
What is Solidity? What is Solidity?
============================= =================
Solidity is the DEV-created (i.e. Ethereum Foundation-created), Solidity is the DEV-created (i.e. Ethereum Foundation-created),
Javascript-inspired language that can be used to create smart contracts Javascript-inspired language that can be used to create smart contracts
@ -28,7 +28,7 @@ There are some `contract examples <https://github.com/fivedogit/solidity-baby-st
there should be a `test contract <https://github.com/ethereum/solidity/blob/develop/test/libsolidity/SolidityEndToEndTest.cpp>`_ for every single feature of Solidity. 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? How do I compile contracts?
============================= ===========================
Probably the fastest way is the `online compiler <https://ethereum.github.io/browser-solidity/>`_. Probably the fastest way is the `online compiler <https://ethereum.github.io/browser-solidity/>`_.
@ -71,8 +71,8 @@ several blockchain explorers.
Contracts on the blockchain should have their original source Contracts on the blockchain should have their original source
code published if they are to be used by third parties. code published if they are to be used by third parties.
Does selfdestruct() free up space in the blockchain? Does :code:`selfdestruct()` free up space in the blockchain?
==================================================== ============================================================
It removes the contract bytecode and storage from the current block It removes the contract bytecode and storage from the current block
into the future, but since the blockchain stores every single block (i.e. into the future, but since the blockchain stores every single block (i.e.
@ -108,8 +108,8 @@ The trick is to create the contract with :code:`{from:someaddress, value: web3.t
See `endowment_retriever.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/30_endowment_retriever.sol>`_. 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 Use a non-constant function (req :code:`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>`_. See `value_incrementer.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/20_value_incrementer.sol>`_.
@ -125,8 +125,8 @@ Its type is the type of the contract. Since any contract type basically inherits
:code:`address` type, :code:`this` is always convertible to :code:`address` and in this case contains :code:`address` type, :code:`this` is always convertible to :code:`address` and in this case contains
its own address. its own address.
What is the difference between a function marked constant and one that is not? What is the difference between a function marked :code:`constant` and one that is not?
============================================================================== ======================================================================================
:code:`constant` functions can perform some action and return a value, but cannot :code:`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 change state (this is not yet enforced by the compiler). In other words, a
@ -139,22 +139,22 @@ with :code:`c.someMethod.sendTransaction({from:eth.accounts[x], gas: 1000000});`
That is, because they can change state, they have to have a gas That is, because they can change state, they have to have a gas
payment sent along to get the work done. payment sent along to get the work done.
Get a contract to return its funds to you (not using selfdestruct(...)). Get a contract to return its funds to you (not using :code:`selfdestruct(...)`).
======================================================================== ================================================================================
This example demonstrates how to send funds from a contract to an address. 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>`_. 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? What is a :code:`mapping` and how do we use them?
========================================= =================================================
A mapping is very similar to a K->V hashmap. A mapping is very similar to a K->V hashmap.
If you have a state variable of type :code:`mapping (string -> uint) x;`, then you can If you have a state variable of type :code:`mapping (string -> uint) x;`, then you can
access the value by :code:`x["somekeystring"]`. access the value by :code:`x["somekeystring"]`.
How can I get the length of a mapping? How can I get the length of a :code:`mapping`?
====================================== ==============================================
Mappings are a rather low-level data structure. It does not store the keys 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, and it is not possible to know which or how many values are "set". Actually,
@ -166,14 +166,14 @@ In this sense, the attribute :code:`length` for a mapping does not really apply.
If you want to have a "sized mapping", you can use the iterable mapping If you want to have a "sized mapping", you can use the iterable mapping
(see below) or just a dynamically-sized array of structs. (see below) or just a dynamically-sized array of structs.
Are mappings iterable? Are :code:`mapping`'s iterable?
====================== ===============================
Mappings themselves are not iterable, but you can use a higher-level 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>`_. datastructure on top of it, for example the `iterable mapping <https://github.com/ethereum/dapp-bin/blob/master/library/iterable_mapping.sol>`_.
Can I put arrays inside of a mapping? How do I make a mapping of a mapping? Can I put arrays inside of a :code:`mapping`? How do I make a :code:`mapping` of a :code:`mapping`?
=========================================================================== ===================================================================================================
Mappings are already syntactically similar to arrays as they are, therefore it doesn't make much sense to store an array in them. Rather what you should do is create a mapping of a mapping. Mappings are already syntactically similar to arrays as they are, therefore it doesn't make much sense to store an array in them. Rather what you should do is create a mapping of a mapping.
@ -192,8 +192,8 @@ An example of this would be::
} }
} }
Can you return an array or a string from a solidity function call? Can you return an array or a :code:`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>`_. Yes. See `array_receiver_and_returner.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/60_array_receiver_and_returner.sol>`_.
@ -203,13 +203,13 @@ 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. Returning variably-sized data as part of an external transaction or call is fine.
How do you represent double/float in Solidity? How do you represent :code:`double`/:code:`float` in Solidity?
============================================== ==============================================================
This is not yet possible. This is not yet possible.
Is it possible to in-line initialize an array like so: string[] myarray = ["a", "b"]; Is it possible to in-line initialize an array like so: :code:`string[] myarray = ["a", "b"];`
======================================================================================= =============================================================================================
Yes. However it should be noted that this currently only works with statically sized memory arrays. You can even create an inline memory Yes. However it should be noted that this currently only works with statically sized memory arrays. You can even create an inline memory
array in the return statement. Pretty cool, huh? array in the return statement. Pretty cool, huh?
@ -223,8 +223,8 @@ Example::
} }
} }
What are events and why do we need them? What are :code:`event`'s and why do we need them?
======================================== =================================================
Let us suppose that you need a contract to alert the outside world when 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 something happens. The contract can fire an event, which can be listened to
@ -246,13 +246,13 @@ Internal function calls have the advantage that you can use
all Solidity types as parameters, but you have to stick to the all Solidity types as parameters, but you have to stick to the
simpler ABI types for external calls. simpler ABI types for external calls.
* external: all, only externally * :code:`external`: all, only externally
* public: all (this is the default), externally and internally * :code:`public`: all (this is the default), externally and internally
* internal: only this contract and contracts deriving from it, only internally * :code:`internal`: only this contract and contracts deriving from it, only internally
* private: only this contract, only internally * :code:`private`: only this contract, only internally
Do contract constructors have to be publicly visible? Do contract constructors have to be publicly visible?
@ -278,8 +278,8 @@ Is a constructor required?
No. If there is no constructor, a generic one without arguments and no actions will be used. No. If there is no constructor, a generic one without arguments and no actions will be used.
Are timestamps (now, block.timestamp) reliable? Are timestamps (:code:`now,` :code:`block.timestamp`) reliable?
=============================================== ===============================================================
This depends on what you mean by "reliable". This depends on what you mean by "reliable".
In general, they are supplied by miners and are therefore vulnerable. In general, they are supplied by miners and are therefore vulnerable.
@ -296,20 +296,20 @@ The value of :code:`now` will be identical to Y and X <= Y <= Z.
Never use :code:`now` or :code:`block.hash` as a source of randomness, unless you know Never use :code:`now` or :code:`block.hash` as a source of randomness, unless you know
what you are doing! what you are doing!
Can a contract function return a struct? Can a contract function return a :code:`struct`?
======================================== ================================================
Yes, but only in "internal" function calls. 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? If I return an :code:`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. 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 You have to do the mapping yourself for now, we might provide some help
later. later.
What is the deal with "function () { ... }" inside Solidity contracts? How can a function not have a name? What is the deal with :code:`function () { ... }` inside Solidity contracts? How can a function not have a name?
========================================================================================================== ================================================================================================================
This function is called "fallback function" and it This function is called "fallback function" and it
is called when someone just sent Ether to the contract without is called when someone just sent Ether to the contract without
@ -368,16 +368,16 @@ Examples::
C c = new C(); C c = new C();
} }
What is the "modifier" keyword? What is the :code:`modifier` keyword?
=============================== =====================================
Modifiers are a way to prepend or append code to a function in order 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. 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>`_. For examples, see the `features.sol <https://github.com/ethereum/dapp-bin/blob/master/library/features.sol>`_.
How do structs work? How do :code:`struct`'s 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>`_. See `struct_and_for_loop_tester.sol <https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/65_struct_and_for_loop_tester.sol>`_.
@ -402,8 +402,8 @@ Solidity is character set agnostic concerning strings in the source code, althou
utf-8 is recommended. Identifiers (variables, functions, ...) can only use utf-8 is recommended. Identifiers (variables, functions, ...) can only use
ASCII. ASCII.
What are some examples of basic string manipulation (substring, indexOf, charAt, etc)? What are some examples of basic string manipulation (:code:`substring`, :code:`indexOf`, :code:`charAt`, etc)?
====================================================================================== ==============================================================================================================
There are some string utility functions at `stringUtils.sol <https://github.com/ethereum/dapp-bin/blob/master/library/stringUtils.sol>`_ 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. which will be extended in the future.
@ -429,8 +429,8 @@ Can I concatenate two strings?
You have to do it manually for now. 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();)? Why is the low-level function :code:`.call()` less favorable than instantiating a contract with a variable (:code:`ContractB b;`) and executing its functions (:code:`b.doSomething();`)?
================================================================================================================================================================= =========================================================================================================================================================================================
If you use actual functions, the compiler will tell you if the types 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 your arguments do not match, if the function does not exist
@ -445,8 +445,8 @@ Is unused gas automatically refunded?
Yes and it is immediate, i.e. done as part of the transaction. 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? When returning a value of say :code:`uint` type, is it possible to return an :code:`undefined` or "null"-like value?
======================================================================================================== ====================================================================================================================
This is not possible, because all types use up the full value range. This is not possible, because all types use up the full value range.
@ -495,8 +495,8 @@ No, a function call from one contract to another does not create its own transac
you have to look in the overall transaction. This is also the reason why several 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. block explorer do not show Ether sent between contracts correctly.
What is the memory keyword? What does it do? What is the :code:`memory` keyword? What does it do?
============================================ ====================================================
The Ethereum Virtual Machine has three areas where it can store items. The Ethereum Virtual Machine has three areas where it can store items.
@ -670,8 +670,8 @@ Note2: Optimizing storage access can pull the gas costs down considerably, becau
currently do not work across loops and also have a problem with bounds checking. currently do not work across loops and also have a problem with bounds checking.
You might get much better results in the future, though. You might get much better results in the future, though.
What does p.recipient.call.value(p.amount)(p.data) do? What does :code:`p.recipient.call.value(p.amount)(p.data)` do?
====================================================== ==============================================================
Every external function call in Solidity can be modified in two ways: Every external function call in Solidity can be modified in two ways:
@ -687,8 +687,8 @@ gas and return your 20 Wei).
In the above example, the low-level function :code:`call` is used to invoke another In the above example, the low-level function :code:`call` is used to invoke another
contract with :code:`p.data` as payload and :code:`p.amount` Wei is sent with that call. contract with :code:`p.data` as payload and :code:`p.amount` Wei is sent with that call.
What happens to a struct's mapping when copying over a struct? What happens to a :code:`struct`'s mapping when copying over a :code:`struct`?
============================================================== ==============================================================================
This is a very interesting question. Suppose that we have a contract field set up like such:: This is a very interesting question. Suppose that we have a contract field set up like such::
@ -731,8 +731,8 @@ Can a contract function accept a two-dimensional array?
This is not yet implemented for external calls and dynamic arrays - This is not yet implemented for external calls and dynamic arrays -
you can only use one level of 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? What is the relationship between :code:`bytes32` and :code:`string`? Why is it that :code:`bytes32 somevar = "stringliteral";` works and what does the saved 32-byte hex value mean?
============================================================================================================================================================== ====================================================================================================================================================================================
The type :code:`bytes32` can hold 32 (raw) bytes. In the assignment :code:`bytes32 samevar = "stringliteral";`, The type :code:`bytes32` can hold 32 (raw) bytes. In the assignment :code:`bytes32 samevar = "stringliteral";`,
the string literal is interpreted in its raw byte form and if you inspect :code:`somevar` and the string literal is interpreted in its raw byte form and if you inspect :code:`somevar` and
@ -753,8 +753,8 @@ characters) and the second byte (not character) of the utf-8 encoded
string, respectively. string, respectively.
Can a contract pass an array (static size) or string or bytes (dynamic size) to another contract? Can a contract pass an array (static size) or string or :code:`bytes` (dynamic size) to another contract?
================================================================================================= =========================================================================================================
Sure. Take care that if you cross the memory / storage boundary, Sure. Take care that if you cross the memory / storage boundary,
independent copies will be created:: independent copies will be created::
@ -782,8 +782,8 @@ to create an independent copy of the storage value in memory
:code:`h(x)` successfully modifies :code:`x` because only a reference :code:`h(x)` successfully modifies :code:`x` because only a reference
and not a copy is passed. 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? Sometimes, when I try to change the length of an array with ex: :code:`arrayname.length = 7;` I get a compiler error :code:`Value must be an lvalue`. Why?
============================================================================================================================================== ==========================================================================================================================================================
You can resize a dynamic array in storage (i.e. an array declared at the You can resize a dynamic array in storage (i.e. an array declared at the
contract level) with :code:`arrayname.length = <some new length>;`. If you get the contract level) with :code:`arrayname.length = <some new length>;`. If you get the
@ -812,8 +812,8 @@ The reason for this is that :code:`T[5]` is always an array of 5 :code:`T`'s,
no matter whether :code:`T` itself is an array or not (this is not the no matter whether :code:`T` itself is an array or not (this is not the
case in C or Java). case in C or Java).
Is it possible to return an array of strings ( string[] ) from a Solidity function? Is it possible to return an array of strings (:code:`string[]`) from a Solidity function?
=================================================================================== ===========================================================================================
Not yet, as this requires two levels of dynamic arrays (:code:`string` is a dynamic array itself). Not yet, as this requires two levels of dynamic arrays (:code:`string` is a dynamic array itself).
@ -825,8 +825,8 @@ individual elements. If you want to return the complete array, you have to
manually write a function to do that. 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 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 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 The gas costs for this depend on the length of the code and it might be
@ -839,8 +839,8 @@ 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. 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()? How do I use :code:`.send()`?
===================== =============================
If you want to send 20 Ether from a contract to the address :code:`x`, you use :code:`x.send(20 ether);`. If you want to send 20 Ether from a contract to the address :code:`x`, you use :code:`x.send(20 ether);`.
Here, :code:`x` can be a plain address or a contract. If the contract already explicitly defines Here, :code:`x` can be a plain address or a contract. If the contract already explicitly defines