solidity/docs/frequently-asked-questions.rst

###########################
Frequently Asked Questions
###########################

This list was originally compiled by `fivedogit <mailto:fivedogit@gmail.com>`_.


***************
Basic Questions
***************

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.

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, it is preferable to **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 `Remix <https://remix.ethereum.org/>`_), 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.

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 function **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.

Is it possible to in-line initialize an array like so: ``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
array in the return statement. Pretty cool, huh?

Example::

    pragma solidity ^0.4.16;

    contract C {
        function f() public pure returns (uint8[5] memory) {
            string[4] memory adaArr = ["This", "is", "an", "array"];
            return ([1, 2, 3, 4, 5]);
        }
    }

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.

Can state variables be initialized in-line?
===========================================

Yes, this is possible for all types (even for structs). However, for arrays it
should be noted that you must declare them as static memory arrays.

Examples::

    pragma solidity ^0.4.0;

    contract C {
        struct S {
            uint a;
            uint b;
        }

        S public x = S(1, 2);
        string name = "Ada";
        string[4] adaArr = ["This", "is", "an", "array"];
    }

    contract D {
        C c = new C();
    }

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. Such as the following example:

``for (uint i = 0; i < a.length; i ++) { a[i] = i; }``

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 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. In addition, Arachnid has written `solidity-stringutils <https://github.com/Arachnid/solidity-stringutils>`_.

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::

    pragma solidity ^0.4.0;

    contract C {
        string s;

        function append(byte c) public {
            bytes(s).push(c);
        }

        function set(uint i, byte c) public {
            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::

    pragma solidity >0.4.23 <0.5.0;

    contract C {
        uint[] counters;

        function getCounter(uint index)
            public
            view
            returns (uint counter, bool error) {
                if (index >= counters.length)
                    return (0, true);
                else
                    return (counters[index], false);
        }

        function checkCounter(uint index) public view {
            (uint counter, bool 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.
You can only send ether along to a function that has the ``payable`` modifier,
otherwise an exception is thrown.

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 every time 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 in memory by default
* local variables of mapping type reference storage by default
* local variables of value type (i.e. neither array, nor struct nor mapping) are stored in the stack

For local variables of struct or array type the storage location has to be stated explicitly.

Example::

    pragma solidity ^0.4.0;

    contract C {
        uint[] data1;
        uint[] data2;

        function appendOne() public {
            append(data1);
        }

        function appendTwo() public {
            append(data2);
        }

        function append(uint[] storage d) internal {
            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``.

.. warning::
    Prior to version 0.5.0, a common mistake was to declare a local variable and assume that it will
    be created in memory, although it will be created in storage. Using such a variable without initializing
    could lead to unexpected behavior. Starting from 0.5.0, however, the storage location for local variables
    has to be specified explicitly and local storage variables have to be initialized, which should prevent
    these kinds of mistakes.

******************
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>`_.

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 happens to a ``struct``'s mapping when copying over a ``struct``?
======================================================================

This is a very interesting question. Suppose that we have a contract field set up like such::

    struct User {
        mapping(string => string) comments;
    }

    function somefunction public {
       User user1;
       user1.comments["Hello"] = "World";
       User user2 = user1;
    }

In this case, the mapping of the struct being copied over into ``user2`` is ignored as there is no "list of mapped keys".
Therefore it is not possible to find out which values should be copied over.

How do I initialize a contract with only a specific amount of wei?
==================================================================

Currently the approach is a little ugly, but there is little that can be done to improve it.
In the case of a ``contract A`` calling a new instance of ``contract B``, parentheses have to be used around
``new B`` because ``B.value`` would refer to a member of ``B`` called ``value``.
You will need to make sure that you have both contracts aware of each other's presence and that ``contract B`` has a ``payable`` constructor.
In this example::

    pragma solidity >0.4.24;

    contract B {
        constructor() public payable {}
    }

    contract A {
        address child;

        function test() public {
            child = (new B).value(10)(); //construct a new B with 10 wei
        }
    }

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::

    pragma solidity ^0.4.16;

    contract C {
        uint[20] x;

        function f() public {
            g(x);
            h(x);
        }

        function g(uint[20] memory y) internal pure {
            y[2] = 3;
        }

        function h(uint[20] storage y) internal {
            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.
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.

::

    // This will not compile

    pragma solidity ^0.4.18;

    contract C {
        int8[] dynamicStorageArray;
        int8[5] fixedStorageArray;

        function f() {
            int8[] memory memArr;        // Case 1
            memArr.length++;             // illegal

            int8[5] storage storageArr = fixedStorageArray;   // Case 2
            storageArr.length++;                             // illegal

            int8[] storage storageArr2 = dynamicStorageArray;
            storageArr2.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 :ref:`getter function<getter-functions>`  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.


What does the following strange check do in the Custom Token contract?
======================================================================

::

    require((balanceOf[_to] + _value) >= balanceOf[_to]);

Integers in Solidity (and most other machine-related programming languages) are restricted to a certain range.
For ``uint256``, this is ``0`` up to ``2**256 - 1``. If the result of some operation on those numbers
does not fit inside this range, it is truncated. These truncations can have
`serious consequences <https://en.bitcoin.it/wiki/Value_overflow_incident>`_, so code like the one
above is necessary to avoid certain attacks.


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>`_.