solidity/docs/contracts.rst

.. index:: ! contract

##########
Contracts
##########

Contracts in Solidity are similar to classes in object-oriented languages. They
contain 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.

A constructor is optional. Only one constructor is allowed, and this means
overloading is not supported.

From ``web3.js``, i.e. the JavaScript
API, this is done as follows::

    // Need to specify some source including contract name for the data param below
    var source = "contract CONTRACT_NAME { function CONTRACT_NAME(uint a, uint b) {} }";

    // 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(source);
    MyContract = web3.eth.contract(MyContract_.CONTRACT_NAME.info.abiDefinition);
    // deploy new contract
    var contractInstance = MyContract.new(
        10,
        11,
        {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.

::

    pragma solidity ^0.4.0;

    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) {
            // State variables are accessed via their name
            // and not via e.g. this.owner. This also applies
            // to functions and especially in the constructors,
            // you can only call them like that ("internall"),
            // because the contract itself does not exist yet.
            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 == address(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 (keccak256(newOwner) & 0xff) == (bytes20(tokenAddress) & 0xff);
        }
    }

.. index:: ! visibility, external, public, private, internal

.. _visibility-and-getters:

**********************
Visibility and Getters
**********************

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 getter
    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.

.. note::
    Everything that is inside a contract is visible to
    all external observers. Making something ``private``
    only prevents other contracts from accessing and modifying
    the information, but it will still be visible to the
    whole world outside of the blockchain.

The visibility specifier is given after the type for
state variables and between parameter list and
return parameter list for functions.

::

    pragma solidity ^0.4.0;

    contract C {
        function f(uint a) private returns (uint b) { return a + 1; }
        function setData(uint a) internal { data = a; }
        uint public data;
    }

In the following example, ``D``, can call ``c.getData()`` to retrieve the value of
``data`` in state storage, but is not able to call ``f``. Contract ``E`` is derived from
``C`` and, thus, can call ``compute``.

::

    // This will not compile

    pragma solidity ^0.4.0;

    contract C {
        uint private data;

        function f(uint a) private returns(uint b) { return a + 1; }
        function setData(uint a) { data = a; }
        function getData() public returns(uint) { return data; }
        function compute(uint a, uint b) internal returns (uint) { return a+b; }
    }


    contract D {
        function readData() {
            C c = new C();
            uint local = c.f(7); // error: member "f" is not visible
            c.setData(3);
            local = c.getData();
            local = c.compute(3, 5); // error: member "compute" is not visible
        }
    }


    contract E is C {
        function g() {
            C c = new C();
            uint val = compute(3, 5);  // acces to internal member (from derivated to parent contract)
        }
    }

.. index:: ! getter;function, ! function;getter
.. _getter_functions:

Getter Functions
================

The compiler automatically creates getter functions for
all **public** state variables. For the contract given below, the compiler will
generate 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.

::

    pragma solidity ^0.4.0;

    contract C {
        uint public data = 42;
    }


    contract Caller {
        C c = new C();
        function f() {
            uint local = c.data();
        }
    }

The getter functions have external visibility. If the
symbol is accessed internally (i.e. without ``this.``),
it is evaluated as a state variable.  If it is accessed externally
(i.e. with ``this.``), it is evaluated as a function.

::

    pragma solidity ^0.4.0;

    contract C {
        uint public data;
        function x() {
            data = 3; // internal access
            uint val = this.data(); // external access
        }
    }

The next example is a bit more complex:

::

    pragma solidity ^0.4.0;

    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

.. _modifiers:

******************
Function Modifiers
******************

Modifiers can be used to easily change the behaviour of functions.  For example,
they can automatically check a condition prior to executing the function. Modifiers are
inheritable properties of contracts and may be overridden by derived contracts.

::

    pragma solidity ^0.4.11;

    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 {
            require(msg.sender == owner);
            _;
        }
    }


    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; }

        // It is important to also provide the
        // "payable" keyword here, otherwise the function will
        // automatically reject all Ether sent to it.
        function register() payable costs(price) {
            registeredAddresses[msg.sender] = true;
        }

        function changePrice(uint _price) onlyOwner {
            price = _price;
        }
    }

    contract Mutex {
        bool locked;
        modifier noReentrancy() {
            require(!locked);
            locked = true;
            _;
            locked = false;
        }

        /// This function is protected by a mutex, which means that
        /// reentrant calls from within msg.sender.call cannot call f again.
        /// The `return 7` statement assigns 7 to the return value but still
        /// executes the statement `locked = false` in the modifier.
        function f() noReentrancy returns (uint) {
            require(msg.sender.call());
            return 7;
        }
    }

Multiple modifiers are applied to a function by specifying them in a
whitespace-separated list and are evaluated in the order presented.

.. warning::
    In an earlier version of Solidity, ``return`` statements in functions
    having modifiers behaved differently.

Explicit returns from a modifier or function body only leave the current
modifier or function body. Return variables are assigned and
control flow 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

************************
Constant State Variables
************************

State variables can be declared as ``constant``. In this case, they have to be
assigned from an expression which is a constant at compile time. Any expression
that accesses storage, blockchain data (e.g. ``now``, ``this.balance`` or
``block.number``) or
execution data (``msg.gas``) or make calls to external contracts are disallowed. Expressions
that might have a side-effect on memory allocation are allowed, but those that
might have a side-effect on other memory objects are not. The built-in functions
``keccak256``, ``sha256``, ``ripemd160``, ``ecrecover``, ``addmod`` and ``mulmod``
are allowed (even though they do call external contracts).

The reason behind allowing side-effects on the memory allocator is that it
should be possible to construct complex objects like e.g. lookup-tables.
This feature is not yet fully usable.

The compiler does not reserve a storage slot for these variables, and every occurrence is
replaced by the respective constant expression (which might be computed to a single value by the optimizer).

Not all types for constants are implemented at this time. The only supported types are
value types and strings.

::

    pragma solidity ^0.4.0;

    contract C {
        uint constant x = 32**22 + 8;
        string constant text = "abc";
        bytes32 constant myHash = keccak256("abc");
    }


.. _constant-functions:

******************
Constant Functions
******************

Functions can be declared constant in which case they promise not to modify the state.

::

    pragma solidity ^0.4.0;

    contract C {
        function f(uint a, uint b) constant returns (uint) {
            return a * (b + 42);
        }
    }

.. note::
  Getter methods are marked constant.

.. warning::
  The compiler does not enforce yet that a constant method is not modifying state.

.. index:: ! fallback function, function;fallback

.. _fallback-function:

*****************
Fallback Function
*****************

A contract can have exactly one unnamed function. This function cannot have
arguments and cannot return anything.
It is executed on a call to the contract if none of the other
functions match the given function identifier (or if no data was supplied at
all).

Furthermore, this function is executed whenever the contract receives plain
Ether (without data).  In such a context, there is usually very little gas available to
the function call (to be precise, 2300 gas), so it is important to make fallback functions as cheap as
possible.

In particular, the following operations will consume more gas than the stipend provided to a fallback function:

- Writing to storage
- Creating a contract
- Calling an external function which consumes a large amount of gas
- Sending Ether

Please ensure you test your fallback function thoroughly to ensure the execution cost is less than 2300 gas before deploying a contract.

.. warning::
    Contracts that receive Ether directly (without a function call, i.e. using ``send`` or ``transfer``)
    but do not define a fallback function
    throw an exception, sending back the Ether (this was different
    before Solidity v0.4.0). So if you want your contract to receive Ether,
    you have to implement a fallback function.

::

    pragma solidity ^0.4.0;

    contract Test {
        // This function is called for all messages sent to
        // this contract (there is no other function).
        // Sending Ether to this contract will cause an exception,
        // because the fallback function does not have the "payable"
        // modifier.
        function() { x = 1; }
        uint x;
    }


    // This contract keeps all Ether sent to it with no way
    // to get it back.
    contract Sink {
        function() payable { }
    }


    contract Caller {
        function callTest(Test test) {
            test.call(0xabcdef01); // hash does not exist
            // results in test.x becoming == 1.

            // The following will not compile, but even
            // if someone sends ether to that contract,
            // the transaction will fail and reject the
            // Ether.
            //test.send(2 ether);
        }
    }

.. index:: ! event

.. _events:

******
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 them).

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 that 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
Keccak-256 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.

.. note::
    Indexed arguments will not be stored themselves.  You can only
    search for the values, but it is impossible to retrieve the
    values themselves.

::

    pragma solidity ^0.4.0;

    contract ClientReceipt {
        event Deposit(
            address indexed _from,
            bytes32 indexed _id,
            uint _value
        );

        function deposit(bytes32 _id) payable {
            // 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
``keccak256("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 name is explicitly given.

When a contract inherits from multiple contracts, only a single
contract is created on the blockchain, and the code from all the base contracts
is copied into the created 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.

::

    pragma solidity ^0.4.0;

    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 by another function with the same name and
        // the same number/types of inputs.  If the overriding function has different
        // types of output parameters, that causes an error.
        // 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::

    pragma solidity ^0.4.0;

    contract owned {
        function owned() { owner = msg.sender; }
        address owner;
    }

    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``::

    pragma solidity ^0.4.0;

    contract owned {
        function owned() { owner = msg.sender; }
        address owner;
    }

    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.  Rather, it 
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 in two ways::

    pragma solidity ^0.4.0;

    contract Base {
        uint x;
        function Base(uint _x) { x = _x; }
    }


    contract Derived is Base(7) {
        function Derived(uint _y) Base(_y * _y) {
        }
    }

One way is directly in the inheritance list (``is Base(7)``).  The other is 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 is 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".

::

    // This will not compile

    pragma solidity ^0.4.0;

    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".

Inheriting Different Kinds of Members of the Same Name
======================================================

When the inheritance results in a contract with a function and a modifier of the same name, it is considered as an error.
This error is produced also by an event and a modifier of the same name, and a function and an event of the same name.
As an exception, a state variable getter can override a public function.

.. 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 ``;``)::

    pragma solidity ^0.4.0;

    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::

    pragma solidity ^0.4.0;

    contract Feline {
        function utterance() returns (bytes32);
    }

    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:: ! contract;interface, ! interface contract

**********
Interfaces
**********

Interfaces are similar to abstract contracts, but they cannot have any functions implemented. There are further restrictions:

#. Cannot inherit other contracts or interfaces.
#. Cannot define constructor.
#. Cannot define variables.
#. Cannot define structs.
#. Cannot define enums.

Some of these restrictions might be lifted in the future.

Interfaces are basically limited to what the Contract ABI can represent, and the conversion between the ABI and
an Interface should be possible without any information loss.

Interfaces are denoted by their own keyword:

::

    interface Token {
        function transfer(address recipient, uint amount);
    }

Contracts can inherit interfaces as they would inherit other contracts.

.. index:: ! library, callcode, delegatecall

.. _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 ``DELEGATECALL``
(``CALLCODE`` until Homestead)
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 no way to name them, otherwise).

Libraries can be seen as implicit base contracts of the contracts that use them.
They will not be explicitly visible in the inheritance hierarchy, but calls
to library functions look just like calls to functions of explicit base
contracts (``L.f()`` if ``L`` is the name of the library). Furthermore,
``internal`` functions of libraries are visible in all contracts, just as
if the library were a base contract. Of course, calls to internal functions
use the internal calling convention, which means that all internal types
can be passed and memory types will be passed by reference and not copied.
To realize this in the EVM, code of internal library functions
and all functions called from therein will be pulled into the calling
contract, and a regular ``JUMP`` call will be used instead of a ``DELEGATECALL``.

.. 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).

::

    pragma solidity ^0.4.11;

    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.
            require(Set.insert(knownValues, value));
        }
        // 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, and they 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 (``DELEGATECALL``) to an external
contract/library. If you use libraries, take care that an
actual external function call is performed.
``msg.sender``, ``msg.value`` and ``this`` will retain their values
in this call, though (prior to Homestead, because of the use of `CALLCODE`, ``msg.sender`` and
``msg.value`` changed, though).

The following example shows how to use memory types and
internal functions in libraries in order to implement
custom types without the overhead of external function calls:

::

    pragma solidity ^0.4.0;

    library BigInt {
        struct bigint {
            uint[] limbs;
        }

        function fromUint(uint x) internal returns (bigint r) {
            r.limbs = new uint[](1);
            r.limbs[0] = x;
        }

        function add(bigint _a, bigint _b) internal returns (bigint r) {
            r.limbs = new uint[](max(_a.limbs.length, _b.limbs.length));
            uint carry = 0;
            for (uint i = 0; i < r.limbs.length; ++i) {
                uint a = limb(_a, i);
                uint b = limb(_b, i);
                r.limbs[i] = a + b + carry;
                if (a + b < a || (a + b == uint(-1) && carry > 0))
                    carry = 1;
                else
                    carry = 0;
            }
            if (carry > 0) {
                // too bad, we have to add a limb
                uint[] memory newLimbs = new uint[](r.limbs.length + 1);
                for (i = 0; i < r.limbs.length; ++i)
                    newLimbs[i] = r.limbs[i];
                newLimbs[i] = carry;
                r.limbs = newLimbs;
            }
        }

        function limb(bigint _a, uint _limb) internal returns (uint) {
            return _limb < _a.limbs.length ? _a.limbs[_limb] : 0;
        }

        function max(uint a, uint b) private returns (uint) {
            return a > b ? a : b;
        }
    }


    contract C {
        using BigInt for BigInt.bigint;

        function f() {
            var x = BigInt.fromUint(7);
            var y = BigInt.fromUint(uint(-1));
            var z = x.add(y);
        }
    }

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 :ref:`commandline-compiler` for 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
- Cannot receive Ether

(These might be lifted at a later point.)

.. 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::

    pragma solidity ^0.4.11;

    // 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)
            require(knownValues.insert(value));
        }
    }

It is also possible to extend elementary types in that way::

    pragma solidity ^0.4.0;

    library Search {
        function indexOf(uint[] storage self, uint value) returns (uint) {
            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.indexOf(_old);
            if (index == uint(-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.