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.

Creating Contracts¶

Contracts can be created “from outside” via Ethereum transactions or from within Solidity contracts.

IDEs, such as Remix, make the creation process seamless using UI elements.

Creating contracts programatically on Ethereum is best done via using the JavaScript API web3.js. As of today it has a method called web3.eth.Contract to facilitate contract creation.

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.

Internally, constructor arguments are passed ABI encoded 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 ^0.4.16;

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) public {
        // 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 ("internally"),
        // 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) public {
        // 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) public {
        // 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)
       public
       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)  public {
        // Again, the external type of `tokenAddress` is
        // simply `address`.
        tokenAddress.changeName(name);
    }

    function isTokenTransferOK(address currentOwner, address newOwner)
        public
        view
        returns (bool ok)
    {
        // Check some arbitrary condition.
        address tokenAddress = msg.sender;
        return (keccak256(newOwner) & 0xff) == (bytes20(tokenAddress) & 0xff);
    }
}

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 ^0.4.16;

contract C {
    function f(uint a) private pure 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) public { 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() public {
        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() public {
        C c = new C();
        uint val = compute(3, 5); // access to internal member (from derived to parent contract)
    }
}

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() public {
        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() public {
        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) public 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.

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() public { 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() public 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) public { 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() public payable costs(price) {
        registeredAddresses[msg.sender] = true;
    }

    function changePrice(uint _price) public 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() public 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).

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");
}

Functions¶

View Functions¶

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

The following statements are considered modifying the state:

Writing to state variables.
Emitting events.
Creating other contracts.
Using selfdestruct.
Sending Ether via calls.
Calling any function not marked view or pure.
Using low-level calls.
Using inline assembly that contains certain opcodes.

pragma solidity ^0.4.16;

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

Note

constant is an alias to view.

Note

Getter methods are marked view.

Warning

The compiler does not enforce yet that a view method is not modifying state.

Pure Functions¶

Functions can be declared pure in which case they promise not to read from or modify the state.

In addition to the list of state modifying statements explained above, the following are considered reading from the state:

Reading from state variables.
Accessing this.balance or <address>.balance.
Accessing any of the members of block, tx, msg (with the exception of msg.sig and msg.data).
Calling any function not marked pure.
Using inline assembly that contains certain opcodes.

pragma solidity ^0.4.16;

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

Warning

The compiler does not enforce yet that a pure method is not reading from the state.

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). Additionally, in order to receive Ether, the fallback function must be marked payable. If no such function exists, the contract cannot receive Ether through regular transactions.

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. Note that the gas required by a transaction (as opposed to an internal call) that invokes the fallback function is much higher, because each transaction charges an additional amount of 21000 gas or more for things like signature checking.

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.

Note

Even though the fallback function cannot have arguments, one can still use msg.data to retrieve any payload supplied with the call.

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.

Warning

A contract without a payable fallback function can receive Ether as a recipient of a coinbase transaction (aka miner block reward) or as a destination of a selfdestruct.

A contract cannot react to such Ether transfers and thus also cannot reject them. This is a design choice of the EVM and Solidity cannot work around it.

It also means that this.balance can be higher than the sum of some manual accounting implemented in a contract (i.e. having a counter updated in the 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() public { x = 1; }
    uint x;
}


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

contract Caller {
    function callTest(Test test) public {
        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);
    }
}

Function Overloading¶

A Contract can have multiple functions of the same name but with different arguments. This also applies to inherited functions. The following example shows overloading of the f function in the scope of contract A.

pragma solidity ^0.4.16;

contract A {
    function f(uint _in) public pure returns (uint out) {
        out = 1;
    }

    function f(uint _in, bytes32 _key) public pure returns (uint out) {
        out = 2;
    }
}

Overloaded functions are also present in the external interface. It is an error if two externally visible functions differ by their Solidity types but not by their external types.

// This will not compile
pragma solidity ^0.4.16;

contract A {
    function f(B _in) public pure returns (B out) {
        out = _in;
    }

    function f(address _in) public pure returns (address out) {
        out = _in;
    }
}

contract B {
}

Both f function overloads above end up accepting the address type for the ABI although they are considered different inside Solidity.

Overload resolution and Argument matching¶

Overloaded functions are selected by matching the function declarations in the current scope to the arguments supplied in the function call. Functions are selected as overload candidates if all arguments can be implicitly converted to the expected types. If there is not exactly one candidate, resolution fails.

Note

Return parameters are not taken into account for overload resolution.

pragma solidity ^0.4.16;

contract A {
    function f(uint8 _in) public pure returns (uint8 out) {
        out = _in;
    }

    function f(uint256 _in) public pure returns (uint256 out) {
        out = _in;
    }
}

Calling f(50) would create a type error since 250 can be implicitly converted both to uint8 and uint256 types. On another hand f(256) would resolve to f(uint256) overload as 256 cannot be implicitly converted to uint8.

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) public 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("0x1234...ab67" /* 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);
});

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

pragma solidity ^0.4.10;

contract C {
    function f() public payable {
        bytes32 _id = 0x420042;
        log3(
            bytes32(msg.value),
            bytes32(0x50cb9fe53daa9737b786ab3646f04d0150dc50ef4e75f59509d83667ad5adb20),
            bytes32(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¶

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, especially concerning multiple inheritance.

Details are given in the following example.

pragma solidity ^0.4.16;

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) public returns (address adr);
}

contract NameReg {
    function register(bytes32 name) public;
    function unregister() public;
 }

// 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() public {
        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) public {
      if (msg.sender == owner) info = newInfo;
   }

   function get() public view 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() public { owner = msg.sender; }
    address owner;
}

contract mortal is owned {
    function kill() public {
        if (msg.sender == owner) selfdestruct(owner);
    }
}

contract Base1 is mortal {
    function kill() public { /* do cleanup 1 */ mortal.kill(); }
}

contract Base2 is mortal {
    function kill() public { /* 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() public { owner = msg.sender; }
    address owner;
}

contract mortal is owned {
    function kill() public {
        if (msg.sender == owner) selfdestruct(owner);
    }
}

contract Base1 is mortal {
    function kill() public { /* do cleanup 1 */ super.kill(); }
}


contract Base2 is mortal {
    function kill() public { /* 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.

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) public { x = _x; }
}

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

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.

Multiple Inheritance and Linearization¶

Languages that allow multiple inheritance have to deal with several problems. One is the Diamond Problem. Solidity follows the path of Python and uses “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.

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() public 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() public returns (bytes32);
}

contract Cat is Feline {
    function utterance() public 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.

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:

pragma solidity ^0.4.11;

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

Contracts can inherit interfaces as they would inherit other contracts.

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.

The following example illustrates how to use libraries (but be sure to check out using for for a more advanced example to implement a set).

pragma solidity ^0.4.16;

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)
      public
      returns (bool)
  {
      if (self.flags[value])
          return false; // already there
      self.flags[value] = true;
      return true;
  }

  function remove(Data storage self, uint value)
      public
      returns (bool)
  {
      if (!self.flags[value])
          return false; // not there
      self.flags[value] = false;
      return true;
  }

  function contains(Data storage self, uint value)
      public
      view
      returns (bool)
  {
      return self.flags[value];
  }
}

contract C {
    Set.Data knownValues;

    function register(uint value) public {
        // 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 ^0.4.16;

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

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

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

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

contract C {
    using BigInt for BigInt.bigint;

    function f() public pure {
        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 Using the 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.)

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 Libraries in this way:

pragma solidity ^0.4.16;

// 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)
      public
      returns (bool)
  {
      if (self.flags[value])
        return false; // already there
      self.flags[value] = true;
      return true;
  }

  function remove(Data storage self, uint value)
      public
      returns (bool)
  {
      if (!self.flags[value])
          return false; // not there
      self.flags[value] = false;
      return true;
  }

  function contains(Data storage self, uint value)
      public
      view
      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) public {
        // 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 ^0.4.16;

library Search {
    function indexOf(uint[] storage self, uint value)
        public
        view
        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) public {
        data.push(value);
    }

    function replace(uint _old, uint _new) public {
        // 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.