# 合約 Solidity中合約有點類似面向對象語言中的類。合約中有用于數據持久化的`狀態變量(state variables)`，和可以操作他們的函數。調用另一個合約實例的函數時，會執行一個EVM函數調用，這個操作會切換執行時的上下文，這樣，前一個合約的`狀態變量(state variables)`就不能訪問了。 ## 創建合約 合約可以通過Solidity，或不通過Solidity創建。當合約創建時，一個和合約同名的函數(構造器函數)會調用一次，用于初始化。 構造器函數是可選的。僅能有一個構造器，所以不支持重載。 如果不通過Solidity，我們可以通過`web3.js`，使用JavaScript的API來完成合約創建: ``` // Need to specify some source including contract name for the data param below var source = "contract CONTRACT_NAME { function CONTRACT_NAME(unit 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} ); ``` 具體內部實現里，構造器的參數是緊跟在合約代碼的后面，但如果你使用`web3.js`，可以不用關心這樣的細節。 如果一個合約要創建另一個合約，它必須要知道源碼。這意味著循環創建依賴是不可能的。 ``` 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); } } ```