合约

Solidity里的合约是面向对象语言里的类。它们持久存放在状态变量和函数中,(在里面)可以修改这些变量。在不同的合约(实例)中调用一个函数(的过程),(实际上)是在EVM(Ether虚拟机)中完成一次调用,并且完成(一次)上下文切换,(此时)状态变量是不可访问的。

创建合约

  合约可以从“外部”创建,也可以由Solidity合约创立。在创建合约时,它的构造函数(函具有与合约名称同名的函数)将被执行。

web3.js,即 JavaScript API, 是这样做的:

// 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(abiArray);// deploy new contractvar contractInstance = MyContract.new(

  10,

  {from: myAccount, gas: 1000000}

);

// The json abi array generated by the compiler  由编译器生成的json abi 数组

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(abiArray);

// deploy new contract  部署一个新合约

var contractInstance = MyContract.new(

  10,

  {from: myAccount, gas: 1000000}

);

在内部,在合约的代码后要接着有构造函数的参数,但如果你使用web3.js,就不必关心这个。

如果是一个合约要创立另外一个合约,被创立的合约的源码(二进制代码)要能被创立者知晓。这意味着:循环创建依赖就成为不可能的事情。

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) {

        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 == 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 (sha3(newOwner) & 0xff) == (bytes20(tokenAddress) & 0xff);

    }}

contract OwnedToken {

    // TokenCreator is a contract type that is defined below.  TokenCreator是在下面定义的合约类型

    // It is fine to reference it as long as it is not used  若它本身不用于创建新的合约的话,它就是一个引用

    // to create a new contract.

    TokenCreator creator;

    address owner;

    bytes32 name;

    // This is the constructor which registers the 这个是一个登记创立者和分配名称的结构函数

    // creator and the assigned name.

    function OwnedToken(bytes32 _name) {

        owner = msg.sender;

        // We do an explicit type conversion from `address` 我们做一次由`address`到`TokenCreator` 的显示类型转换,,确保调用合约的类型是 TokenCreator, (因为没有真正的方法来检测这一点)

        // 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 --  仅仅是创立者可以改变名称--

        // the comparison is possible since contracts  因为合约是隐式转换到地址上,这种比较是可能的

        // are implicitly convertible to addresses.

        if (msg.sender == creator) name = newName;

    }

    function transfer(address newOwner) {

        // Only the current owner can transfer the token.  仅仅是 仅仅是当前(合约)所有者可以转移 token

        if (msg.sender != owner) return;

        // We also want to ask the creator if the transfer  我们可以询问(合约)创立者"转移是否成功"

        // is fine. Note that this calls a function of the  注意下面定义的合约的函数调用

        // contract defined below. If the call fails (e.g.     如果函数调用失败,(如gas用完了等原因)

        // due to out-of-gas), the execution here stops  程序的执行将立刻停止

        // 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.  创立一个新的Token合约,并且返回它的地址

        // From the JavaScript side, the return type is simply  从 JavaScript观点看,返回的地址类型是"address"

        // "address", as this is the closest type available in   这个是和ABI最接近的类型

        // the ABI.

        return new OwnedToken(name);

    }

    function changeName(OwnedToken tokenAddress, bytes32 name) {

        // Again, the external type of "tokenAddress" is     "tokenAddress" 的外部类型也是 简单的"address".

        // simply "address".

        tokenAddress.changeName(name);

    }

    function isTokenTransferOK(

        address currentOwner,

        address newOwner

    ) returns (bool ok) {

        // Check some arbitrary condition. 检查各种条件

        address tokenAddress = msg.sender;

        return (sha3(newOwner) & 0xff) == (bytes20(tokenAddress) & 0xff);

    }

}

可见性和访问限制符

因为Solidity可以理解两种函数调用(“内部调用”,不创建一个真实的EVM调用(也称为“消息调用”);“外部的调用”-要创建一个真实的EMV调用), 有四种的函数和状态变量的可见性。

函数可以被定义为external, public, internal or private,缺省是 public。对状态变量而言, external是不可能的,默认是 internal。

external: 外部函数是合约接口的一部分,这意味着它们可以从其他合约调用, 也可以通过事务调用。外部函数f不能被内部调用(即 f()不执行,但this.f()执行)。外部函数,当他们接收大数组时,更有效。

public:公共函数是合约接口的一部分,可以通过内部调用或通过消息调用。对公共状态变量而言,会有的自动访问限制符的函数生成(见下文)。

internal:这些函数和状态变量只能内部访问(即在当前合约或由它派生的合约),而不使用(关键字)this 。

private:私有函数和状态变量仅仅在定义该合约中可见, 在派生的合约中不可见。

请注意

在外部观察者中,合约的内部的各项均可见。用 private 仅仅防止其他合约来访问和修改(该合约中)信息, 但它对blockchain之外的整个世界仍然可见。

可见性说明符是放在在状态变量的类型之后,(也可以放在)参数列表和函数返回的参数列表之间。

contract c {

    function f(uint a) private returns (uint b) { return a + 1; }

    function setData(uint a) internal { data = a; }

    uint public data;

}

其他合约可以调用c.data()来检索状态存储中data的值,但不能访问(函数)f。由c派生的合约可以访问(合约中)setData(函数),以便改变data的值(仅仅在它们自己的范围里)。

访问限制符函数

编译器会自动创建所有公共状态变量的访问限制符功能。下文中的合约中有一个称作data的函数,它不带任何参数的,它返回一个uint类型, 状态变量的值是data。可以在声明里进行状态变量的初始化。

访问限制符函数有外部可见性。如果标识符是内部可访问(即没有this),则它是一个状态变量,如果外部可访问的(即 有this),则它是一个函数。

contract test {

    uint public data = 42;}

下面的例子复杂些:

contract complex {

    struct Data { uint a; bytes3 b; mapping(uint => uint) map; }

    mapping(uint => mapping(bool => Data[])) public data;}

它生成了如下形式的函数:

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;}

注意 结构体的映射省略了,因为没有好的方法来提供映射的键值。

函数修饰符

修饰符可以用来轻松改变函数的行为, 例如,在执行的函数之前自动检查条件。他们是可继承合约的属性,也可被派生的合约重写。

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 { if (msg.sender != owner) throw; _ }}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; }

    function register() costs(price) {

        registeredAddresses[msg.sender] = true;

    }

    function changePrice(uint _price) onlyowner {

        price = _price;

    }}

contract owned {

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

    address owner;

    // This contract only defines a modifier but does not use 这个合约仅仅定义了修饰符,但没有使用它

    // it - it will be used in derived contracts. 在派生的合约里使用

    // The function body is inserted where the special symbol  ,函数体插入到特殊的标识 "_"定义的地方 

    // "_" in the definition of a modifier appears.

    // This means that if the owner calls this function, the 这意味着若它自己调用此函数,则函数将被执行

    // function is executed and otherwise, an exception is 否则,一个异常将抛出

    // thrown.

    modifier onlyowner { if (msg.sender != owner) throw; _ }

}

contract mortal is owned {

    // This contract inherits the "onlyowner"-modifier from   该合约是从"owned" 继承的"onlyowner"修饰符,

    // "owned" and applies it to the "close"-function, which    并且应用到"close"函数, 如果他们存储owner

    // 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:  修饰符可以接收参数

    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; }

    function register() costs(price) {

        registeredAddresses[msg.sender] = true;

    }

    function changePrice(uint _price) onlyowner {

        price = _price;

    }

}

多个修饰符可以被应用到一个函数中(用空格隔开),并顺序地进行计算。当离开整个函数时,显式返回一个修饰词或函数体, 同时在“_”之后紧接着的修饰符,直到函数尾部的控制流,或者是修饰体将继续执行。任意表达式允许修改参数,在修饰符中,所有函数的标识符是可见的。在此函数由修饰符引入的标识符是不可见的(虽然他们可以通过重写,改变他们的值)。

常量

状态变量可以声明为常量(在数组和结构体类型上仍然不可以这样做,映射类型也不可以)。

contract C {

    uint constant x = 32*\*22 + 8;

    string constant text = "abc";

}

编译器不保留这些变量存储块, 每到执行到这个语句时,常量值又被替换一次。

表达式的值只能包含整数算术运算。

回退函数

一个合约可以有一个匿名函数。若没有其他函数和给定的函数标识符一致的话,该函数将没有参数,将执行一个合约的调用(如果没有提供数据)。

此外,当合约接收一个普通的Ether时,函数将被执行(没有数据)。在这样一个情况下,几乎没有gas用于函数调用,所以调用回退函数是非常廉价的,这点非常重要。

contract Test {

    function() { x = 1; }

    uint x;}

//  This contract rejects any Ether sent to it. It is good

// practise to  include such a function for every contract

// in order not to loose  Ether.

contract Rejector {

    function() { throw; }

}

contract Caller {

  function callTest(address testAddress) {

      Test(testAddress).call(0xabcdef01); // hash does not exist

      // results in Test(testAddress).x becoming == 1.

      Rejector r = Rejector(0x123);

      r.send(2 ether);

      // results in r.balance == 0

  }

}

contract Test {

    function() { x = 1; }

    uint x;}

//  This contract rejects any Ether sent to it. It is good   这个合约拒绝任何发给它的Ether.

// practise to  include such a function for every contract  为了严管Ether,在每个合约里包含一个这样的函数,是非常好的做法

// in order not to loose  Ether.

contract Rejector {

    function() { throw; }

}

contract Caller {

  function callTest(address testAddress) {

      Test(testAddress).call(0xabcdef01); // hash does not exist hash值不存在

      // results in Test(testAddress).x becoming == 1.  Test(testAddress).x的结果  becoming == 1

      Rejector r = Rejector(0x123);

      r.send(2 ether);

      // results in r.balance == 0     结果里r.balance == 0  

  }

}

事件

事件允许EMV写日志功能的方便使用, 进而在dapp的用户接口中用JavaScript顺序调用,从而监听这些事件。

事件是合约中可继承的成员。当他们调用时,会在导致一些参数在事务日志上的存储--在blockchain上的一种特殊的数据结构。这些日志和合约的地址相关联, 将被纳入blockchain中,存储在block里以便访问( 在Frontier Homestead里是永久存储,但在Serenity里有些变化)。在合约内部,日志和事件数据是不可访问的(从创建该日志的合约里)。

SPV日志证明是可行的, 如果一个外部实体提供一个这样的证明给合约, 它可以检查blockchain内实际存在的日志(但要注意这样一个事实,最终要提供block的headers, 因为合约只能看到最近的256块hash值)。

最多有三个参数可以接收属性索引,它将对各自的参数进行检索: 可以对用户界面中的索引参数的特定值进行过滤。

如果数组(包括string和 bytes)被用作索引参数, 就会以sha3-hash形式存储,而不是topic。

除了用anonymous声明事件之外,事件的指纹的hash值都将是topic之一。这意味着,不可能通过名字来过滤特定的匿名事件。

所有非索引参数将被作为数据日志记录的一部分进行存储。

contract ClientReceipt {

    event Deposit(

        address indexed _from,

        bytes32 indexed _id,

        uint _value

    );

    function deposit(bytes32 _id) {

        // 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);

    }

}

contract ClientReceipt {

    event Deposit(

        address indexed _from,

        bytes32 indexed _id,

        uint _value

    );

    function deposit(bytes32 _id) {

        // Any call to this function (even deeply nested) can 任何对这个函数的调用都能通过JavaScipt API , 用`Deposit` 过滤来检索到(即使深入嵌套)

        // be detected from the JavaScript API by filtering

        // for `Deposit` to be called.

        Deposit(msg.sender, _id, msg.value);

    }

}

JavaScript API 的使用如下:

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);

});

var abi = /\ abi as generated by the compiler /;     /\ 由编译器生成的abi /;  

var ClientReceipt = web3.eth.contract(abi);

var clientReceipt = ClientReceipt.at(0x123 /\ address /);   /\ 地址 /); 

var event = clientReceipt.Deposit();

// watch for changes   观察变化

event.watch(function(error, result){

    // result will contain various information   结果包含不同的信息: 包括给Deposit调用的参数

    // including the argumets given to the Deposit

    // call.

    if (!error)

        console.log(result);});

// Or pass a callback to start watching immediately 或者通过callback立刻开始观察

var event = clientReceipt.Deposit(function(error, result) {

    if (!error)

        console.log(result);

});

底层日志的接口

还可以通过函数log0 log1,log2,log3 log4到 logi,共i+1个bytes32类型的参数来访问底层日志机制的接口。第一个参数将用于数据日志的一部分,其它的参数将用于topic。上面的事件调用可以以相同的方式执行。.

log3(

    msg.value,

    0x50cb9fe53daa9737b786ab3646f04d0150dc50ef4e75f59509d83667ad5adb20,

    msg.sender,

    _id

);

很长的十六进制数等于

sha3(“Deposit(address,hash256,uint256)”), 这个就是事件的指纹。

理解事件的额外的资源

继承

通过包括多态性的复制代码,Solidity支持多重继承。

除非合约是显式给出的,所有的函数调用都是虚拟的,绝大多数派生函数可被调用。

即使合约是继承了多个其他合约, 在blockchain上只有一个合约被创建, 基本合约代码总是被复制到最终的合约上。

通用的继承机制非常类似于Python里的继承,特别是关于多重继承方面。

下面给出了详细的例子。

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, 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;

}

contract owned {

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

    address owner;}

// Use "is" to derive from another contract. Derived   用"is"是从其他的合约里派生出

// contracts can access all non-private members including   派生出的合约能够访问所有非私有的成员,包括内部函数和状态变量。  它们不能从外部用'this'来访问。

// 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  这些抽象的合约仅仅是让编译器知道已经生成了接口,

// interface known to the compiler. Note the function   注意:函数没有函数体。如果合约不做实现的话,它就只能当作接口。

// 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 多重继承也是可以的,注意"owned" 也是mortal的基类, 虽然 仅仅有"owned"的单个实例,(和C++里的virtual继承一样)

// 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, both local and  函数被重写,本地和基于消息的函数调用把这些override带入账户里。

    // 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  还可以调用特定的override函数

            // overridden function.

            mortal.kill();

        }

    }}

// If a constructor takes an argument, it needs to be  如果构造器里带有一个参数,有必要在头部给出,(或者在派生合约的构造器里使用修饰符调用方式modifier-invocation-style(见下文))

// 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;

}

注意:在上文中,我们使用mortal.kill() 来“forward” 析构请求。这种做法是有问题的,请看下面的例子:

contract mortal is owned {

    function kill() {

        if (msg.sender == owner) selfdestruct(owner);

    }

}

contract Base1 is mortal {

    function kill() { /\ do cleanup 1  清除1 / mortal.kill(); 

}

}

contract Base2 is mortal {

    function kill() { /\ do cleanup 2  清除2 / mortal.kill(); 

}

}

contract Final is Base1, Base2 {

}

Final.kill() 将调用Base2.kill作为最后的派生重写,但这个函数绕开了Base1.kill。因为它不知道有Base1。这种情况下要使用 super

contract mortal is owned {

    function kill() {

        if (msg.sender == owner) selfdestruct(owner);

    }

}

contract Base1 is mortal {

    function kill() { /\ do cleanup 1   清除1  \/    super.kill(); }

}

contract Base2 is mortal {

    function kill() { /\ do cleanup 2  清除2  \/    super.kill(); }

}

contract Final is Base2, Base1 {

}

若Base1 调用了super函数,它不是简单地调用基本合约之一的函数, 它是调用最后继承关系的下一个基本合约的函数。所以它会调用 base2.kill()(注意,最后的继承顺序是–从最后的派生合约开始:Final, Base1, Base2, mortal, owned)。当使用类的上下文中super不知道的情况下,真正的函数将被调用,虽然它的类型已经知道。这个和普通的virtual方法的查找相似。

基本构造函数的参数

派生的合约需要为基本构造函数提供所有的参数。这可以在两处进行:

contract Base {

    uint x;

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

}

contract Derived is Base(7) {

    function Derived(uint _y) Base(_y * _y) {

    }

}

第一种方式是直接在继承列表里实现(是 Base(7)),第二种方式是在派生的构造器的头部,修饰符被调用时实现(Base(_y * _y))。如果构造函数参数是一个常量,并且定义了合约的行为或描述了它的行为,第一种方式比较方便。 如果基本构造函数参数依赖于派生合约的构造函数,则必须使用第二种方法。如果在这个荒谬的例子中,这两个地方都被使用,修饰符样式的参数优先。

多继承和线性化

允许多重继承的编程语言,要处理这样几个问题,其中一个是Diamond问题。Solidity是沿用Python的方式, 使用“C3线性化”,在基类的DAG强制使用特定的顺序。这导致单调但不允许有一些的继承关系。特别是,在其中的基础类的顺序是直接的,这点非常重要。在下面的代码中,Solidity会报错:“继承关系的线性化是不可能的”。

contract X {}

contract A is X {}

contract C is A, X {}

这个原因是,C要求X来重写A(定义A,X这个顺序),但A本身的要求重写X,这是一个矛盾,不能解决。

一个简单的规则是要指定基类中的顺序,从“最基本”到“最近派生”。

抽象契约

合约函数可以缺少实现(请注意,函数声明头将被终止),见下面的例子:

contract feline {

    function utterance() returns (bytes32);

}

这样的合约不能被编译(即使它们包含实现的函数和非实现的函数),但它们可以用作基本合约:

contract Cat is feline {

    function utterance() returns (bytes32) { return "miaow"; }

}

如果一个合约是从抽象合约中继承的,而不实现所有非执行功能,则它本身就是抽象的。

库和合约类似,但是它们的目的主要是在给定地址上部署,以及用EVM的CALLCODE特性来重用代码。这些代码是在调用合约的上下文里执行的,例如调用合约的指针和调用合约的存储能够被访问。由于库是一片独立的代码,如果它们显示地提供的话,就仅仅能访问到调用合约的状态变量(有方法命名它们)

下面的例子解释了怎样使用库(确保用using for 来实现)

library Set {

  // We define a new struct datatype that will be used to   我们定义了一个新的结构体数据类型,用于存放调用合约中的数据

  // hold its data in the calling contract.

  struct Data { mapping(uint => bool) flags; }

  // Note that the first parameter is of type "storage 注意第一个参数是 “存储引用”类型,这样仅仅是它的地址,而不是它的内容在调用中被传入 这是库函数的特点, 

  // 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  若第一个参数用"self"调用时很笨的的,如果这个函数可以被对象的方法可见。

  // 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 已经在那里

      self.flags[value] = true;

      return true;

  }

  function remove(Data storage self, uint value)

      returns (bool)

  {

      if (!self.flags[value])

          return false; // not there 不在那里

      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  这个库函数没有特定的函数实例被调用,因为“instance”是当前的合约

        // specific instance of the library, since the

        // "instance" will be the current contract.

        if (!Set.insert(knownValues, value))

            throw;

    }

    // In this contract, we can also directly access knownValues.flags, if we want 在这个合约里,如果我们要的话,也可以直接访问 knownValues.flags

.*}

当然,你不必这样使用库--他们也可以事前不定义结构体数据类型,就可以使用。 没有任何存储引入参数,函数也可以执行。也可以在任何位置,有多个存储引用参数。

Set.contains, Set.insert and Set.remove都可编译到(CALLCODE)外部合约/库。如果你使用库,注意真正进行的外部函数调用,所以`msg.sender不再指向来源的sender了,而是指向了正在调用的合约。msg.value包含了调用库函数中发送的资金。

因为编译器不知道库将部署在哪里。这些地址不得不由linker填进最后的字节码(见使用命令行编译器如何使用命令行编译器链接)。如果不给编译器一个地址做参数,编译的十六进制码就会包含Set 这样的占位符(Set是库的名字)。通过替换所有的40个字符的十六进制编码的库合约的地址,地址可以手动进行填充。

比较合约和库的限制:

  • 无状态变量
  • 不能继承或被继承

(这些可能在以后会被解除)

库的常见“坑”

msg.sender的值

msg.sender的值将是调用库函数的合约的值。

例如,如果A调用合约B,B内部调用库C。在库C库的函数调用里,msg.sender将是合约B的地址。

表达式LibraryName.functionName() 用CALLCODE完成外部函数调用, 它映射到一个真正的EVM调用,就像otherContract.functionName() 或者 this.functionName()。这种调用可以一级一级扩展调用深度(最多1024级),把msg.sender存储为当前的调用者,然后执行库合约的代码,而不是执行当前的合约存储。这种执行方式是发生在一个完全崭新的内存环境中,它的内存类型将被复制,并且不能绕过引用。

转移Ether

原则上使用LibraryName.functionName.value(x)()来转移Ether。但若使用CALLCODE,Ether会在当前合约里用完。

Using For

指令 using A for B; 可用于附加库函数(从库A)到任何类型(B)。这些函数将收到一个作为第一个参数的对象(像Python中self变量)。

using A for *;,是指函数从库A附加到任何类型。

在这两种情况下,所有的函数将被附加,(即使那些第一个参数的类型与对象的类型不匹配)。该被调用函数的入口类型将被检查,并进行函数重载解析。

using A for B; 指令在当前的范围里是有效的,作用范围限定在现在的合约里。但(出了当前范围)在全局范围里就被移除。因此,通过 including一个模块,其数据类型(包括库函数)都将是可用的,而不必添加额外的代码。

让我们用这种方式重写库中的set示例:

// 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)

        if (!knownValues.insert(value))

            throw;

    }

}

// This is the same code as before, just without comments   这个代码和之前的一样,仅仅是没有注释

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 已经在那里

      self.flags[value] = true;

      return true;

  }

  function remove(Data storage self, uint value)

      returns (bool)

  {

      if (!self.flags[value])

          return false; // not there 没有

      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   这个是关键的变化

    Set.Data knownValues;

    function register(uint value) {

        // Here, all variables of type Set.Data have   这里,所有Set.Data 的变量都有相应的成员函数

        // corresponding member functions.

        // The following function call is identical to  下面的函数调用和Set.insert(knownValues, value) 作用一样

        // Set.insert(knownValues, value)

        if (!knownValues.insert(value))

            throw;

    }

}

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

这个也是一种扩展基本类型的(方式)

library Search {

    function indexOf(uint[] storage self, uint value) {

        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   这样完成了库函数的调用

        uint index = data.find(_old);

        if (index == -1)

            data.push(_new);

        else

            data[index] = _new;

    }}

注意:所有的库函数调用都是调用实际的EVM。这意味着,如果你要使用内存或值类型,就必须执行一次拷贝操作,即使是self变量。拷贝没有完成的情况可能是存储引用变量已被使用。

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