繼承 · 智能合約Solidity編程語言中文文檔

# 繼承(Inheritance) Solidity通過復制包括多態的代碼來支持多重繼承。所有函數調用是`虛擬(virtual)`的，這意味著最遠的派生方式會被調用，除非明確指定了合約。當一個合約從多個其它合約那里繼承，在區塊鏈上僅會創建一個合約，在父合約里的代碼會復制來形成繼承合約。基本的繼承體系與`python`有些類似，特別是在處理多繼承上面。下面用一個例子來詳細說明： ``` 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; } ``` 上面的例子的`named`合約的`kill()`方法中，我們調用了`motal.kill()`調用父合約的`銷毀函數(destruction)`。但這樣可能什么引發一些小問題。 ``` pragma solidity ^0.4.0; 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 { } ``` 對`Final.kill()`的調用只會調用`Base2.kill()`，因為派生重寫，會跳過`Base1.kill`，因為它根本就不知道有`Base1`。一個變通方法是使用`super`。 ``` pragma solidity ^0.4.0; 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 { } ``` 如果`Base1`調用了函數`super`，它不會簡單的調用基類的合約函數，它還會調用繼承關系圖譜上的下一個基類合約，所以會調用`Base2.kill()`。需要注意的最終的繼承圖譜將會是：Final，Base1，Base2，mortal，owned。使用super時會調用的實際函數在使用它的類的上下文中是未知的，盡管它的類型是已知的。這類似于普通虛函數查找`(ordinary virtual method lookup)` ## 基類構造器的方法(Arguments for Base Constructors) 派生的合約需要提供所有父合約需要的所有參數，所以用兩種方式來做，見下面的例子： ``` 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) { } } ``` 或者直接在繼承列表中使用`is Base(7)`，或像`修改器(modifier)`使用方式一樣，做為派生構造器定義頭的一部分`Base(_y * _y)`。第一種方式對于構造器是常量的情況比較方便，可以大概說明合約的行為。第二種方式適用于構造的參數值由派生合約的指定的情況。在上述兩種都用的情況下，第二種方式優先(一般情況只用其中一種方式就好了)。 ## 多繼承與線性化(Multiple Inheritance and Linearization) 實現多繼承的編程語言需要解決幾個問題，其中之一是`菱形繼承問題`又稱`鉆石問題`，如下圖。<br/> <img src="media/14825855458212/14826257952604.png" alt=""/> Solidity的解決方案參考`Python`，使用[C3_linearization](https://en.wikipedia.org/wiki/C3_linearization)來強制將基類合約轉換一個有向無環圖(DAG)的特定順序。結果是我們希望的單調性，但卻禁止了某些繼承行為。特別是基類合約在`is`后的順序非常重要。下面的代碼，Solidity會報錯`Linearization of inheritance graph impossible`。 ``` pragma solidity ^0.4.0; contract X {} contract A is X {} contract C is A, X {} ``` 原因是`C`會請求`X`來重寫`A`(因為繼承定義的順序是`A,X`)，但`A`自身又是重寫`X`的，所以這是一個不可解決的矛盾。一個簡單的指定基類合約的繼承順序原則是從`most base-like`到`most derived`。 ## 繼承有相同名字的不同類型成員當繼承最終導致一個合約同時存在多個相同名字的修改器或函數，它將被視為一個錯誤。同新的如果事件與修改器重名，或者函數與事件重名都將產生錯誤。作為一個例外，狀態變量的`getter`可以覆蓋一個`public`的函數。