inheritance
1.Solidity 支持多重继承。合约可以通过is关键字继承其他合约
将被子合约覆盖的函数必须声明为virtual.
- 要覆盖父函数的函数必须使用关键字override。
- 继承顺序很重要。您必须按照从“最基础”到“最衍生”的顺序列出父合约。
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;
/* Graph of inheritance
A
/ \
B C
/ \ /
F D,E
*/
contract A {
function foo() public pure virtual returns (string memory) {
return "A";
}
}
// Contracts inherit other contracts by using the keyword 'is'.
contract B is A {
// Override A.foo()
function foo() public pure virtual override returns (string memory) {
return "B";
}
}
contract C is A {
// Override A.foo()
function foo() public pure virtual override returns (string memory) {
return "C";
}
}
// Contracts can inherit from multiple parent contracts.
// When a function is called that is defined multiple times in
// different contracts, parent contracts are searched from
// right to left, and in depth-first manner.
contract D is B, C {
// D.foo() returns "C"
// since C is the right most parent contract with function foo()
function foo() public pure override(B, C) returns (string memory) {
return super.foo();
}
}
contract E is C, B {
// E.foo() returns "B"
// since B is the right most parent contract with function foo()
function foo() public pure override(C, B) returns (string memory) {
return super.foo();
}
}
// Inheritance must be ordered from “most base-like” to “most derived”.
// Swapping the order of A and B will throw a compilation error.
contract F is A, B {
function foo() public pure override(A, B) returns (string memory) {
return super.foo();
}
}
2.隐藏继承的状态变量
与函数不同,状态变量不能通过在子合约中重新声明来覆盖。
让我们学习如何正确覆盖继承的状态变量。
A constructor is an optional function that is executed upon contract creation.
Here are examples of how to pass arguments to constructors.
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;
contract A {
string public name = "Contract A";
function getName() public view returns (string memory) {
return name;
}
}
// Shadowing is disallowed in Solidity 0.6
// This will not compile
// contract B is A {
// string public name = "Contract B";
// }
contract C is A {
// This is the correct way to override inherited state variables.
constructor() {
name = "Contract C";
}
// C.getName returns "Contract C"
}
3.调用父合约
- 父合约可以直接调用
- 也可以使用关键字调用super。
通过使用关键字super,将调用所有直接父合约。 Parent contracts can be called directly, or by using the keyword super.
By using the keyword super, all of the immediate parent contracts will be called.
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;
/* Inheritance tree
A
/ \
B C
\ /
D
*/
contract A {
// This is called an event. You can emit events from your function
// and they are logged into the transaction log.
// In our case, this will be useful for tracing function calls.
event Log(string message);
function foo() public virtual {
emit Log("A.foo called");
}
function bar() public virtual {
emit Log("A.bar called");
}
}
contract B is A {
function foo() public virtual override {
emit Log("B.foo called");
A.foo();
}
function bar() public virtual override {
emit Log("B.bar called");
super.bar();
}
}
contract C is A {
function foo() public virtual override {
emit Log("C.foo called");
A.foo();
}
function bar() public virtual override {
emit Log("C.bar called");
super.bar();
}
}
contract D is B, C {
// Try:
// - Call D.foo and check the transaction logs.
// Although D inherits A, B and C, it only called C and then A.
// - Call D.bar and check the transaction logs
// D called C, then B, and finally A.
// Although super was called twice (by B and C) it only called A once.
function foo() public override(B, C) {
super.foo();
}
function bar() public override(B, C) {
super.bar();
}
}
1.规则
- virtual: 父合约中的函数,如果希望子合约重写,需要加上virtual关键字。
- override:子合约重写了父合约中的函数,需要加上override关键字。
注意:用override修饰public变量,会重写与变量同名的getter函数,例如:
2.简单继承
写一个简单的爷爷合约Yeye,里面包含1个Log事件和3个function: hip(), pop(), yeye(),输出都是”Yeye ”
定义一个爸爸合约Baba,让他继承Yeye合约,语法就是contract Baba is Yeye,非常直观。
在Baba合约里,我们重写一下hip()和pop()这两个函数,加上override关键字,并将他们的输出改为”Baba”;并且加一个新的函数baba,输出也是”Baba”:
我们部署合约,可以看到Baba合约里有4个函数,其中hip()和pop()的输出被成功改写成”Baba”,而继承来的yeye()的输出仍然是”Yeye”。
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;
// 合约继承
contract Yeye {
event Log(string msg);
// 定义3个function: hip(), pop(), man(),Log值为Yeye。
function hip() public virtual{
emit Log("Yeye");
}
function pop() public virtual{
emit Log("Yeye");
}
function yeye() public virtual {
emit Log("Yeye");
}
}
contract Baba is Yeye{
// 继承两个function: hip()和pop(),输出改为Baba。
function hip() public virtual override{
emit Log("Baba");
}
function pop() public virtual override{
emit Log("Baba");
}
function baba() public virtual{
emit Log("Baba");
}
}
3.多重继承
规则:
- 继承时要按辈分最高到最低的顺序排。比如我们写一个Erzi合约,继承Yeye合约和Baba合约,那么就要写成contract Erzi is Yeye, Baba,而不能写成contract Erzi is Baba, Yeye,不然就会报错。
- 如果某一个函数在多个继承的合约里都存在,比如例子中的hip()和pop(),在子合约里必须重写,不然会报错。
- 重写在多个父合约中都重名的函数时,override关键字后面要加上所有父合约名字,例如override(Yeye, Baba)。
例子: 我们可以看到,Erzi合约里面重写了hip()和pop()两个函数,将输出改为”Erzi”,并且还分别从Yeye和Baba合约继承了yeye()和baba()两个函数。
contract Erzi is Yeye, Baba{
// 继承两个function: hip()和pop(),输出改为Erzi。
function hip() public virtual override(Yeye, Baba){
emit Log("Erzi");
}
function pop() public virtual override(Yeye, Baba) {
emit Log("Erzi");
}
function callParent() public{
Yeye.pop();
}
function callParentSuper() public{
super.pop();
}
}
4.修饰器的继承
Solidity中的修饰器(Modifier)同样可以继承,用法与函数继承类似,在相应的地方加 virtual 和 override 关键字即可。
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;
contract Base1 {
modifier exactDividedBy2And3(uint _a) virtual {
require(_a % 2 == 0 && _a % 3 == 0);
_;
}
}
contract Identifier is Base1 {
//计算一个数分别被2除 和被3除的值,但是传入的参数必须是2和3的倍数
function getExactDividedBy2And3(uint _dividend) public exactDividedBy2And3(_dividend) pure returns(uint, uint) {
return getExactDividedBy2And3WithoutModifier(_dividend);
}
//计算一个数分别被2除和被3除的值
function getExactDividedBy2And3WithoutModifier(uint _dividend) public pure returns(uint, uint){
uint div2 = _dividend / 2;
uint div3 = _dividend / 3;
return (div2, div3);
}
//重写Modifier: 不重写时,输入9调用getExactDividedBy2And3,会revert,因为无法通过检查
//删掉下面三行注释重写Modifier,这时候输入9调用getExactDividedBy2And3, 会调用成功
// modifier exactDividedBy2And3(uint _a) override {
// _;
// }
}
构造函数的继承
子合约有两种方法继承父合约的构造函数。举个简单的例子,父合约A里面有一个状态变量a,并由构造函数的参数来确定:
- 在继承时声明父构造函数的参数,例如:contract B is A(1)
- 在子合约的构造函数中声明构造函数的参数,例如
contract C is A {
constructor(uint _c) A(_c * _c) {}
}
调用父合约的函数
子合约有两种方式调用父合约的函数
- 直接调用,子合约可以直接用父合约名.函数名()的方式来调用父合约函数,例如Yeye.pop()。
function callParent() public{
Yeye.pop();
}
- 利用super关键字 super关键字:子合约可以利用super.函数名()来调用最近的父合约函数。solidity继承关系按声明时从右到左的顺序是:contract Erzi is Yeye, Baba,那么Baba是最近的父合约,super.pop()将调用Baba.pop()而不是Yeye.pop():
function callParentSuper() public{
// 将调用最近的父合约函数,Baba.pop()
super.pop();
}
钻石继承:指一个派生类同时有两个或两个以上的基类
我们先写一个合约God,再写Adam和Eve两个合约继承God合约,最后让创建合约people继承自Adam和Eve,每个合约都有foo和bar两个函数。
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.13;
/* 继承树:
God
/ \
Adam Eve
\ /
people
*/
contract God {
event Log(string message);
function foo() public virtual {
emit Log("God.foo called");
}
function bar() public virtual {
emit Log("God.bar called");
}
}
contract Adam is God {
function foo() public virtual override {
emit Log("Adam.foo called");
}
function bar() public virtual override {
emit Log("Adam.bar called");
super.bar();
}
}
contract Eve is God {
function foo() public virtual override {
emit Log("Eve.foo called");
super.foo();
}
function bar() public virtual override {
emit Log("Eve.bar called");
super.bar();
}
}
contract people is Adam, Eve {
function foo() public override(Adam, Eve) {
super.foo();
}
function bar() public override(Adam, Eve) {
super.bar();
}
}
在这个例子中,调用合约people中的super.bar()会依次调用Eve、Adam,最后是God合约。
虽然Eve、Adam都是God的子合约,但整个过程中God合约只会被调用一次。原因是Solidity借鉴了Python的方式,强制一个由基类构成的DAG(有向无环图)使其保证一个特定的顺序。