继承、抽象合约与接口

在面向对象编程中，继承是指一个程序能够使用另一个程序的功能。在 Solidity 中，继承允许我们通过复用已被验证为安全的代码来扩展合约，从而保持代码的组织性和可维护性。提供这些功能的合约称为基合约，而继承该功能的合约则称为派生合约。

我们使用 is 关键字声明一个合约从某个源合约继承，这相当于表示派生合约是基合约的一个实例。

1pragma solidity >=0.4.0 <0.7.0;
2contract X {}
3contract Y is X {}
4contract Z is Y {
5    // 这使得 Z 也继承自 X
6}

1pragma solidity >=0.5.0 <0.9.0; 
2contract sum { 
3    function computeSum(uint num1, uint num2) internal pure returns(uint) { 
4        return num1 + num2;
5    } 
6} 
7contract product { 
8    function computeProduct(uint num1, uint num2) internal pure returns(uint) { 
9        return num1 * num2;
10    } 
11}  
12contract operations is sum, product {
13    uint public mySum;
14    uint public myProduct;
15    function setSum(uint num1, uint num2) external returns(uint) { 
16        mySum = computeSum(num1, num2);
17        return mySum;
18    } 
19    function setProduct(uint num1, uint num2) external returns(uint) { 
20        myProduct = computeProduct(num1, num2);
21        return myProduct;
22    } 
23}

1// SPDX-License-Identifier: MIT
2pragma solidity >=0.6.0 <0.9.0; 
3abstract contract sumAndProduct{ 
4    function computeSum(uint num1, uint num2) internal pure returns(uint) { 
5        return num1 + num2;
6    } 
7    function computeProduct(uint num1, uint num2) internal virtual pure returns(uint);
8} 
9contract operations is sumAndProduct{
10    uint public mySum;
11    uint public myProduct;
12    function setSum(uint num1, uint num2) external returns(uint) { 
13        mySum = computeSum(num1, num2);
14        return mySum;
15    } 
16    function computeProduct(uint num1, uint num2) internal override pure returns(uint){
17        return num1 * num2;
18    }
19    function setProduct(uint num1, uint num2) external returns(uint) { 
20        myProduct = computeProduct(num1, num2);
21        return myProduct;
22    } 
23}

1// SPDX-License-Identifier: MIT
2pragma solidity >=0.5.0 <0.9.0; 
3interface sumAndProduct{ 
4    function computeSum(uint num1, uint num2) external pure returns(uint);
5    function computeProduct(uint num1, uint num2) external pure returns(uint);
6} 
7contract operations is sumAndProduct{
8    uint public mySum;
9    uint public myProduct;
10    function computeSum(uint num1, uint num2) public override pure returns(uint){
11        return num1 + num2;
12    }
13    function setSum(uint num1, uint num2) external returns(uint) { 
14        mySum = computeSum(num1, num2);
15        return mySum;
16    } 
17    function computeProduct(uint num1, uint num2) public override pure returns(uint){
18        return num1 * num2;
19    }
20    function setProduct(uint num1, uint num2) external returns(uint) { 
21        myProduct = computeProduct(num1, num2);
22        return myProduct;
23    } 
24}