Solidity极简入门: 9. 常量 constant和immutable
0x25df
April 28th, 2022

我最近在重新学solidity,巩固一下细节,也写一个“Solidity极简入门”,供小白们使用(编程大佬可以另找教程),每周更新1-3讲。

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所有代码和教程开源在github(1024个star发课程认证,2048个star发社群NFT): github.com/AmazingAng/WTFSolidity


这一讲,我们介绍solidity中两个关键字,constant(常量)和immutable(不变量)。状态变量声明这个两个关键字之后,不能在合约后更改数值;并且还可以节省gas。另外,只有数值变量可以声明constantimmutableStringbytes可以声明为constant,但不能为immutable

constant和immutable

constant

constant变量必须在声明的时候初始化,之后再也不能改变。尝试改变的话,编译不通过。

    // constant变量必须在声明的时候初始化,之后不能改变
    uint256 constant CONSTANT_NUM = 10;
    string constant CONSTANT_STRING = "0xAA";
    bytes constant CONSTANT_BYTES = "WTF";
    address constant CONSTANT_ADDRESS = 0x0000000000000000000000000000000000000000;

immutable

immutable变量可以在声明时或构造函数中初始化,因此更加灵活。

    // immutable变量可以在constructor里初始化,之后不能改变
    uint256 public immutable IMMUTABLE_NUM = 9999999999;
    address public immutable IMMUTABLE_ADDRESS;
    uint256 public immutable IMMUTABLE_BLOCK;
    uint256 public immutable IMMUTABLE_TEST;

你可以使用全局变量例如address(this)block.number ,或者自定义的函数给immutable变量初始化。在下面这个例子,我们利用了test()函数给IMMUTABLE_TEST初始化为9:

    // 利用constructor初始化immutable变量,因此可以利用
    constructor(){
        IMMUTABLE_ADDRESS = address(this);
        IMMUTABLE_BLOCK = block.number;
        IMMUTABLE_TEST = test();
    }

    function test() public pure returns(uint256){
        uint256 what = 9;
        return(what);
    }

总结

这一讲,我们介绍solidity中两个关键字,constant(常量)和immutable(不变量),让不应该变的变量保持不变。

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