# Solidity 课程 14: 常量


By [Novar](https://paragraph.com/@novar) · 2022-06-08

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合约的状态变量可以声明为不可变常数，数值定义后**不能再次编辑**，可以**节约gas fee**。有两种定义命令，`constant` 和 `immutable` ，区别是

1.  `constant`只支持值类型和字符串，`immutable`可以；
    
2.  `constant`必须声明时就赋值，`immutable`可以推迟到构造函数里赋值 。
    

举例：

        //1、constant只支持值类型和字符串，immutable可以；
    
        address public constant MY_ADDRESS = 0x777788889999AaAAbBbbCcccddDdeeeEfFFfCcCc;
        uint public constant MY_UINT = 123;
        string public constant MY_STRING = "123";
        bytes public constant MY_BYTES = "123";
    
        address public immutable MY_ADDRESS_IM = 0x777788889999AaAAbBbbCcccddDdeeeEfFFfCcCc;
        uint public immutable MY_UINT_IM = 123;
        // string public immutable MY_STRING_IM = 123;
        //bytes public immutable MY_BYTES = "123";
    
    
       //2、constant必须声明时就赋值，immutable可以推迟到构造函数里赋值
        uint public immutable MY_UINT_CONSTRUCTOR;
        uint256 public immutable IMMUTABLE_TEST;
    
        constructor(){
            MY_UINT_CONSTRUCTOR = 123;
            IMMUTABLE_TEST = test();
        }
    
        function test() public pure returns(uint256){
            uint256 what = 9;
            return(what);
        }
    

这节学习了不可变常量的两种定义方式，`constant` 和 `immutable`，可以**节约gas fee。**

### Code:

[https://github.com/Luca-Hsu/SuperSolidity/blob/main/14\_Constants/Constants.sol](https://github.com/Luca-Hsu/SuperSolidity/blob/main/14_Constants/Constants.sol)

### Reference:

[https://solidity-by-example.org/constants](https://solidity-by-example.org/constants)

[https://solidity-by-example.org/immutable](https://solidity-by-example.org/immutable)

[https://docs.soliditylang.org/en/v0.8.4/contracts.html#constant-and-immutable-state-variables](https://docs.soliditylang.org/en/v0.8.4/contracts.html#constant-and-immutable-state-variables)

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*Originally published on [Novar](https://paragraph.com/@novar/solidity-14)*