# Solidity 课程 14: 常量


**Published by:** [Novar](https://paragraph.com/@novar/)
**Published on:** 2022-06-08
**URL:** https://paragraph.com/@novar/solidity-14

## Content

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constant只支持值类型和字符串，immutable可以；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.solReference:https://solidity-by-example.org/constants https://solidity-by-example.org/immutable https://docs.soliditylang.org/en/v0.8.4/contracts.html#constant-and-immutable-state-variables

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