存储位置

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.7.0 <0.9.0;
/*
    Data locations
    storage     状态变量
    memory      局部变量
    calldata    类似memory，但只能用于输入的参数中
*/
contract DataLocations {
    struct MyStruct {
        uint foo;
        string text;
    }
    mapping(address => MyStruct) public myStructs;
    function examples() external {
        myStructs[msg.sender] = MyStruct({foo: 123, text: "bar"});
        // storage 状态变量，方法结束后可以保存修改结果，即 myStructs[msg.sender] 为 MyStruct({foo: 123, text: "foo"});
        // MyStruct storage myStruct = myStructs[msg.sender];
        // myStruct.text = "foo";
        // memory 局部变量，方法结束后不可以保存修改结果，即 myStructs[msg.sender] 为 MyStruct({foo: 123, text: "bar"});
        MyStruct memory readOnly = myStructs[msg.sender];
        readOnly.foo = 456;
    }
}