三种类型变量-常量-读取写入状态变量

1.Solidity中有3种类型的变量

• 当地的
  • 在函数内部声明
  • 不存储在区块链上
• 状态
  • 在函数外部声明
  • 存储在区块链上
• 全球（提供有关区块链的信息）

There are 3 types of variables in Solidity local declared inside a function not stored on the blockchain

state declared outside a function stored on the blockchain global (provides information about the blockchain)

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

contract Variables {
    // State variables are stored on the blockchain.
    string public text = "Hello";
    uint public num = 123;

    function doSomething() public {
        // Local variables are not saved to the blockchain.
        uint i = 456;

        // Here are some global variables
        uint timestamp = block.timestamp; // Current block timestamp
        address sender = msg.sender; // address of the caller
    }
}

2.常量:不能修改的变量

它们的值是硬编码的，使用常量可以节省天然气成本。

Constants are variables that cannot be modified.

Their value is hard coded and using constants can save gas cost.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

contract Constants {
    // coding convention to uppercase constant variables
    address public constant MY_ADDRESS = 0x777788889999AaAAbBbbCcccddDdeeeEfFFfCcCc;
    uint public constant MY_UINT = 123;
}

3.读取和写入状态变量

要写入或更新状态变量，您需要发送交易。

另一方面，您可以免费读取状态变量，无需任何交易费用。

To write or update a state variable you need to send a transaction.

On the other hand, you can read state variables, for free, without any transaction fee.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

contract SimpleStorage {
    // State variable to store a number
    uint public num;

    // You need to send a transaction to write to a state variable.
    function set(uint _num) public {
        num = _num;
    }

    // You can read from a state variable without sending a transaction.
    function get() public view returns (uint) {
        return num;
    }
}