2023-03-02

call与fallback

fallback函数被调用的情况

Which function is called, fallback() or receive()?

           send Ether
               |
         msg.data is empty?
              / \
            yes  no
            /     \
receive() exists?  fallback()
/   \
yes   no
/      \
receive()   fallback()

• 函数名称不存在

Receive函数没有 function 关键字，没有参数也没有返回值，且必须是 external 可见性（允许外部合约调用）并具有 payable 可支付属性。

转账的时候会先调用receive，没有receive的时候会调用fallback

address可以唯一标识一个合约，某种意义上来说，address就是合约

接收及发送以太币（send、transfer、call）

How to send Ether?

如何发送以太币？

You can send Ether to other contracts by

你可以通过以下方式发送以太币给其它合约

• transfer (2300 gas, throws error)
• send (2300 gas, returns bool)
• call (forward all gas or set gas, returns bool)

How to receive Ether?

如何接收以太币？

A contract receiving Ether must have at least one of the functions below

接收以太币的合约必须拥有至少一个以下函数：

• receive() external payable
• fallback() external payable

receive() is called if is empty, otherwise is called.msg.datafallback()

当receive()是空的的时候会被调用，否则msg.data会调用fallback()

Which method should you use?

call in combination with re-entrancy guard is the recommended method to use after December 2019.

call函数是2019年12月份之后被推荐使用的，它结合了重入防御机制

Guard against re-entrancy by

通过以下方式防止重入

• making all state changes before calling other contracts / 在调用其他合约之前先做完所有状态的改变

• using re-entrancy guard modifier / 使用防重入修饰符

call能省一些gas（详见-> 视频）

如下代码：

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;

contract ReceiveEther {

    // Function to receive Ether. msg.data must be empty
    receive() external payable {}

    // Fallback function is called when msg.data is not empty
    fallback() external payable {}

    function getBalance() public view returns (uint) {
        return address(this).balance;
    }
}

contract SendEther {
    function sendViaTransfer(address payable _to) public payable {
        // This function is no longer recommended for sending Ether.
        _to.transfer(msg.value);
    }

    function sendViaSend(address payable _to) public payable {
        // Send returns a boolean value indicating success or failure.
        // This function is not recommended for sending Ether.
        bool sent = _to.send(msg.value);
        require(sent, "Failed to send Ether");
    }

    function sendViaCall(address payable _to) public payable {
        // Call returns a boolean value indicating success or failure.
        // This is the current recommended method to use.
        (bool sent, bytes memory data) = _to.call{value: msg.value}("");
        require(sent, "Failed to send Ether");
    }
}