fallback

fallback是特殊的函数，无参数，无返回值

何时会被调用：

1. 当被调用的方法不存在时，fallback会被调用，属于default函数；
2. 当向合约转ether但是合约不存在receive函数时；
3. 当向合约转ether但是msg.data不为空时。（即使receive存在）

当使用transfer或者send对合约进行转账时，fallback函数的gaslimit限定为2300 gas

// SPDX-License-Identifier: SEE LICENSE IN LICENSE
pragma solidity ^0.8.9;

contract Fallback {
    event Log(uint gas);

    fallback() external payable {
        //gasleft 当前还剩的gas
        emit Log(gasleft());
    }
    function getBalance() public view returns (uint) {
        return address(this).balance;
    }
}

contract SendToFallback {
    function transferToFallback(address payable _to) public payable {
        _to.transfer(msg.value);
    }

    function callFallback(address payable _to) public payable {
        (bool sent, ) = _to.call{value: msg.value}("");
        require(sent, "Failed to send Ether");
    }

    function callNoExistFunc(address payable _to) public payable {
        (bool sent, ) = _to.call{value: msg.value}(
            abi.encodeWithSignature("noExistFunc()")
        );
        require(sent, "Failed to call");
    }
}