fallback

fallback是一个特殊函数，在以下情况下执行

• 调用不存在的函数或
• 以太直接发送到合约但receive()不存在或msg.data不为空

fallbacktransfer当由或调用时，有 2300 Gas 限制send。

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

contract Fallback {
    event Log(string func, uint gas);

    // Fallback function must be declared as external.
    fallback() external payable {
        // send / transfer (forwards 2300 gas to this fallback function)
        // call (forwards all of the gas)
        emit Log("fallback", gasleft());
    }

    // Receive is a variant of fallback that is triggered when msg.data is empty
    receive() external payable {
        emit Log("receive", gasleft());
    }

    // Helper function to check the balance of this contract
    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");
    }
}

fallback可以选择bytes输入和输出

pragma solidity ^0.8.20;

// TestFallbackInputOutput -> FallbackInputOutput -> Counter
contract FallbackInputOutput {
    address immutable target;

    constructor(address _target) {
        target = _target;
    }

    fallback(bytes calldata data) external payable returns (bytes memory) {
        (bool ok, bytes memory res) = target.call{value: msg.value}(data);
        require(ok, "call failed");
        return res;
    }
}

contract Counter {
    uint public count;

    function get() external view returns (uint) {
        return count;
    }

    function inc() external returns (uint) {
        count += 1;
        return count;
    }
}

contract TestFallbackInputOutput {
    event Log(bytes res);

    function test(address _fallback, bytes calldata data) external {
        (bool ok, bytes memory res) = _fallback.call(data);
        require(ok, "call failed");
        emit Log(res);
    }

    function getTestData() external pure returns (bytes memory, bytes memory) {
        return (abi.encodeCall(Counter.get, ()), abi.encodeCall(Counter.inc, ()));
    }
}