【Fallback】
fallback
是一個沒有任何參數也不會回傳任何值的函數
他在以下情況會被執行:
當一筆非由呼叫函式引起的交易被送往合約時
當呼叫的函式不存在且
fallback
函式存在時合約直接匯出 ether 但是
receive()
不存在或msg.data
非為空時
若是合約接收到了ether卻沒有任何的Fallback Function也沒有任何呼叫函式的行為,將會Throw Exception
fallback
有 2300 gas 限制
pragma solidity ^0.8.10;
contract Fallback {
event Log(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(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 Function 有以下的特性:
可視性只能為
external
當我們沒有任何
payable
的 Function 符合調用,那就會觸發例外處理(exception),除非我們擁有 Fallback functionFallback Function 就像
catch
,當我們沒有和任何payable function互動,或沒有任何函式符合交易的encoded data field,就會觸發
pragma solidity ^0.8.11;
contract FunctionsExample{
mapping(address => uint) public balanceReceived;
address payable owner;
constructor() public {
owner = payable(msg.sender);
}
function getOwner () public view returns(address){
return owner;
}
function convertWeiToEther(uint _amountInWei) public pure returns(uint){
return _amountInWei / 1 ether;
// alternatively:
// return _amountInWei / 10**18;
// pure function call only interacte with the variables in this scope like _amountInWei, but not the state variables outside the scope
}
function destroyContract() public {
require(msg.sender == owner, "You are not the owner");
selfdestruct(owner);
}
function receiveMoney() public payable {
assert(balanceReceived[msg.sender] + msg.value >= balanceReceived[msg.sender]);
balanceReceived[msg.sender] += msg.value;
}
function withdrawMoney(address payable _to, uint _amount) public {
require(balanceReceived[msg.sender] <= _amount, "You don't have enough ether");
assert(balanceReceived[msg.sender] >= balanceReceived[msg.sender] - _amount);
balanceReceived[msg.sender] -= _amount;
_to.transfer(_amount);
}
fallback () external payable{
receiveMoney();
// fallback function will have input fill(in remix IDE) even we didn't declare any input arguments in function
// because the fallback function is triggered automatically no matter have arguments or not.
// arguments data is in msg.data
}
}
Fallback Function存在最大的原因是我們沒辦法避免接收ether,常見情況下我們有至少三種的方式可以接收來自外部的ether:
當我們呼叫了
selfdistructor
解構其他合約並把自己的合約地址當作參數傳入挖礦,我們將智能合約的地址設為礦工地址
在智能合約被部屬之前就先將ether傳至其地址(機率不高但有可能發生)
最糟的情況下,我們還可以依賴2300 gas的限制:
_contractAddress.trasfer(1 ether);
當Contract Data被呼叫時使其強制地避免函式執行
require(msg.data.length == 0)
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