异常

try-catch

try / catch只能捕获来自外部函数调用和合约创建的错误。

try / catch can only catch errors from external function calls and contract creation.

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

// External contract used for try / catch examples
contract Foo {
    address public owner;

    constructor(address _owner) {
        require(_owner != address(0), "invalid address");
        assert(_owner != 0x0000000000000000000000000000000000000001);
        owner = _owner;
    }

    function myFunc(uint x) public pure returns (string memory) {
        require(x != 0, "require failed");
        return "my func was called";
    }
}

contract Bar {
    event Log(string message);
    event LogBytes(bytes data);

    Foo public foo;

    constructor() {
        // This Foo contract is used for example of try catch with external call
        foo = new Foo(msg.sender);
    }

    // Example of try / catch with external call
    // tryCatchExternalCall(0) => Log("external call failed")
    // tryCatchExternalCall(1) => Log("my func was called")
    function tryCatchExternalCall(uint _i) public {
        try foo.myFunc(_i) returns (string memory result) {
            emit Log(result);
        } catch {
            emit Log("external call failed");
        }
    }

    // Example of try / catch with contract creation
    // tryCatchNewContract(0x0000000000000000000000000000000000000000) => Log("invalid address")
    // tryCatchNewContract(0x0000000000000000000000000000000000000001) => LogBytes("")
    // tryCatchNewContract(0x0000000000000000000000000000000000000002) => Log("Foo created")
    function tryCatchNewContract(address _owner) public {
        try new Foo(_owner) returns (Foo foo) {
            // you can use variable foo here
            emit Log("Foo created");
        } catch Error(string memory reason) {
            // catch failing revert() and require()
            emit Log(reason);
        } catch (bytes memory reason) {
            // catch failing assert()
            emit LogBytes(reason);
        }
    }
}

例子2

在solidity中，try-catch只能被用于external函数或创建合约时constructor（被视为external函数）的调用。基本语法如下：

try externalContract.f() {
    // call成功的情况下 运行一些代码
} catch {
    // call失败的情况下 运行一些代码
}

其中externalContract.f()是某个外部合约的函数调用，try模块在调用成功的情况下运行，而catch模块则在调用失败时运行。

同样可以使用this.f()来替代externalContract.f()，this.f()也被视作为外部调用，但不可在构造函数中使用，因为此时合约还未创建。

如果调用的函数有返回值，那么必须在try之后声明returns(returnType val)，并且在try模块中可以使用返回的变量；如果是创建合约，那么返回值是新创建的合约变量。

try externalContract.f() returns(returnType val){
    // call成功的情况下 运行一些代码
} catch {
    // call失败的情况下 运行一些代码
}

另外，catch模块支持捕获特殊的异常原因：

try externalContract.f() returns(returnType){
    // call成功的情况下 运行一些代码
} catch Error(string memory reason) {
    // 捕获失败的 revert() 和 require()
} catch (bytes memory reason) {
    // 捕获失败的 assert()
}

1.try-catch实战:

1-1.OnlyEven

我们创建一个外部合约OnlyEven，并使用try-catch来处理异常：

contract OnlyEven{
    constructor(uint a){
        require(a != 0, "invalid number");
        assert(a != 1);
    }

    function onlyEven(uint256 b) external pure returns(bool success){
        // 输入奇数时revert
        require(b % 2 == 0, "Ups! Reverting");
        success = true;
    }
}

OnlyEven合约包含一个构造函数和一个onlyEven函数。

• 构造函数有一个参数a，当a=0时，require会抛出异常；当a=1时，assert会抛出异常；其他情况均正常。
• onlyEven函数有一个参数b，当b为奇数时，require会抛出异常。

1-2.处理外部函数调用异常

首先，在TryCatch合约中定义一些事件和状态变量：

// 成功event
event SuccessEvent();

// 失败event
event CatchEvent(string message);
event CatchByte(bytes data);

// 声明OnlyEven合约变量
OnlyEven even;

constructor() {
    even = new OnlyEven(2);
}

SuccessEvent是调用成功会释放的事件，而CatchEvent和CatchByte是抛出异常时会释放的事件，分别对应require/revert和assert异常的情况。even是个OnlyEven合约类型的状态变量。

然后我们在execute函数中使用try-catch处理调用外部函数onlyEven中的异常：

// 在external call中使用try-catch
function execute(uint amount) external returns (bool success) {
    try even.onlyEven(amount) returns(bool _success){
        // call成功的情况下
        emit SuccessEvent();
        return _success;
    } catch Error(string memory reason){
        // call不成功的情况下
        emit CatchEvent(reason);
    }
}

1.error是solidity 0.8.4版本新加的内容

方便且高效（省gas）地向用户解释操作失败的原因，同时还可以在抛出异常的同时携带参数，帮助开发者更好地调试。

错误将撤消事务期间对状态所做的所有更改。 require您可以通过调用,revert或 来引发错误assert。

• require用于在执行之前验证输入和条件。
• revert类似于require。详情请参阅下面的代码。
• assert用于检查不应该为假的代码。断言失败可能意味着存在错误。

An error will undo all changes made to the state during a transaction.

You can throw an error by calling require, revert or assert.

• require is used to validate inputs and conditions before execution.
• revert is similar to require. See the code below for details.
• assert is used to check for code that should never be false. Failing assertion probably means that there is a bug.

使用自定义错误来节省gas。

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

contract Error {
    function testRequire(uint _i) public pure {
        // Require should be used to validate conditions such as:
        // - inputs
        // - conditions before execution
        // - return values from calls to other functions
        require(_i > 10, "Input must be greater than 10");
    }

    function testRevert(uint _i) public pure {
        // Revert is useful when the condition to check is complex.
        // This code does the exact same thing as the example above
        if (_i <= 10) {
            revert("Input must be greater than 10");
        }
    }

    uint public num;

    function testAssert() public view {
        // Assert should only be used to test for internal errors,
        // and to check invariants.

        // Here we assert that num is always equal to 0
        // since it is impossible to update the value of num
        assert(num == 0);
    }

    // custom error
    error InsufficientBalance(uint balance, uint withdrawAmount);

    function testCustomError(uint _withdrawAmount) public view {
        uint bal = address(this).balance;
        if (bal < _withdrawAmount) {
            revert InsufficientBalance({balance: bal, withdrawAmount: _withdrawAmount});
        }
    }
}

这是另一个例子

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

contract Account {
    uint public balance;
    uint public constant MAX_UINT = 2 ** 256 - 1;

    function deposit(uint _amount) public {
        uint oldBalance = balance;
        uint newBalance = balance + _amount;

        // balance + _amount does not overflow if balance + _amount >= balance
        require(newBalance >= oldBalance, "Overflow");

        balance = newBalance;

        assert(balance >= oldBalance);
    }

    function withdraw(uint _amount) public {
        uint oldBalance = balance;

        // balance - _amount does not underflow if balance >= _amount
        require(balance >= _amount, "Underflow");

        if (balance < _amount) {
            revert("Underflow");
        }

        balance -= _amount;

        assert(balance <= oldBalance);
    }
}

人们可以在contract之外定义异常。下面，我们定义一个TransferNotOwner异常，当用户不是代币owner的时候尝试转账，会抛出错误：

error TransferNotOwner(); // 自定义error

我们也可以定义一个携带参数的异常，来提示尝试转账的账户地址

error TransferNotOwner(address sender); // 自定义的带参数的error

2.在执行当中，error必须搭配revert（回退）命令使用。

我们定义了一个transferOwner1()函数，它会检查代币的owner是不是发起人，如果不是，就会抛出TransferNotOwner异常；如果是的话，就会转账。

function transferOwner1(uint256 tokenId, address newOwner) public {
    if(_owners[tokenId] != msg.sender){
        revert TransferNotOwner();
        // revert TransferNotOwner(msg.sender);
    }
    _owners[tokenId] = newOwner;
}

3.Assert

assert命令一般用于程序员写程序debug，因为它不能解释抛出异常的原因（比require少个字符串）。它的用法很简单，assert(检查条件），当检查条件不成立的时候，就会抛出异常。

我们用assert命令重写一下上面的transferOwner函数：

function transferOwner3(uint256 tokenId, address newOwner) public {
    assert(_owners[tokenId] == msg.sender);
    _owners[tokenId] = newOwner;
}