Solidity - contract structure - error - ^0.8.4 NEW

We know , The contract structure includes   State variables 、 function 、 Function modifier （modifier）、 event （event）、 Structure （struct） And enumeration types （enum）.

stay ^0.8.4 edition , The contract structure has increased error （error）, In response to failure , Mistakes can be made in revert Use in . Compared with the error string ,error It costs less gas（ That is, cheaper ）, And allow encoding of additional data , You can also use natspec Annotation form .

Take a chestnut , The code is as follows ：

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4; 

/// @dev  Not enough money 
/// @param _balance Indicates the current contract account balance 
/// @param _amount  Transfer amount 
error NotEnoughFunds(uint _balance,uint _amount);

contract TestError {
    /// @dev  event  -  Record transfer information 
    event Tansfer(address _from, address _to, uint _amount);

    /// @dev  When the balance of the contract account is less than the transfer amount , Use string  revert
    function transfer(address _to, uint _amount) external {
         if (address(this).balance < _amount) {
            revert("no enough funds");
        }
        emit Tansfer(msg.sender, _to, _amount);
    }

    /// @dev  When the balance of the contract account is less than the transfer amount , Use error revert
    function transferWithError(address _to, uint _amount) external {
        if (address(this).balance < _amount) {
            revert NotEnoughFunds(address(this).balance, _amount);
        }
        emit Tansfer(msg.sender, _to, _amount);
    }
}

error Same as struct equally , It can be written in the contract , It can also be written outside the contract , The above code is written outside the contract . At the same time natspec Annotation form . 

Use Remix in JavaScript VM Deployment contract , Execute two functions in the contract （ A use error structure , The other uses the wrong string ）, Cost when validation fails gas：

（1） perform transfer function （ Use the wrong string ）

After execution, you can see the error message prompt “no enough funds”, Spent gas by 22513

（2） perform transferWithError function （ Use error structure ）

After execution, you can see the parameter value output in the error , Feeling and front-end console.log equally , It can be used as debugging information , Spent gas by 22497, Less than above 16 individual gas

If deployed to the test network , Such as Popsten, Executing the function will directly prompt （ As shown in the figure below ）, If the transaction is forced , The result is the same as the above .