Solidity - truncated and checked modes of arithmetic operations - new features of 0.8.0

0.8.0 Version start , Arithmetic operation has two calculation modes , One is “wrapping”（ truncation ） Mode or “unchecked”（ Do not check ） Pattern , That is, in case of overflow “ truncation ”, Failure exception will not be triggered , Therefore, we have to introduce additional checklists to solve this problem （ Such as OpenZeppelin Medium SafeMath library ）; The other is "checked"（ Check ） Pattern . By default , Arithmetic operations use “checked” Pattern , Overflow check will be performed , If the result overflows , Failure exception fallback occurs .

Take a chestnut ：

uint8 The maximum value is 255,255 Add a greater than 0 The integer of will overflow （ Overflow ）, add unchecked{} Do not check for overflow , Overflow truncates the display ; Otherwise, the truncation mode is used by default to check for overflow , Abnormal fallback occurs .

test 0.8.0 Or above  

Test code

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.7; 

/// @dev  Example contract ：uint8 The maximum value is 255,255 Add a greater than 0 The integer of will overflow , add unchecked Does not check for overflow 
contract TestUncheck {  
    /// @dev  Check for spillage （checked Pattern ）
    function withoutUnchecked() external pure returns (uint8) {
        uint8 a = 255;
        uint8 b = 5;
        return (a + b);
    }

    /// @dev  Do not check for overflow （unchecked Mode, or truncation mode ）
    function withUnchecked() external pure returns (uint8) { 
        uint8 a = 255;
        uint8 b = 5;
        //  Do not check for overflow , Output overflow result 
        unchecked { 
            return (a + b);
        }
    }
}

Test check mode （checked） 

Check mode , Due to data overflow , Abnormal fallback occurs .

Test truncation mode  （unchecked or wrapping）

Truncation mode , although 255+5 Overflowed , But the overflow result is output 4.

unchecked  Code blocks can be used anywhere in a code block , But it can't replace the whole function code block , It can't be nested either .

This setting only affects the syntax at  unchecked  The statement within a block . The function invoked in the block does not affect this .

test 0.8.0 The previous version

0.8.0 The previous version was “wrapping”（ truncation ） Pattern , That is, in case of overflow “ truncation ”, Failure exception will not be triggered , Therefore, we have to introduce additional checklists to solve this problem （ Such as OpenZeppelin Medium SafeMath library ）

Test code  

Test a distance 0.8.0 The latest version 0.7.6, The contract code is as follows ：

// SPDX-License-Identifier: MIT
pragma solidity ^0.7.6; 

/// @dev  test 0.8.0 Previous version , Overflow is in truncation mode 
contract TestOverflow {  
    /// @dev  Do not check for overflow （wrapping Pattern ）
    function overflow() external pure returns (uint8) {
        uint8 a = 255;
        uint8 b = 5;
        return (a + b);
    }
}

test result （wrapping Pattern ）

Output truncated data , That is, do not check mode