Require vs Assert vs Revert in Solidity: Key Differences and Usage

require checks conditions on inputs or external calls and refunds unused gas on failure, assert checks for internal errors and uses all gas on failure, while revert explicitly stops execution and refunds gas with a custom error message.

Quick Comparison

Here is a quick table comparing require, assert, and revert in Solidity based on key factors.

Factor	require	assert	revert
Purpose	Validate inputs and external calls	Check for internal errors	Explicitly stop execution with error
Gas Refund	Refunds unused gas	Consumes all gas	Refunds unused gas
Use Case	User input validation	Detect bugs and invariants	Custom error handling
Error Message	Supports custom messages	No custom messages	Supports custom messages
When to Use	Check conditions before execution	Check for impossible states	Replace require for complex logic

Key Differences

require is used to validate conditions like user inputs or external contract calls. If the condition fails, it stops execution, reverts state changes, and refunds unused gas. It also allows a custom error message to explain the failure.

assert is meant for checking internal errors or invariants that should never fail if the contract is correct. When an assert fails, it consumes all remaining gas and signals a serious bug. It does not support custom error messages.

revert explicitly stops execution and reverts state changes like require, but it is more flexible. It is often used for complex error handling or to replace require when multiple conditions or detailed error messages are needed. It also refunds unused gas.

Code Comparison

solidity

pragma solidity ^0.8.0;

contract Example {
    function checkRequire(uint x) public pure returns (string memory) {
        require(x > 10, "x must be greater than 10");
        return "Passed require check";
    }
}

Output

If x <= 10, transaction reverts with error 'x must be greater than 10'. Otherwise, returns 'Passed require check'.

Revert Equivalent

solidity

pragma solidity ^0.8.0;

contract Example {
    function checkRevert(uint x) public pure returns (string memory) {
        if (x <= 10) {
            revert("x must be greater than 10");
        }
        return "Passed revert check";
    }
}

Output

If x <= 10, transaction reverts with error 'x must be greater than 10'. Otherwise, returns 'Passed revert check'.

When to Use Which

Choose require when you want to validate user inputs or external calls and want to provide clear error messages with gas refund. Use assert to check for conditions that should never fail, like internal bugs or invariants, since it uses all gas on failure signaling a critical error. Use revert when you need flexible error handling with custom messages, especially for complex logic or multiple checks where require might be less clear.

Key Takeaways

require is for input validation and refunds unused gas on failure.

assert checks for internal errors and consumes all gas if it fails.

revert explicitly stops execution with custom errors and refunds gas.

Use require for user errors, assert for bugs, and revert for complex error handling.