Require, assert, and revert in Blockchain / Solidity

Introduction

These commands help check if things are right in your smart contract. If something is wrong, they stop the contract from running and can show a message.

Check if a user has enough money before buying something.

Make sure a number is not zero before dividing.

Stop the contract if an unexpected error happens.

Verify that only the owner can change important settings.

Syntax

Blockchain / Solidity

require(condition, "error message");
assert(condition);
revert("error message");

require checks conditions and shows a message if false.

assert checks for errors that should never happen.

Examples

Stops if balance is less than amount, showing a message.

Blockchain / Solidity

require(balance >= amount, "Not enough balance");

Checks that total supply equals sum of balances, stops if not.

Blockchain / Solidity

assert(totalSupply == balancesSum);

Stops and shows message if caller is not owner.

Blockchain / Solidity

if (msg.sender != owner) {
    revert("Only owner can call this");
}

Sample Program

This contract lets users deposit and withdraw money. It uses require to check if users have enough money before withdrawing. It uses assert to make sure only the owner can do emergency withdrawal. It uses revert to stop anyone but the owner from calling a special function.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract SimpleBank {
    mapping(address => uint) public balances;
    address public owner;

    constructor() {
        owner = msg.sender;
    }

    function deposit() public payable {
        balances[msg.sender] += msg.value;
    }

    function withdraw(uint amount) public {
        require(balances[msg.sender] >= amount, "Insufficient balance");
        balances[msg.sender] -= amount;
        payable(msg.sender).transfer(amount);
    }

    function emergencyWithdraw() public {
        assert(msg.sender == owner);
        // Owner can withdraw all funds in emergency
        payable(owner).transfer(address(this).balance);
    }

    function onlyOwnerFunction() public {
        if (msg.sender != owner) {
            revert("Only owner allowed");
        }
        // Some owner-only logic here
    }
}

Important Notes

require refunds unused gas and returns your error message.

assert is for checking things that should never fail; if it fails, it uses all gas.

revert lets you stop execution and show a message anywhere in your code.

Summary

require checks inputs and conditions, stops with a message if false.

assert checks for bugs, stops if something impossible happens.

revert stops execution and shows a message, useful for custom checks.

Practice

1. Which statement best describes the purpose of require in Solidity smart contracts?

easy

A. It checks user inputs or conditions and stops execution with an error message if the condition is false.

B. It is used to check for internal errors that should never happen in the code.

C. It immediately stops execution without any error message.

D. It logs information but does not stop the contract execution.

Solution

1. Step 1: Understand the role of require

   require is used to validate inputs or conditions before continuing execution.

2. Step 2: Compare with other statements

   assert checks internal errors, and revert stops execution with a message, but require is specifically for input validation.

3. Final Answer:

   It checks user inputs or conditions and stops execution with an error message if the condition is false. -> Option A

4. Quick Check:

   require = input check with error [OK]

Hint: Require is for input checks and stops if false [OK]

Common Mistakes:

• Confusing require with assert for internal errors
• Thinking require does not stop execution
• Believing require logs info without stopping

2. Which of the following is the correct syntax to use assert in Solidity?

easy

A. assert(condition);

B. assert(condition, "Error message");

C. assert {condition};

D. assert condition;

Solution

1. Step 1: Recall assert syntax

   In Solidity, assert takes only a condition without an error message.

2. Step 2: Check each option

   assert(condition, "Error message"); wrongly includes an error message, C and D have invalid syntax.

3. Final Answer:

   assert(condition); -> Option A

4. Quick Check:

   assert syntax = assert(condition); [OK]

Hint: Assert takes only a condition, no message [OK]

Common Mistakes:

• Adding error messages to assert
• Using braces or missing parentheses
• Writing assert as a statement without parentheses

3. What will happen when the following Solidity code runs?

function test(uint x) public {
    require(x > 10, "x must be greater than 10");
    assert(x != 15);
    if (x == 20) {
        revert("x cannot be 20");
    }
}

Input: x = 15

medium

A. Execution stops at require with error 'x must be greater than 10'.

B. Execution stops at revert with error 'x cannot be 20'.

C. Execution stops at assert because x equals 15.

D. Function completes successfully without errors.

Solution

1. Step 1: Check require condition

   Input x=15 is greater than 10, so require passes.

2. Step 2: Check assert condition

   assert(x != 15) fails because x is 15, so execution stops here.

3. Final Answer:

   Execution stops at assert because x equals 15. -> Option C

4. Quick Check:

   assert fails if condition false [OK]

Hint: Assert fails if condition is false, stopping execution [OK]

Common Mistakes:

• Thinking require fails for x=15
• Assuming revert triggers for x=15
• Believing function runs fully without errors

4. Identify the error in this Solidity function:

function check(uint value) public {
    require(value > 0, "Value must be positive");
    assert(value < 100, "Value too large");
}

medium

A. The require statement is missing a message string.

B. There is no error; the code is correct.

C. The function should use revert instead of require.

D. The assert statement incorrectly includes an error message.

Solution

1. Step 1: Review require usage

   Require correctly uses a condition and an error message.

2. Step 2: Review assert usage

   Assert does not accept an error message; including one causes a syntax error.

3. Final Answer:

   The assert statement incorrectly includes an error message. -> Option D

4. Quick Check:

   Assert has no message parameter [OK]

Hint: Assert never takes error messages, only conditions [OK]

Common Mistakes:

• Adding messages to assert
• Confusing require and revert usage
• Assuming no error in assert syntax

5. You want to write a Solidity function that checks if a user has enough balance to withdraw an amount. If the balance is insufficient, it should stop with a clear error message. If an unexpected internal error occurs, it should stop immediately without a message. Which combination of require, assert, and revert is best to use?

hard

A. Use assert for balance check and require for internal errors.

B. Use require to check balance and assert for internal errors; no need for revert.

C. Use revert to check balance with message and assert for internal errors.

D. Use only revert for both balance check and internal errors.

Solution

1. Step 1: Identify correct use of require

   Require is ideal for checking user inputs like balance with error messages.

2. Step 2: Identify correct use of assert

   Assert is for internal errors that should never happen, stopping immediately without messages.

3. Step 3: Consider revert usage

   Revert can be used for complex checks but is not necessary here if require and assert cover cases.

4. Final Answer:

   Use require to check balance and assert for internal errors; no need for revert. -> Option B

5. Quick Check:

   Require for inputs, assert for internal errors [OK]

Hint: Require for inputs, assert for internal errors, revert optional [OK]

Common Mistakes:

• Using assert for input validation
• Using revert unnecessarily for simple checks
• Mixing assert and require roles