Require, assert, and revert in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the require statement do in a smart contract?

require checks a condition and stops execution if the condition is false. It refunds remaining gas and returns an error message.

intermediate

How is assert different from require?

assert is used to check for conditions that should never be false. If it fails, it consumes all remaining gas and indicates a serious error.

beginner

What happens when revert is called in a smart contract?

revert immediately stops execution, undoes all changes, and refunds remaining gas. It can include a custom error message.

intermediate

When should you use require versus assert?

Use require to check user inputs or external conditions. Use assert to check for internal errors or invariants that should never fail.

advanced

Why is gas refunded when require or revert fails but not when assert fails?

require and revert indicate expected errors and refund gas to save users money. assert signals a critical bug, so it consumes all gas to highlight the problem.

Which statement immediately stops execution and refunds remaining gas if a condition is false?

A continue

B assert

C revert

D require

require stops execution and refunds gas when its condition is false.

Which statement is used to check for conditions that should never happen and consumes all gas if it fails?

A assert

B require

C revert

D throw

assert is for internal errors and consumes all gas on failure.

What does revert do in a smart contract?

A Continues execution ignoring errors

B Stops execution and refunds gas with an optional error message

C Consumes all gas and stops execution

D Logs a warning but continues

revert stops execution, undoes changes, and refunds gas with an error message.

When should you use require in your smart contract?

A To check user inputs and external conditions

B To check internal invariants

C To consume all gas on error

D To log events

require is for validating inputs and external conditions.

Why does assert consume all remaining gas on failure?

A Because it is used for user input validation

B Because it refunds gas to the user

C Because it signals a critical bug that should never happen

D Because it logs an error

assert failures indicate serious bugs, so all gas is consumed to highlight the issue.

Explain the differences between require, assert, and revert in smart contracts.

Think about when each is used and how gas is handled.

When should you use require instead of assert in your code?

Consider the source of the error.

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