What is the difference between require and revert in Solidity?

require(condition, message) is syntactic sugar for if (!condition) revert(message). Both use the REVERT opcode and refund remaining gas. require is idiomatic for input validation, while explicit revert is preferred for complex conditional logic or custom errors.

Does assert still consume all gas in Solidity 0.8+?

No. Since Solidity 0.8, assert uses the REVERT opcode (instead of INVALID) and refunds remaining gas. It emits Panic(uint256) error data with a code indicating the failure type. Pre-0.8, assert consumed ALL remaining gas.

How much gas do custom errors save compared to require strings?

Custom errors typically save 50%+ on error data encoding because they use only 4 bytes for the selector plus ABI-encoded parameters, versus ~132+ bytes for Error(string) encoding. The savings increase with longer error messages.

Require / Revert / Assert

Explore Solidity error handling types — syntax, encoding, gas behavior, and usage guidance.

require()Input Validation Guard

1// Basic require
2require(msg.sender == owner);
3
4// With message
5require(amount > 0, "Amount must be positive");
6
7// Multi-condition
8require(balance >= amount, "Insufficient balance");
9require(block.timestamp >= unlockTime, "Too early");
10
11// Custom error (0.8.26+)
12require(amount > 0, InsufficientAmount());

require

Error(string) encoding:

08c379a0

0000...0020

0000...length

"Insufficient balance" (UTF-8 padded)

Error(string) selector

Offset to string data

String length in bytes

UTF-8 string data (padded to 32B)

Total: 132B

Properties

EVM OpcodeREVERT (0xfd)

SinceSolidity 0.4.22

Gas on FailureRefunds unused gas

Gas Overhead~200 gas overhead

Use CaseInput validation, access control, preconditions

BytecodeAdds REVERT opcode with Error(string) ABI encoding

Error DataABI-encoded Error(string) or none

Use require when:

•Validating function inputs and parameters
•Checking access control (msg.sender == owner)
•Verifying preconditions before state changes
•Checking external call return values

Don't use require for:

•Internal invariant checks (use assert)
•Complex error data (use custom errors)
•Gas-sensitive operations (string messages cost extra)

What is require/revert?

require() validates conditions and reverts with a message. revert() explicitly aborts execution. Both use the REVERT opcode and refund remaining gas.

Assert vs Require

require is for expected failures (bad input), assert is for bugs (broken invariants). Since 0.8, both refund gas, but assert signals a code defect.

Custom Errors

Introduced in 0.8.4, custom errors are gas-efficient alternatives to string messages. They use only 4 bytes for the selector plus ABI-encoded parameters.

Error Encoding

Error data is ABI-encoded: Error(string) for require/revert messages, Panic(uint256) for assert, and custom selectors for user-defined errors.

Fun Fact

Before Solidity 0.8.0, assert() consumed ALL remaining gas — making it essentially a gas bomb if triggered accidentally.

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