Custom Errors · Revert Data · Ethereum & Contracts Deep Lesson 28

Pre-0.8.4 the only way to revert with context was require(cond, "long message"). The string is embedded in the contract bytecode and re-encoded into revert data each call, costing real gas. Custom errors compile to a 4-byte selector (like a function call) plus ABI-encoded args, dropping the bytes-on-revert from dozens to a fixed handful.

They also carry structured data: revert InsufficientBalance(uint256 needed, uint256 actual) can be decoded by the frontend to display rich error messages. The demo compares the bytes-on-revert of a require-string vs the equivalent custom error.

The Reentrancy Reaper attacks — common mistakes

Mixing custom errors with require-string throughout the codebase

Putting too much data in error args — they are still on-chain bytes

Forgetting custom errors need to be declared (interface or contract)

Assuming custom errors save gas in the success path (they only matter on revert)

Boss battleCompare the calldata-on-revert bytes for require("...long string...") vs revert CustomError(needed, actual).

Example code

<!doctype html><html><head><meta charset="utf-8"></head> <body style="background:#06040d;color:#e6e0ff;font-family:monospace;padding:20px"><pre id="o"></pre> <script> const errStr = 'ERC20: transfer amount exceeds balance'; const strLen = errStr.length; // require("string"): Error(string) selector + offset(32) + length(32) + ⌈len/32⌉*32 bytes of string data const stringDataBytes = Math.ceil(strLen / 32) * 32; const requireBytes = 4 + 32 + 32 + stringDataBytes; // custom error InsufficientBalance(uint256 needed, uint256 actual): 4 + 32 + 32 const customErrBytes = 4 + 32 + 32; const savings = requireBytes - customErrBytes; document.getElementById('o').textContent = [ 'message: "' + errStr + '" (' + strLen + ' chars)', '', 'require(cond, message) revert data:', ' 4 (Error selector) + 32 (offset) + 32 (length) + ' + stringDataBytes + ' (string) = ' + requireBytes + ' bytes', '', 'revert InsufficientBalance(uint256, uint256) revert data:', ' 4 (selector) + 32 (needed) + 32 (actual) = ' + customErrBytes + ' bytes', '', 'savings per revert: ' + savings + ' bytes (~' + Math.round(100*savings/requireBytes) + '%)', ' + structured args your frontend can decode' ].join('\n'); </script></body></html>

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custom errors · revert data

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The Reentrancy Reaper attacks — common mistakes

Example code