unchecked in Solidity#

📖 Definition#

• The unchecked { ... } block disables automatic overflow and underflow checks introduced in Solidity 0.8.0+.
• Inside this block, arithmetic behaves like older Solidity versions:
  • Overflow wraps back to zero.
  • Underflow wraps back to the maximum value.

Purpose#

• Gas Optimization: Saves gas by skipping safety checks.
• Intentional Wraparound: Useful when cyclic behavior is desired (e.g., circular buffers, modulo arithmetic).

Example: With and Without unchecked#

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pragma solidity ^0.8.24;
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contract SafeExample {
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    uint8 public value = 255;
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    function increment() public {
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        value += 1;
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        //  Reverts: overflow detected
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    }
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}

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pragma solidity ^0.8.24;
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contract UncheckedExample {
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    uint8 public value = 255;
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    function increment() public {
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        unchecked {
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            value += 1;
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            //  Overflow wraps to 0
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        }
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    }
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}

Security Considerations#

• Using unchecked removes protections - errors won’t revert.

• Attackers could exploit wraparound to manipulate balances, counters, or logic.

• Should only be used when:

  • Gas savings are critical and
  • Logic guarantees safety (e.g., loops bounded by conditions).

Gas Optimization Example#

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pragma solidity ^0.8.24;
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contract LoopExample {
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    function sum(uint256 n) public pure returns (uint256 total) {
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        for (uint256 i = 0; i < n; ) {
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            total += i;
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            unchecked { i++; } // gas optimized increment
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        }
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    }
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}

• Without unchecked, Solidity inserts overflow checks in every i++.
• With unchecked, checks are skipped, saving gas.

Best Practices#

• Default to safe arithmetic - use unchecked only when necessary.
• Add comments explaining why unchecked is safe.
• Never use it in user balance updates or sensitive math unless wraparound is intentional.

Notes#

• unchecked = manual override of safety.
• Great for gas optimization, dangerous if misused.
• Always justify its usage in audits and documentation.