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What is the difference between undefined, unspecified, and implementation-defined behavior? Why is reading an uninitialized variable undefined behavior? What happens if you dereference a null pointer? What are common causes of undefined behavior? How do you detect and prevent undefined behavior? What is the strict aliasing rule and how can violating it cause UB? How can undefined behavior cause 'impossible' optimizations and bugs? What is a data race and why is it undefined behavior?
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C++ › Undefined Behavior

Why is signed integer overflow undefined but unsigned overflow defined?

Intermediate

Answer

Unsigned overflow is defined to wrap modulo 2^n. Signed overflow is undefined because the standard doesn't fix a representation-independent result, and leaving it UB lets compilers optimize (e.g., assume x+1 > x). This means relying on signed wraparound is unsafe; use unsigned types or explicit checks when wraparound semantics are needed.
int x = INT_MAX; x + 1;   // signed overflow: UB
unsigned u = UINT_MAX; u + 1; // defined: wraps to 0
Real-world example An overflow check written as 'if (x + 1 < x)' is optimized away because signed overflow is UB.

Common follow-ups: How does unsigned overflow behave? | Why does UB enable optimization here?

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