All 10 Undefined Behavior questions
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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? Why is signed integer overflow undefined but unsigned overflow defined? 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

What is undefined behavior (UB) in C++?

Beginner

Answer

Undefined behavior is program behavior the C++ standard places no requirements on—anything can happen: a crash, wrong results, or seemingly working code that breaks later or under optimization. The compiler may assume UB never occurs and optimize accordingly. UB arises from invalid operations like out-of-bounds access, use-after-free, or signed overflow.
int a[3]; a[5] = 1;   // out-of-bounds: undefined behavior
Real-world example Code that 'worked' in debug crashes in release because the optimizer exploited an assumption of no UB.

Common follow-ups: Can the compiler assume UB never happens? | Give an example of UB.

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All 10 Undefined Behavior questions