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What is undefined behavior (UB) in C++? What is the difference between undefined, unspecified, and implementation-defined 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

Why is reading an uninitialized variable undefined behavior?

Beginner

Answer

An uninitialized local variable holds an indeterminate value; reading it is UB because its bits are arbitrary and, for some types, may not even be a valid value. The program may appear to work by luck, then fail unpredictably. Always initialize variables (e.g., int x = 0; or brace-init) before use.
int x;
if (x > 0) { /* UB: x is uninitialized */ }
Real-world example An uninitialized flag intermittently enables a code path, causing a bug that only appears on some runs.

Common follow-ups: How do you avoid it? | Why can it appear to work?

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All 10 Undefined Behavior questions