All 10 Undefined Behavior questions
More in Undefined Behavior
What is undefined behavior (UB) in C++? 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? 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 are common causes of undefined behavior?

Intermediate

Answer

Common UB: out-of-bounds array/container access, dereferencing null or dangling pointers, use-after-free/double-free, reading uninitialized variables, signed integer overflow, data races (unsynchronized concurrent access), violating strict aliasing, invalid downcasts with static_cast, and returning references to locals. Many are memory- or lifetime-related.
int x; std::cout << x;   // reading uninitialized value: UB
Real-world example A data race between two threads on the same variable is UB and produces intermittent, hard-to-reproduce bugs.

Common follow-ups: Why is a data race UB? | Name several sources of UB.

Memory Management Concurrency Undefined Behavior
All 10 Undefined Behavior questions