What is undefined behavior in C++? Give examples.

Undefined behavior (UB) is code for which the standard gives no rules. The compiler may assume it never happens, so the result is anything: a crash, a wrong answer, or “it works on my machine”.

• Reading a variable which is not initialized.
• An array index out of range, or dereferencing nullptr.
• Signed integer overflow (unsigned overflow is defined: it wraps).
• Using an object after it is deleted (dangling pointer).
• Two threads changing the same data without a lock (data race).
• Modifying a const object, or a missing return in a function which returns a value.

Tools find UB while testing. We compile this code with the undefined behavior sanitizer:

#include <climits>
#include <iostream>

int main() {
    int x = INT_MAX;
    x = x + 1;                          // signed overflow: undefined behavior
    std::cout << x << '\n';
}

Use -fsanitize=address,undefined and -Wall -Wextra in every test build.

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