Undefined Behavior / UB

CORE DEFINITION

In computer programming, undefined behavior refers to the result of executing certain computer code whose behavior, in the current program state, is not specified by the language standard. It commonly occurs when a translator (compiler) makes certain assumptions about the source code, and these assumptions do not hold at runtime. Some programming languages have undefined behavior in certain situations, most notably C and C++. In the standards of these languages, the semantics of certain operations are defined as undefined, typical examples being program errors such as out-of-bounds array access. The standard allows a specific implementation of the language to assume that any conforming program will not exhibit such behavior. Specifically in C/C++, the compiler may optionally issue a diagnostic, but it is not required to: for undefined behavior, any reaction by the language implementation is considered correct, similar to don't-care terms in digital logic. Although compiler implementations may issue diagnostics for undefined behavior, it is the programmer's responsibility to ensure that the code they write does not trigger undefined behavior.