undef

Undef, in the context of programming, refers most often to the #undef directive used in C and C++ style preprocessors. It removes a macro definition that was previously established with #define. The directive is written as #undef macro-name. After its appearance, the macro name ceases to be defined for the rest of the current translation unit, and subsequent uses of that name are no longer replaced by the macro’s replacement text. #undef is commonly used to prevent macro name collisions, to disable a macro that was temporarily enabled, or to allow conditional redefinition within complex header files.

Scope and behavior: Macro definitions in C and C++ exist only within a translation unit. #undef affects only the current translation unit; it cannot influence macros defined in other source files. It does not disable language keywords or other non-macro tokens. Practically, it is often used after a macro has served a specific purpose, or within include guards and feature-toggle patterns to manage macro hygiene and avoid unintended expansions.

Examples: Define a macro, use it, then undefine it.

#define MAX 1024

int a = MAX; // expands to 1024

#undef MAX

int b = MAX; // MAX is no longer defined; further use would not expand

Other uses: In other languages or contexts, undef can refer to the concept of an undefined value. In Perl, undef is a built-in function used to assign an undefined value to a variable or to clear elements, and defined() tests whether a value is defined. The general idea of “undefined” values also appears in many languages as nil, null, or undefined.

See also: #define, macro (preprocessor), preprocessor directives, defined (Perl).