ifdef

ifdef is a preprocessor directive used in C and C++ to control conditional compilation. It tests whether a given macro name has been defined, and based on that, includes or excludes the following code until the matching #endif. The condition considers only the existence of the macro, not its value. If the macro is defined (even with a value like 0), the code between #ifdef MACRO and #endif is compiled; if it is not defined, that block is skipped.

Syntax and related forms:

#ifdef MACRO

// code included if MACRO is defined

#endif

You can also pair it with an #else to provide an alternative block:

#ifdef MACRO

// code if MACRO is defined

#else

// code if MACRO is not defined

#endif

Additionally, #ifndef MACRO and #if defined(MACRO) are equivalent to their counterparts:

#ifndef MACRO is the same as #if !defined(MACRO)

#ifdef MACRO is the same as #if defined(MACRO)

Common usage: include guards in header files to prevent multiple inclusion. For example:

#ifndef HEADER_H

#define HEADER_H

// header contents

#endif

Macros can be defined by the source code or supplied by the compiler command line (for example, using -DDEBUG). The #ifdef directive plays a central role in portable code, enabling or disabling features based on the build environment. Limitations include its lack of awareness of a macro’s value and potential complexity in large codebases; for value-based conditions, #if or #if defined(...) is preferable.