definedMACRO

definedMACRO is not a formal language feature with a fixed definition, but a term used to describe the practice of using preprocessor macros to detect whether another macro is defined. In languages with a C-style preprocessor, this detection is performed with the defined operator in conditional compilation directives.

The most common syntax uses #if defined(MACRO) or its equivalent #ifdef MACRO. These checks evaluate at compile time and do not inspect the value of the macro, only its presence. For example, code inside a block guarded by #if defined(HAVE_LIBXML) will be compiled only if HAVE_LIBXML has been defined, regardless of any value it might have. Tests can be combined with other conditions, such as #if defined(HAVE_LIBXML) && HAVE_LIBXML >= 2, to incorporate both presence and value when a macro expands to a numeric or boolean-like value.

Use cases for definedMACRO patterns include platform-specific optimizations, feature toggles, and enabling or disabling support for optional libraries. Build systems frequently generate configuration headers (for example, config.h or feature.h) that define macros like HAVE_OPENSSL, HAVE_ZLIB, or PLATFORM_WINDOWS. Source files then use defined checks to select appropriate code paths without pulling in runtime checks.

Limitations include that the defined operator is a preprocessor construct and only reflects the state of macros in a given translation unit. It does not convey macro values automatically, and inconsistent macro definitions across headers can lead to non-portable code. Proper documentation and centralized configuration headers helpManage these checks across a codebase.

See also: C preprocessor, conditional compilation, macro definitions, feature detection.