proposifdef

proposifdef is a preprocessor directive used in some C and C++ compilers, notably in older or embedded systems environments. It functions similarly to the more common #ifdef directive but is used to define a macro only if a preceding macro has not been defined.

The syntax is #proposifdef MACRO_NAME. If MACRO_NAME has not been defined by any prior #define directives or compiler flags, the code block that follows until a #endif or #else directive is processed. If MACRO_NAME has been defined, the code block is skipped.

This directive offers a way to conditionally compile code based on the *absence* of a defined symbol. It can be useful in situations where you want to provide a default implementation or configuration unless a specific feature or setting is explicitly enabled. For instance, a library might use #proposifdef to enable an experimental feature only if a particular macro is *not* defined, suggesting it's off by default.

While functional, #proposifdef is less frequently encountered in modern C++ development compared to #ifdef, #ifndef, #if, and #else. Its usage is often associated with legacy codebases or specific toolchains where it was a standard feature. Developers working with such systems may need to understand its behavior for maintenance or modification.