#ifdef

A C preprocessor switch that includes or excludes code depending on a compile-time flag.

#ifdef ('if defined') tells the C/C++ compiler to keep a block of code only when a named symbol is set, which is the classic way to write one source file that compiles differently per platform. Overusing it scatters an #ifdef for every operating system through the code, so good abstractions hide those differences behind a common interface instead. It runs before compilation, choosing which text the compiler even sees.