Preprocessor

In Graphics, preprocessors are the directives to tell the compiler before turning them into codes.

#include "file.h" essentially copy-pastes that file to here. That is why we separate the codes into files. We don't want extra copies of a function.

#ifdef _DEBUG and #endif also exclude that block when compiling the production version.

#define CONSTANT also copy-pastes.

Rule of thumb — only #include the headers you necessarily need. Or, if you make an everything.h, precompile it!