Conditional Compilation

Before the compiler ever sees your code, the preprocessor runs. It handles every line starting with #: includes, macros, and conditionals.

Conditional compilation lets you include or exclude blocks of code based on conditions evaluated at this stage, producing different builds from one source.

#define creates a macro. With no value it is just a flag you can test later.

Object-like macros replace a name with text; flags simply mark that something is set.