Preprocessor & Macros

Before the C++ compiler sees a single line of code, a different program runs over the source file and rewrites it. That program is the preprocessor, and the directives that drive it (#include, #define, #undef, and the conditional family) operate as pure text substitution, with no awareness of types, scopes, or even C++ syntax. This chapter covers what the preprocessor does, how #define builds object-like and function-like macros, the stringize (#) and token-paste (##) operators, the classic macro pitfalls, and why modern C++ replaces most macros with constexpr, inline, and enum class.