Notes

Thoughts, write-ups, and things worth remembering.

Preprocessor directives

#cpp#compilation Updated Sep 16, 2026

Runs before the compiler ever sees real C++. Pure text substitution — the preprocessor doesn’t know what a type is, what scope is, or what a function is.

#define — text substitution, not a constant

#define MAX_HEALTH 100
#define SQUARE(x) x * x

int a = SQUARE(2 + 3);   // expands to 2 + 3 * 2 + 3 = 11, NOT 25

Function-like macros paste their arguments in verbatim, no precedence protection. Wrap every parameter (and the whole expression) in parens: #define SQUARE(x) ((x) * (x)). Better yet, use a constexpr function instead and skip macros for this entirely.

Include guards

#ifndef NODE_H
#define NODE_H
// ... contents ...
#endif

Stops a header from being pasted into the same file twice, which would redefine every class/function in it and fail to compile. NODE_H is just a name you pick — convention is <FILENAME>_H.

#pragma once

Same job, one line, nothing to name or accidentally collide with. Not technically standard C++, but every compiler you’ll actually use supports it. Prefer this for new code.

Conditional compilation

#ifdef _DEBUG
    logDetailedState();
#endif

#if VERSION >= 2
    newFeature();
#else
    oldFeature();
#endif

Code inside a false branch isn’t just skipped at runtime — it’s never even compiled. Useful for platform-specific paths or stripping debug-only logic from release builds entirely.

Predefined macros

__FILE__, __LINE__, __DATE__, __TIME__, __cplusplus (tells you which standard is active) — the compiler fills these in for you. Handy for assert messages and version checks.

Gotchas

Macros ignore scope and namespaces — a dumb find-and-replace across the whole file. A macro named min will silently wreck any function or variable also named min.
No type checking, no debugger stepping into them, and a function-like macro can evaluate an argument more than once (SQUARE(x++) increments x twice).