Preprocessor Macros

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\( \newcommand{\P}[]{\unicode{xB6}} \newcommand{\AA}[]{\unicode{x212B}} \newcommand{\empty}[]{\emptyset} \newcommand{\O}[]{\emptyset} \newcommand{\Alpha}[]{Α} \newcommand{\Beta}[]{Β} \newcommand{\Epsilon}[]{Ε} \newcommand{\Iota}[]{Ι} \newcommand{\Kappa}[]{Κ} \newcommand{\Rho}[]{Ρ} \newcommand{\Tau}[]{Τ} \newcommand{\Zeta}[]{Ζ} \newcommand{\Mu}[]{\unicode{x039C}} \newcommand{\Chi}[]{Χ} \newcommand{\Eta}[]{\unicode{x0397}} \newcommand{\Nu}[]{\unicode{x039D}} \newcommand{\Omicron}[]{\unicode{x039F}} \DeclareMathOperator{\sgn}{sgn} \def\oiint{\mathop{\vcenter{\mathchoice{\huge\unicode{x222F}\,}{\unicode{x222F}}{\unicode{x222F}}{\unicode{x222F}}}\,}\nolimits} \def\oiiint{\mathop{\vcenter{\mathchoice{\huge\unicode{x2230}\,}{\unicode{x2230}}{\unicode{x2230}}{\unicode{x2230}}}\,}\nolimits} \)

Some common preprocessor macros

Include Guard

For most modern compilers (MS Visual C++, GCC, Clang/llvm), you can just add a #pragma once at the top.
Note that this is not defined in the standard.
See Wikipedia: pragma once#Portability to see a list of compilers which support this.

For older compilers:

#ifndef GRANDPARENT_H
#define GRANDPARENT_H
... contents of grandparent.h
#endif /* !GRANDPARENT_H */

GLSL

#define PI 3.141592653589793
#define PI_2 1.5707963267948966
#define DEG2RAD 0.017453292519943295
#define RAD2DEG 57.29577951308232

#define sign(x) (int((x) > 0) - int((x) < 0))
#define imin(x, y) (int(x) * int(int(x) < int(y)) + int(y) * int(int(x) >= int(y)))
#define imax(x, y) (int(x) * int(int(x) > int(y)) + int(y) * int(int(x) <= int(y)))
#define iabs(x) ((x) * int((x) >= 0) - (x) * int((x) < 0))