C++ | A Few Useful Preprocessor Directives

(#x): Viewing a variable’s name

How it works: (#x) can get the name of x; the reason is that (#x) tells the preprocessor to keep this content as-is and treat it as a string literal
Usage: watch(x) you don’t even need a semicolon

#define watch(x) std::cout << (#x) << ": " << x << std::endl;

__cplusplus: The C++ version number

How it works: __cplusplus defines the version number, which can be compared, e.g. #if __cplusplus > 201403L
Usage: combined with #if, this can avoid CE caused by some crappy OJs like POJ having too low a C++ version, as well as the WARNING caused by C++17 deprecating the register keyword

• Avoiding a WARNING when the version is too high

#if __cplusplus > 201403L
#define r
#else
#define r register
#endif

• Avoiding a CE when the version is too low

#if __cplusplus >= 201103L
template <typename T, typename... Args>
void read(T &x, Args &... args)
{
    read(x);
    read(args...);
}
#endif

#ifndef to avoid the CE caused by including the same header multiple times

How it works: conditional compilation, #ifndef meaning “if not defined”
Usage: you can refer to any header in the STL; here is an example from the <algorithm> header on my computer

// copyright notice trimmed

#ifndef _GLIBCXX_ALGORITHM
#define _GLIBCXX_ALGORITHM 1

#pragma GCC system_header

#include <utility> // UK-300.
#include <bits/stl_algobase.h>
#include <bits/stl_algo.h>

#ifdef _GLIBCXX_PARALLEL
# include <parallel/algorithm>
#endif

#endif

#pragma GCC optimize(2): forcing extra oxygen

How it works: a compiler directive that forces extra “oxygen” (aggressive optimization)
Usage: #pragma GCC optimize(whatever level of oxygen you want). Through experimentation, stacking O2, O3, and Ofast together has a curious effect.

• Example

#pragma GCC optimize(2) // oxygen
#pragma GCC optimize(3) // ozone
#pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math") // everything else

• Everyday constant-factor squeezing (卡常)

#pragma GCC optimize(2)
#pragma GCC optimize(3)
#pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4.1,sse4.2,avx,avx2,popcnt,tune=native")

• The “locomotive” (a well-known combo of all the flags at once)

#pragma GCC optimize(3)
#pragma GCC target("avx,sse2,sse3,sse4,mmx")
#pragma GCC optimize("Ofast")
#pragma GCC optimize("inline")
#pragma GCC optimize("-fgcse")
#pragma GCC optimize("-fgcse-lm")
#pragma GCC optimize("-fipa-sra")
#pragma GCC optimize("-ftree-pre")
#pragma GCC optimize("-ftree-vrp")
#pragma GCC optimize("-fpeephole2")
#pragma GCC optimize("-ffast-math")
#pragma GCC optimize("-fsched-spec")
#pragma GCC optimize("unroll-loops")
#pragma GCC optimize("-falign-jumps")
#pragma GCC optimize("-falign-loops")
#pragma GCC optimize("-falign-labels")
#pragma GCC optimize("-fdevirtualize")
#pragma GCC optimize("-fcaller-saves")
#pragma GCC optimize("-fcrossjumping")
#pragma GCC optimize("-fthread-jumps")
#pragma GCC optimize("-funroll-loops")
#pragma GCC optimize("-fwhole-program")
#pragma GCC optimize("-freorder-blocks")
#pragma GCC optimize("-fschedule-insns")
#pragma GCC optimize("inline-functions")
#pragma GCC optimize("-ftree-tail-merge")
#pragma GCC optimize("-fschedule-insns2")
#pragma GCC optimize("-fstrict-aliasing")
#pragma GCC optimize("-fstrict-overflow")
#pragma GCC optimize("-falign-functions")
#pragma GCC optimize("-fcse-skip-blocks")
#pragma GCC optimize("-fcse-follow-jumps")
#pragma GCC optimize("-fsched-interblock")
#pragma GCC optimize("-fpartial-inlining")
#pragma GCC optimize("no-stack-protector")
#pragma GCC optimize("-freorder-functions")
#pragma GCC optimize("-findirect-inlining")
#pragma GCC optimize("-fhoist-adjacent-loads")
#pragma GCC optimize("-frerun-cse-after-loop")
#pragma GCC optimize("inline-small-functions")
#pragma GCC optimize("-finline-small-functions")
#pragma GCC optimize("-ftree-switch-conversion")
#pragma GCC optimize("-foptimize-sibling-calls")
#pragma GCC optimize("-fexpensive-optimizations")
#pragma GCC optimize("-funsafe-loop-optimizations")
#pragma GCC optimize("inline-functions-called-once")
#pragma GCC optimize("-fdelete-null-pointer-checks")