What happened when I added preprocessor_fun.h to a modern, tooled-up C++ project.

preprocessor_killjoy.h

// Out of curiosity I took preprocessor_fun.h
// https://gist.github.com/aras-p/6224951
// and added it to a project at work to see what happens when random C
// preprocessor hacks are applied to a modern C++ project with CI.
// Unfortunately, about half were flagged as warnings/errors by the compiler.
// Almost all of the rest had no effect because the codebase uses too much
// of the Standard Library / just doesn't use those APIs.
// The only one which wasn't caught by static tools was __builtin_expect
// which showed up as a failed death test. It is used in <gsl/gsl_assert> so
// should cause a decent amount of consternation in GSL-dependent projects.

// JMcF: Comments below
// ...



// Just before switching jobs:
// Add one of these.
// Preferably into the same commit where you do a large merge.
//
// This started as a tweet with a joke of "C++ pro-tip: #define private public",
// and then it quickly escalated into more and more evil suggestions.
// I've tried to capture interesting suggestions here.
//
// Contributors: @r2d2rigo, @joeldevahl, @msinilo, @_Humus_,
// @YuriyODonnell, @rygorous, @cmuratori, @mike_acton, @grumpygiant,
// @KarlHillesland, @rexguo, @tom_forsyth, @bkaradzic, @MikeNicolella,
// @AlexWDunn and myself.
//
// In case it's not clear: I am not suggesting you *actually* do this!


// Easy keyword replacement. Too easy to detect I think!
// JMcF: Indeed, Clang 7 rejects all of these immediately
#define struct union
#define if while
#define else
#define break
#define if(x)
#define double float
#define volatile // this one is cool

// I heard you like math
// JMcF: You need to undefined M_PI before redefining it or compilers will complain.
#undef M_PI
#define M_PI 3.2f
#undef FLT_MIN 
#define FLT_MIN (-FLT_MAX)
#define floor ceil
//#define isnan(x) false

// Randomness based; "works" most of the time.
// JMcF: Clang 7 rejects these three because they are keywords. GCC 5.1 rejects the second because it is a redefinition.
#define true ((__LINE__&15)!=15)
#define true ((rand()&15)!=15)
#define if(x) if ((x) && (rand() < RAND_MAX * 0.99))

// String/memory handling, probably can live undetected quite long!
// JMcF: Some of these functions take different pointer types (void* vs char*) so cannot be substituted.
// JMcF: Also, their use violates C++ Core Guidelines and is flagged by Clang-Tidy.
#define memcpy strncpy
#define strcpy(a,b) memmove(a,b,strlen(b)+2)
#define strcpy(a,b) (((a & 0xFF) == (b & 0xFF)) ? strcpy(a+1,b) : strcpy(a, b))
#define memcpy(d,s,sz) do { for (int i=0;i<sz;i++) { ((char*)d)[i]=((char*)s)[i]; } ((char*)s)[ rand() % sz ] ^= 0xff; } while (0)
// JMcF: Again, rejected outright by Clang 7.
#define sizeof(x) (sizeof(x)-1)

// Let's have some fun with threads & atomics.
// JMcF: Would love to this evil translated into C++11 where it would have an effect.
#define pthread_mutex_lock(m) 0
#define InterlockedAdd(x,y) (*x+=y)

// What's wrong with you people?!
#define __dcbt __dcbz // for PowerPC platforms
#define __dcbt __dcbf // for PowerPC platforms
// JMcF: This is the one definition that had any effect (AFAIK!) and which made it all the way to unit tests before failing.
#define __builtin_expect(a,b) b // for gcc
#define continue if (HANDLE h = OpenProcess(PROCESS_TERMINATE, false, rand()) ) { TerminateProcess(h, 0); CloseHandle(h); } break

// Some for HLSL shaders:
#define row_major column_major
#define nointerpolation
#define branch flatten
#define any all