[c++] How to detect reliably Mac OS X, iOS, Linux, Windows in C preprocessor?

If there's some cross-platform C/C++ code that should be compiled on Mac OS X, iOS, Linux, Windows, how can I detect them reliably during preprocessor process?

5 Jan 2021: link update thanks to @Sadap's comment.

Kind of a corollary answer: the people on this site have taken the time to make tables of macros defined for every OS/compiler pair.

For example, you can see that _WIN32 is NOT defined on Windows with Cygwin (POSIX), while it IS defined for compilation on Windows, Cygwin (non-POSIX), and MinGW with every available compiler (Clang, GNU, Intel, etc.).

Anyway, I found the tables quite informative and thought I'd share here.

As Jake points out, TARGET_IPHONE_SIMULATOR is a subset of TARGET_OS_IPHONE.

Also, TARGET_OS_IPHONE is a subset of TARGET_OS_MAC.

So a better approach might be:

#ifdef _WIN64
   //define something for Windows (64-bit)
#elif _WIN32
   //define something for Windows (32-bit)
#elif __APPLE__
    #include "TargetConditionals.h"
    #if TARGET_OS_IPHONE && TARGET_IPHONE_SIMULATOR
        // define something for simulator   
    #elif TARGET_OS_IPHONE
        // define something for iphone  
    #else
        #define TARGET_OS_OSX 1
        // define something for OSX
    #endif
#elif __linux
    // linux
#elif __unix // all unices not caught above
    // Unix
#elif __posix
    // POSIX
#endif