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C++ compilers use the same C preprocessor. The preprocessor is a part of the compiler which performs preliminary operations (conditionally compiling code, including files etc...) to your code before the compiler sees it. These transformations are lexical, meaning that the output of the preprocessor is still text.
In computer science, a preprocessor (or precompiler) is a program that processes its input data to produce output that is used as input to another program. The output is said to be a preprocessed form of the input data, which is often used by some subsequent programs like compilers.
To check the operating system of the host in a C or C++ code, we need to check the macros defined by the compiler (GNU GCC or G++). For example, on Windows platform, the compiler has a special macro named _WIN32 defined. So, if the macro _WIN32 is defined, we are on Windows.
Macro: a word defined by the #define preprocessor directive that evaluates to some other expression. Preprocessor directive: a special #-keyword, recognized by the preprocessor. Show activity on this post. preprocessor modifies the source file before handing it over the compiler.
For Mac OS:
#ifdef __APPLE__
For MingW on Windows:
#ifdef __MINGW32__
For Linux:
#ifdef __linux__
For other Windows compilers, check this thread and this for several other compilers and architectures.
See: http://predef.sourceforge.net/index.php
This project provides a reasonably comprehensive listing of pre-defined #defines for many operating systems, compilers, language and platform standards, and standard libraries.
Here's what I use:
#ifdef _WIN32 // note the underscore: without it, it's not msdn official!
// Windows (x64 and x86)
#elif __unix__ // all unices, not all compilers
// Unix
#elif __linux__
// linux
#elif __APPLE__
// Mac OS, not sure if this is covered by __posix__ and/or __unix__ though...
#endif
EDIT: Although the above might work for the basics, remember to verify what macro you want to check for by looking at the Boost.Predef reference pages. Or just use Boost.Predef directly.
If you're writing C++, I can't recommend using the Boost libraries strongly enough.
The latest version (1.55) includes a new Predef library which covers exactly what you're looking for, along with dozens of other platform and architecture recognition macros.
#include <boost/predef.h>
// ...
#if BOOST_OS_WINDOWS
#elif BOOST_OS_LINUX
#elif BOOST_OS_MACOS
#endif
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