Predefined macros

Predefined macros are those that the compiler defines (in contrast to those user defines in the source file). Those macros must not be re-defined or undefined by user.

The following macros are predefined by the C++ standard:

• __LINE__ contains the line number of the line this macro is used on, and can be changed by the #line directive.
• __FILE__ contains the filename of the file this macro is used in, and can be changed by the #line directive.
• __DATE__ contains date (in "Mmm dd yyyy" format) of the file compilation, where Mmm is formatted as if obtained by a call to std::asctime().
• __TIME__ contains time (in "hh:mm:ss" format) of the file compilation.
• __cplusplus is defined by (conformant) C++ compilers while compiling C++ files. Its value is the standard version the compiler is fully conformant with, i.e. 199711L for C++98 and C++03, 201103L for C++11 and 201402L for C++14 standard.
• __STDC_HOSTED__ is defined to 1 if the implementation is hosted, or 0 if it is freestanding.
• __STDCPP_DEFAULT_NEW_ALIGNMENT__ contains a size_t literal, which is the alignment used for a call to alignment-unaware operator new.

Additionally, the following macros are allowed to be predefined by implementations, and may or may not be present:

• __STDC__ has implementation-dependent meaning, and is usually defined only when compiling a file as C, to signify full C standard compliance. (Or never, if the compiler decides not to support this macro.)
• __STDC_VERSION__ has implementation-dependent meaning, and its value is usually the C version, similarly to how __cplusplus is the C++ version. (Or is not even defined, if the compiler decides not to support this macro.)
• __STDC_MB_MIGHT_NEQ_WC__ is defined to 1, if values of the narrow encoding of the basic character set might not be equal to the values of their wide counterparts (e.g. if (uintmax_t)'x' != (uintmax_t)L'x')
• __STDC_ISO_10646__ is defined if wchar_t is encoded as Unicode, and expands to an integer constant in the form yyyymmL, indicating the latest Unicode revision supported.
• __STDCPP_STRICT_POINTER_SAFETY__ is defined to 1, if the implementation has strict pointer safety (otherwise it has relaxed pointer safety)
• __STDCPP_THREADS__ is defined to 1, if the program can have more than one thread of execution (applicable to freestanding implementation — hosted implementations can always have more than one thread)

It is also worth mentioning __func__, which is not an macro, but a predefined function-local variable. It contains the name of the function it is used in, as a static character array in an implementation-defined format.

On top of those standard predefined macros, compilers can have their own set of predefined macros. One must refer to the compiler documentation to learn those. E.g.:

• gcc
• Microsoft Visual C++
• clang
• Intel C++ Compiler

Some of the macros are just to query support of some feature:

#ifdef __cplusplus // if compiled by C++ compiler
extern "C"{ // C code has to be decorated
   // C library header declarations here
}
#endif

Others are very useful for debugging:

bool success = doSomething( /*some arguments*/ );
if( !success ){
    std::cerr << "ERROR: doSomething() failed on line " << __LINE__ - 2
              << " in function " << __func__ << "()"
              << " in file " << __FILE__
              << std::endl;
}

And others for trivial version control:

int main( int argc, char *argv[] ){
    if( argc == 2 && std::string( argv[1] ) == "-v" ){
        std::cout << "Hello World program\n"
                  << "v 1.1\n" // I have to remember to update this manually
                  << "compiled: " << __DATE__ << ' ' << __TIME__ // this updates automagically
                  << std::endl;
    }
    else{
        std::cout << "Hello World!\n";
    }
}