Predefined macros

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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:

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

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.:

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

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;
        std::cout << "Hello World!\n";

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