The GNU compiler provides these extensions to the C++ language (and you
can also use most of the C language extensions in your C++ programs). If you
want to write code that checks whether these features are available, you can
test for the GNU compiler the same way as for C programs: check for a
predefined macro __GNUC__. You can also use __GNUG__ to
test specifically for GNU C++ (see Predefined Macros in The GNU C Preprocessor).