Gnu C++ macro __cplusplus standard conform?

问题

The Gnu C++ compiler seems to define __cplusplus to be 1

#include <iostream> 
int main() {
  std::cout << __cplusplus << std::endl;
}

This prints 1 with gcc in standard c++ mode, as well as in C++0x mode, with gcc 4.3.4, and gcc 4.7.0.

The C++11 FDIS says in "16.8 Predefined macro names [cpp.predefined]" that

The name __cplusplus is defined to the value 201103L when compiling a C++ translation unit. (Footnote: It is intended that future versions of this standard will replace the value of this macro with a greater value. Non-conforming com- pilers should use a value with at most five decimal digits.)

The old std C++03 had a similar rule.

Is the GCC deliberatly setting this to 1, because it is "non-conforming"?

By reading through that list I thought that I could use __cplusplus to check in a portable way if I have a C++11 enabled compiler. But with g++ this does not seem to work. I know about the ...EXPERIMENTAL... macro, but got curious why g++ is defining __cplusplus this way.

My original problem was switch between different null-pointer-variants. Something like this:

#if __cplusplus > 201100L
#  define MYNULL nullptr
#else
#  define MYNULL NULL
#endif

Is there a simple and reasonably portable way to implement such a switch?

回答1:

This was fixed about a month ago (for gcc 4.7.0). The bug report makes for an interesting read: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=1773

回答2:

If I recall correctly this has to do with Solaris 8 causing issues when __cplusplus is set as it should. The gcc team decided at the time to support the Solaris 8 platform rather than be compliant in this particular clause. But I noticed that the latest version of gcc ends the Solaris 8 support, and I guess this is a first step in the right direction.