GNU C++ how to check when -std=c++0x is in effect?

My system compiler (gcc42) works fine with the TR1 features that I want, but trying to support newer compiler versions other than the systems, trying to accessing TR1 headers an #error demanding the -std=c++0x option because of how it interfaces with library or some hub bub like that.

/usr/local/lib/gcc45/include/c++/bits/c++0x_warning.h:31:2: error: #error This file requires compiler and library support for the upcoming ISO C++ standard, C++0x. This support is currently experimental, and must be enabled with the -std=c++0x or -std=gnu++0x compiler options.

Having to supply an开发者_如何学Python extra switch is no problem, to support GCC 4.4 and 4.5 under this system (FreeBSD), but obviously it changes the picture!

Using my system compiler (g++ 4.2 default dialect):

#include <tr1/foo>
using std::tr1::foo;

Using newer (4.5) versions of the compiler with -std=c++0x:

#include <foo>
using std::foo;

Is there anyway using the pre processor, that I can tell if g++ is running with C++0x features enabled?

Something like this is what I'm looking for:

#ifdef __CXX0X_MODE__
#endif

but I have not found anything in the manual or off the web.

At this rate, I'm starting to think that life would just be easier, to use Boost as a dependency, and not worry about a new language standard arriving before TR4... hehe.

There seems, with gcc 4.4.4, to be only one predefined macro hinting that -std=c++0x is in effect:

#define __GXX_EXPERIMENTAL_CXX0X__ 1

I don't have access to gcc 4.5.0 , but you can check that one yourself:

[16:13:41 0 ~] $ g++ -E -dM -std=c++0x -x c++ /dev/null >b
[16:13:44 0 ~] $ g++ -E -dM -std=c++98 -x c++ /dev/null >a
[16:13:50 0 ~] $ diff -u a b
--- a   2010-06-02 16:13:50.200787591 +0200
+++ b   2010-06-02 16:13:44.456912378 +0200
@@ -20,6 +20,7 @@
 #define __linux 1
 #define __DEC32_EPSILON__ 1E-6DF
 #define __unix 1
+#define __GXX_EXPERIMENTAL_CXX0X__ 1
 #define __LDBL_MAX_EXP__ 16384
 #define __linux__ 1
 #define __SCHAR_MAX__ 127

For one-line command do,

g++ -E -dM -std=c++98 -x c++ /dev/null > std1 && g++ -E -dM -std=c++0x -x c++ /dev/null > std2 && diff -u std1 std2 | grep '[+|-]^*#define' && rm std1 std2

gives you something like:

+#define __GXX_EXPERIMENTAL_CXX0X__ 1

If you compile with -std=c++0x, then __GXX_EXPERIMENTAL_CXX0X__ will be defined.

Well, from gcc-4.7 onwards you'll be able to check __cplusplus:

"G++ now sets the predefined macro __cplusplus to the correct value, 199711L for C++98/03, and 201103L for C++11"

This should be the correct, standard-compliant way to do it. Unfortunately, it doesn't work for most gcc installed in the wild.