how to enable __BEGIN_NAMESPACE_STD in stdlib.h - c++

Question

I am now trying to build a c++ library in linux with cmake. If I do not enable -std=c++0x option, I always get compilation errors error: 'div_t' was not declared in this scope for the following codes:
int xPos;
div_t divResult;
divResult = div(xPos,8);
Then if I enable -std-c++0x options with cmake: set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++0x", then everything is fine. However, in my library I did not use any c++0x features, so I am reluctant to set std=c++0x option. So I search the head file that defines div_t and find it is defined in stdlib.h within the following MACRO:
__BEGIN_NAMESPACE_STD
typedef struct
{
int quot;
int rem;
} div_t;
....
....
__END_NAMESPACE_STD
It seems to me that if I can enable these macros I can build the library without enabling c++0x feature. So my question is what I can do in this situation.
By the way, I can build the library very well without enabling c++0x feature if only g++4.4 is installed in the linux machine. When I also install g++4.6 and make g++4.6 the default g++, then the compilation error began to occur. Even I changed the default g++ to g++4.4, the compilation error still exists if I do not enable c++0x feature.

Answer 1

The macros expand to namespace std { and } respectively if the code is pulled in through a C++ standard library header. This leads me to believe that you're not #including stdlib.h directly (which is good!).
Earlier versions of libstdc++ pulled symbols from C legacy headers into the global namespace even if the C++ versions of these headers (e.g. <cstdlib> instead of <stdlib.h>) were used; newer ones place them only in namespace std.
The cleanest way to fix this is to
#include <cstdlib>
in all translation units where the problem occurs and to use std::div instead of div. If you're lazy, you can also
#include <stdlib.h>
in all translation units that use div, but mixing C and C++ is always icky. Not terribad in this particular case, though.