C ++-通过宏定义的名称空间-如何在标头中使用它

Question

I have some code, that has namespace configurable by user as this:

#pragma once

#ifdef GRAPHICS_NAMESPACE
namespace GRAPHICS_NAMESPACE {
#endif

class Foo {
}

#ifdef GRAPHICS_NAMESPACE
}
#endif

And in cpp file, there is

#ifdef GRAPHICS_NAMESPACE
using namespace GRAPHICS_NAMESPACE;
#endif

Now I have a problem. I have defined -DGRAPHICS_NAMESPACE=Graphics and I have a header with:

#pragma once

#include "Foo.h"

class Bar {
   Foo foo;
}

But Foo foo gives me error, since Foo is in namespace now named Graphics . I can solve this with adding

#ifdef GRAPHICS_NAMESPACE
using namespace GRAPHICS_NAMESPACE;
#endif

to my header, but that is not very safe since it will be using namespace everywhere where I include my header. Is there any other solution?

Answer 1

I would do it like this:

#ifdef GRAPHICS_NAMESPACE 
#define GRAPHICS GRAPHICS_NAMESPACE 
#else
#define GRAPHICS 
#endif

...

GRAPHICS::Foo 

Answer 2

Although I would say this is not the optimal way to arrange things, I've seen worse and the solution to your problem is rather straightforward:

class Bar {

#ifdef GRAPHICS_NAMESPACE
GRAPHICS_NAMESPACE::
#endif
   Foo foo;
}

If you insist on sticking with this design, you can clean this up, somewhat, by defining a 2nd macro:

#ifdef GRAPHICS_NAMESPACE
#define GRAPHICS_CLASS(x)  GRAPHICS_NAMESPACE:: x
#else
#define GRAPHICS_CLASS(x)  x
#endif

And then declare things along the lines of:

class Bar {
   GRAPHICS_CLASS(Foo) foo;
}

Finally, if you insist on relying on the preprocessor for this kind of critical functionality, you may want to consider spending a little bit more time reading what your C++ book says on this topic. These are fairly basic, straightforward uses of the C++ preprocessor, which I expect should be fully explained in any introduction on what a C++ preprocessor is, and how it works.