C++ macro "if class is defined"

Question

Is there such macro in C++ (cross-compiler or compiler-specific):

#if isclass(NameSpace::MyClass)

Would be useful.

Answer 1

No. Preprocessing directives and macros are evaluated by the preprocessor, which completes its tasks before the code is parsed as C++. The preprocessor has no knowledge of classes or namespaces.

Answer 2

If you do not care about portability, the __if_exists statement in VC++ meets your needs.

Answer 3

There is no such thing at the preprocessing stage, so no macro.

However you can have a look at the is_class type traits available in Boost or in C++0x that enable you to take decisions at compile time.