C ++; 如何在名称空间中使用全局令牌

Question

I am extending an existing component which defines global TRUE and FALSE tokens. My extension is defining, within a new namespace 'new_namespace' an enum 'new_enum' which has values TRUE and FALSE. The compiler is failing to compile this because of a namespace collision.

#define FALSE (0)
#define TRUE (1)

namespace new_namespace {
  class new_class {
  public :
  enum new_enum {
    TRUE = (0),
    FALSE = (1)
  };
  };
}

I would have hoped this would have been supported ... am I missing something.

Best Regards

Answer 1

Macros are handled by the preprocessor, which (as the name implies) runs before the compiler proper. What it does is replace the macro "call" with the text in the macro body. So what your enum looks like after the preprocessor step, and what the actual compiler sees, is:

enum new_enum {
    (1) = (0),
    (0) = (1)
};

This is not legal C++, and the reason the compiler complains.

Answer 2

Joachim has identified your problem correctly so I won't repeat the answer. But I want to comment on how to solve it. There is really no reason to have those tokens as defines, I would strongly advocate replacing them like this:

const bool TRUE = true;
const bool FALSE = false;

It's just possible that this will result in type problems, although it's most likely that they will gracefully by implicitly cast. If you do see typing problems, try this instead:

const int TRUE = 1;
const int FALSE = 0;

Either way these constants will obey the normal scoping rules and no longer conflict.