Please have a look at the following example
and send comments.

Thanks in advance!
- Stefan

// n.h: only declarations!?
namespace A {
  class X { /* ... */ };
  extern X x;  // ok
  X y;         // error, causes double definition
  int i;       // ok, but why doesn't it cause an error like X y!?
}

// n.cpp: definitions
#include "n.h"
int A::i = 1;
A::X A::x;

// main.cpp
#include "n.h"
main() {
  // use A::i and A::x
}

>// n.h: only declarations!?
>namespace A {
>  class X { /* ... */ };
>  extern X x;  // ok
>  X y;         // error, causes double definition

 No error yet.

>  int i;       // ok, but why doesn't it cause an error like X y!?
>}

 There's no error yet.
>
>// n.cpp: definitions
>#include "n.h"
>int A::i = 1;

 A::i is defined the second time here in the SAME
translation unit.  It's an error. The compiler will be
required to issue a diganostic on this one.

>A::X A::x;
>
>// main.cpp
>#include "n.h"

 Now y is defined twice in n.cpp and main.cpp. No diagnostic
is required. i is define THREE times: twice in n.cpp and once in main.cpp.

[BTW: my answers are based on my paper on the One Definition Rule.]

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