namespaces in different file, preprocessor directives and static variables

[samusamu]

Hello, I'm new in this forum.
I was trying to understand the following behaviour under Visual Studio C++ 2008 express edition.
I have defined my own class (myClass) in myClass.hpp & myClass.cpp

I have create a namespace in MyNameSp.hpp with the following sintax

#ifndef myNameSpace_hpp
#define myNameSpace_hpp

namespace myNameSpace 
{myClass importantVal;}

#endif 

I need to know the value in (let's say) file1.cpp and file2.cpp.
If I include the "MyNameSp.hpp" at the beginning of those files, it compiles, but it won't link:

error LNK2005: "myClass myNameSpace::importantVal" (?ImportantVal@myNameSpace@@3HA) already defined in file1.obj.
(and some others errors; it might be slightly different, my output is in italian)

After many different tentatives I figured out that modifying the code in MyNameSp.hpp with

namespace myNameSpace 
{static myClass importantVal;}

make it works (it both compiles and links).
And this is the behaviour I actually want (only one copy of importantVal should exist)

Side note: I have tried also with int, and I had the same problem.

my question is:
Shouldn't the preprocessor directive prevent a double definition of the namespace (as it does with classes)? What am I not understanding?

Since static variables must be initialized, where should I do that?
Will be automatically used the default constructor? (which is fine, for me)
Thanks in advance for your help.

[richardforc]

Declaring object in header is not good idea. This is not namespace error, its normal c/c++ link error.

Hope below will fix your issue,

in myClass.cpp, do like following

myClass myNameSpace::importantVal;

in myClass.h

namespace myNameSpace {
extern myClass importantVal;
}

[samusamu]

thanks richardforc, I will give it a try later.
Why shouldn't I declare object in a header?
Which would be a better solution?
Moving the namespace in a .cpp file?
If so, how to guarantee access to the namespace from the other files?
Thanks for the help.

[samusamu]

with more googling I finally figured out why richardforc solution is right.
I just add here another reference I found and that clarified to me what I was doing wrong.

http://stackoverflow.com/questions/6109224/c-namespace-member-access-in-different-files-how-to-how-namespace-std-imple