extern local variable for global variable inside namespace

Question

The following code works correctly:

file1.cpp

//global variable
int g_myvar1 = 5;

file2.cpp

int myfunc()
{
   extern int g_myvar1;
   g_myvar1++
}

How can I do file2.cpp if file1.cpp is as follows:

file1.cpp

namespace myns
{
    //global variable
    int g_myvar1 = 5;
}

NOTE1, the following gives compilation error on GCC 4.7 "invalid use of qualified-name". I tried 'using namespace' with no luck also.

int myfunc()
{
   extern int myns::g_myvar1;
   g_myvar1++
}

NOTE2, The following works, but I am looking for only-local variable definition.

namespace myns
{
    //global variable
    extern int g_myvar1;
}
int myfunc()
{
   myns::g_myvar1++
}

Solution 1

Use using:

void f()
{ 
   using myns::g_myvar1;

   ++g_myvar1;
}

You've declare the variables (with extern keyword) in .h file in a namespace myns, and define them in .cpp file. And include the header file wherever you want to use the variables.

Solution 2

Put the namespace with the extern declaration in a header file, and include that header file in all source files needing that variable.

Comments

[Yousf]

I know this is the best solution, but I was looking for a solution which don't need this include.

[Yousf]

+1 for the new usage of using keyword. But this still needs declaration of "namespace mynm{extern int g_myvar1;}" before void f()

[Nawaz]

@Yousf: Yes, it is better to declare them in .h file, and define them in .cpp file.

[Some programmer dude]

@Yousf In C and C++ things have to be declared before they are used, it's as simple as that. If you don't have some kind of declaration of the namespace or the data inside it, the compiler will not know of those identifier names.

[Ant]

@Yousf Thank you. It took me a long time to find that syntax. With your suggestion you don't need the using.