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++
}

Answer 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.

Answer 2

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