Invalid use of qualified name

Question

I'm trying the following:

#include <iostream>

namespace A
{
    extern int j;
}

int main()
{
    int A::j=5;
    std::cout << A::j;
}

But I've error: invalid use of qualified-name ‘A::j’. Please explain why this error occurred?

Answer 1

Please explain why this error occurred?

The language simply doesn't allow you to define namespace-scope variables inside functions. The definition has to be either in namespace A:

namespace A {
    int j = 5;
}

or in the surrounding (global) namespace:

int A::j = 5;

You can, of course, assign a value to the variable inside the function:

int main() {
    A::j = 5;
    // ...
}

but you'll also need a definition somewhere, since your program doesn't have one.