Template class's static variable initialization, c++

Question

consider the following code:

//header.h
template<class T>
class A
{
    static int x;
};

template<class T>
int A<T>::x = 0;

//source1.cpp
#include "header.h"
void f(){} // dummy function


//main.cpp
#include "header.h"
int main(){}

In this case code compiles perfectly without errors, but if I remove the template qualifier from class

class A
{
    static int x;
};

int A::x = 0;

1. In this case compiler erred with multiple definition of x. Can anybody explain this behavior?
2. And when the template class's static variable is initialized / instantiated??

Answer 1

Compiler will remove duplicate template instantiations on its own. If you turn your template class into regular one, then its your duty to make sure only one definition of static variable exists (otherwise linker error will appear). Also remember that static data members are not shared between instatiations of templates for different types. With c++11 you can control instatiations on your own using extern templates: using extern template (C++11).

As for the point of instatiation for static members:

14.6.4.1 Point of instantiation [temp.point] 1 For a function template specialization, a member function template specialization, or a specialization for a member function or static data member of a class template, if the specialization is implicitly instantiated because it is referenced from within another template specialization and the context from which it is referenced depends on a template parameter, the point of instantiation of the specialization is the point of instantiation of the enclosing specialization. Otherwise, the point of instantiation for such a specialization immediately follows the namespace scope declaration or definition that refers to the specialization.

so point of instatiation should be ie. right after main() if you use your type for the first time inside main().