Q:

friend function in class templates and error LNK2019

I'm trying to overload operator << in my template class and
I get error where i want to 
NSizeNatural<30> a(101);
cout << a;

Without this whole program compiles
Error:
error LNK2019: unresolved external symbol "class   std::basic_ostream<char,struct std::char_traits<char> > & __cdecl operator<<<30,unsigned int,unsigned __int64,10>(class std::basic_ostream<char,struct std::char_traits<char> > &,class NSizeNatural<30,unsigned int,unsigned __int64,10> const &)" (?? $? 6$0BO@I_K$09@@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@std@@AAV01@ABV?     $NSizeNatural@$0BO@I_K$09@@@Z) referenced in function _main

My Numbers.h file :
template<int size, typename basic_type = unsigned int, typename long_type = unsigned long long, long_type base = bases(DEC)> 
class NSizeNatural
{
       // this is how I decelerate friend function  
     friend ostream& operator<<(ostream& str, const NSizeNatural<size, basic_type, long_type, base> & n);
}

In  Numbers.cpp file :
template<int size, typename basic_type, typename long_type, long_type base>
std::ostream& operator<<(std::ostream& out, const NSizeNatural<size, basic_type,   long_type, base> &y)
{
   // For now i want to have my code compile-able 
   return out << "gg"; 
}

I have no idea how to do it correctly.  And where is a bug ...

A:

You have two problems in your code, first one is, you should move the implementation of operator << into the header file (as the comments on your question). 
But, second problem is your definition of friend operator <<, you must define it as a template function without default parameter:
template<int size, typename basic_type = unsigned int, typename long_type = unsigned long long, long_type base = bases(DEC)>
class NSizeNatural
{
    template<int size1, typename basic_type1, typename long_type1, long_type base1>
    friend ostream& operator<< (ostream& str, const NSizeNatural<size, basic_type, long_type, base> & n);
};

