Friend template overloaded operator <<: unresolved external symbol

I'm having issues with the error

Error LNK2019 unresolved external symbol "class std::basic_ostream > & __cdecl cop4530::operator<<(class std::basic_ostream > &,class rob::Stack const &)" (??6rob@@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@std@@AAV12@ABV?$Stack@H@0@@Z) referenced in function _main Project7 c:\Users\Robrik\documents\visual studio 2015\Projects\Project7\Project7\post.obj 1

Right now, all that post is doing is calling the operator<<

The declaration

namespace rob {     

template < typename T> class Stack {
    friend std::ostream& operator<< (std::ostream& os, const Stack<T>& a);
    void print(std::ostream& os, char ofc = ' ') const;
private:
    std::vector<T> arr;
};

The definition

template < typename T>
inline std::ostream & rob::operator<<(std::ostream & os, const Stack<T>& a)                {     
    return a.print(os, ' ');
}
template<typename T>
inline void rob::Stack<T>::print(std::ostream & os, char c) const
{
    for (int i = 0; i != arr.size(); i++)
    {
        os << c << arr[i];
    }
    os << '\n';
}

They are located in a .h file and a .hpp respectively, I require that the operator is not a member function (for assignment).