c++ - How to declare friend function of non-member function declared in a class by friend keywork -

i have 2 questions friend declaration in following code. (1) declared friend function. program looks working got warning compiler. original function non-member function defined in class friend declaration. need care of it? or have problem? (2) if possible, declare friendship template class functions owns non-member function. tried declare friend ship class failed. please give me solution or suggestions. thank much.

#include <vector>  template <typename derived, typename t> class function0 { public:     friend bool operator== (derived& org, derived& cmp) {  // ***** line gives warning         derived& org_value = static_cast<derived&>(org);         derived& cmp_value = static_cast<derived&>(cmp);         return *(org_value.value_) == *(cmp_value.value_);     } };  template <typename t, template <typename derived, typename t_t> class functions> class pointer : public functions<pointer<t,functions>, t> { public:     pointer() {};     pointer(t* new_value) : value_(new_value) {};     bool operator== (pointer& cmp) = delete;  // ***** line gives warning     virtual ~pointer() { }; private: //  friend class functions<pointer<t,functions>, t>;  // *** did not work.     friend bool operator== (pointer<t,functions>& org, pointer<t,functions>& cmp);  // *** looks work gave me warning     t* value_ = nullptr; };  class testa { public:     testa(unsigned int id) : id_(id) {};     virtual ~testa() {};     unsigned int id(void) { return id_; }      bool operator== (testa& cmp) {         return (id_ == cmp.id()) ? true : false;     }  private:     unsigned int id_ = 0; };  template <typename element> element findcorrespondingfirst(element& obj, std::vector<element>& vec) {     (unsigned int = 0; < vec.size(); ++i) {         auto o = vec[i];         if (obj == o) {  // dispatches error massage             return o;         }     }     return element(); }  void test_pointer_class(void) {      std::vector<pointer<testa, function0>> ptr_vector;      testa* raw_ptr0 = new testa(1);     testa* raw_ptr1 = nullptr;     testa* raw_ptr2 = new testa(2);      pointer<testa, function0> ptr0 = pointer<testa, function0>(raw_ptr0);     pointer<testa, function0> ptr1 = pointer<testa, function0>(raw_ptr1);     pointer<testa, function0> ptr2 = pointer<testa, function0>(raw_ptr2);      testa* raw_ptr3 = new testa(1);     pointer<testa, function0> ptr3 = pointer<testa, function0>(raw_ptr3);      ptr_vector.push_back(ptr0);     ptr_vector.push_back(ptr1);     ptr_vector.push_back(ptr2);     ptr_vector.push_back(ptr3);      auto result1 = findcorrespondingfirst(ptr3, ptr_vector);      delete raw_ptr0;     delete raw_ptr1;     delete raw_ptr2;     delete raw_ptr3;  } 

the warning message gcc following,

friend declaration ‘bool operator==(pointer<t, functions>&, pointer<t, functions>&)’ declares non-template function -[wnon-template-friend]