c++ - Friend class not able to access private data -


here class declaration:

template <class t> class sptr {     template<typename u> friend class sptr;      template <typename t1, typename t2>     friend bool operator==(const sptr<t1> &a, const sptr<t2> &b);      template <typename u>     friend sptr<t> static_pointer_cast(const sptr<u> &sp);  private:      rc* ref; //reference counter     t* obj;//pointer current obj     std::function<void()> destroydata;      bool ok_;  public:      sptr();     ~sptr();      template <typename u>      sptr(u *);      sptr(const sptr &);      template <typename u>      sptr(const sptr<u> &);      template <typename u>      sptr<t> &operator=(const sptr<u> &);      sptr<t> &operator=(const sptr<t> &);      void reset();      t* operator->() const     {return obj;};      t& operator*() const     {return *obj;};      t* get() const     {return obj;};      explicit operator bool() const {           return ok_;     }  };

below code thats complaining of access problem

    template <typename t, typename u> sptr<t> static_pointer_cast(const sptr<u> &sp) {     //do     sptr<u> answer;      answer.obj = sp.obj;     answer.ref = sp.ref;     answer.destroydata = sp.destroydata;     answer.ok_ = sp.ok_;      return answer; }

when compile following code:

    sptr<derived> sp(new derived);     sptr<base1> sp2(sp);      // sptr<derived> sp3(sp2); // should give syntax error.     sptr<derived> sp3(static_pointer_cast<derived>(sp2));     // sptr<derived> sp4(dynamic_pointer_cast<derived>(sp2)); // should give syntax error polymorphism.

i have made friend function. why not able access variables , how correct it?

this bit tricky. code compiles if replace

template <typename u> friend sptr<t> static_pointer_cast(const sptr<u> &sp);

with

template <typename t1, typename t2> friend sptr<t1> static_pointer_cast(const sptr<t2> &sp);

the reason following (i not 100% sure, please approve / dis-approve):

when sptr<t> instantiated, e.g. t = derived, resulting class definition (sptr<derived>) defines friend function having following signature:

template <typename u> friend sptr<derived> static_pointer_cast(const sptr<u> &sp);

so function has one template parameter. function defined has two template parameters.

your call templated function specialization looks this:

sptr<derived> static_pointer_cast(const sptr<base1> &sp) {     //do     sptr<derived> answer;      answer.obj = sp.obj;     answer.ref = sp.ref;     answer.destroydata = sp.destroydata;     answer.ok_ = sp.ok_;      return answer; }

so tries access both base1 and derived, it's friend of derived, not of base1. last sentence important thing, , changing friend function two template parameters solves issue.


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