Re: Friends to select(ed) members? (Just checking)

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Sun, 03 Feb 2008 20:42:57 +0100

Message-ID:

<13qc6dvanjq2498@corp.supernews.com>

* Olumide:

I know friends normally gives a named class or function (hitherto
called a guest) access to *all* (private and protected) members of the
class that makes the declaration (hitherto called a host). ... I
wonder if there's a way to limit the direct access of the guest class
or function to a subset of the host's member function. For example

class guest{

};

class host{
private:
      friend class guest int number; // friendship restricted
      char* name;
      char sex;
};

Thank you for not flaming me :-)

No direct support for that in language.

You could do some contorted thing like

     #include <string>

     class Host;

     class Guest { void foo( Host& ); };

     class NumberHolder
     {
     friend class Guest;
     protected:
         int myNumber;
         NumberHolder( int v = 0 ): myNumber( v ) {}
     };

     enum GenderEnum { male, female, androgynous };

     class Host: public NumberHolder
     {
     private:
         std::string myName;
         GenderEnum myGender;
     };

     void Guest::foo( Host& h )
     {
         h.myNumber = 666;
     }

     int main()
     {}

Actually I was surprised that this compiled without requiring accessing
the h argument via a reference to NumberHolder type, but, enjoy.

Or -- you really shouldn't be doing this!

Think about whether that "partial friendship" doesn't imply a missing
abstraction in the design, e.g. a class, or some other design issue.

Cheers, & hth.,

- Alf

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