Re: Private Member Access failed through Friend function

From:

Richard Smith <richard@ex-parrot.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 2 May 2007 16:09:08 CST

Message-ID:

<1178127561.126395.258790@y80g2000hsf.googlegroups.com>

On May 2, 10:42 am, Cumhur Guzel <guzelcum...@gmail.com> wrote:

I think you could get better encapsulation plus flexibility by using
the template method pattern with NVI,

namespace X {
        class Base {
        public:
                void print(std::ostream& os) {
                        myprint(os);
                }
        private:
                virtual void myprint(std::ostream&)=0;
        };

        class D1: Base {
        private:
                virtual void myprint(std::ostream&) {};
        };

        class D2: Base {
        private:
                virtual void myprint(std::ostream&) {};
        };

        std::ostream& operator<<(std::ostream& os, Base& b) {
          b.print(os); return os;
        }

(I assume the non-cost Base parameter and the private inheritance are
typos.)

And this is back to an example where I think a friend function is
preferable to a public print() function:

  class Base {
  private:
    virtual void do_print(std::ostream&) = 0;

    friend ostream& operator<<( ostream& os, Base const& b ) {
      b.do_print(os); return os;
    }
  };

Why introduce the redundant print() function when you can do it
directly with the more idiomatic operator<< simply by making it a
friend?

--
Richard Smith

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