Re: Private Member Access failed through Friend function

Richard Smith <>
Wed, 2 May 2007 16:09:08 CST
On May 2, 10:42 am, Cumhur Guzel <> wrote:

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

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

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

        class D2: Base {
                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

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

  class Base {
    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

Richard Smith

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