Re: Private Member Access failed through Friend function

Seungbeom Kim <>
Wed, 2 May 2007 16:06:24 CST
Cumhur Guzel wrote:

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On Apr 30, 7:12 pm, Seungbeom Kim <> wrote:

I agree, but I'd also like to point out a situation where defining a
separate print() function would be useful: polymorphic classes. Because
calls to operator<< cannot be dispatched virtually depending on the
right-hand side operand, defining virtual print() functions and calling
that of the base class in operator<<(std::ostream&, const Base&) is
almost "the" right answer.

I think you could get better encapsulation plus flexibility by using
the template method pattern with NVI,
as given below. As far as I remember, something similar to given below
is suggested in Meyer's Effective C++
or More Effective C++;

Can you elaborate on "better encapsulation plus flexibility" we can get

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

It's basically the same idea; the non-virtual interface pattern you
showed is something to add on top of the "virtual member + non-member
operator<<" pattern, not a replacement of it. The basic idea that the
non-member operator<< calls a virtual member remains the same.

Furthermore, the purpose of the NVI pattern is to enforce some pre-
and/or post-condition to the virtual functions. Since there's no
additional conditions to enforce in your print function (and it's not
very likely that you'll have any in the future), there's little point in
having a separate non-virtual member that calls the virtual member.

Still, we could consider my example as a variation of the NVI pattern,
for a different purpose: to provide a different syntax (os << obj
instead of obj.print(os)). The non-member operator<< here serves as the
non-virtual interface in the NVI pattern. (For a complete analogy, you
could make the virtual member private and make the non-member a friend.)

Seungbeom Kim

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