Re: design problem with inheritance

From:

Paul N <gw7rib@aol.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 23 Aug 2011 15:09:39 -0700 (PDT)

Message-ID:

<49ab3aa1-8035-4369-9a34-189a75cb5552@o26g2000vbi.googlegroups.com>

On Aug 23, 3:09 pm, alessio211734 <alessio211...@yahoo.it> wrote:

I am reading a design pattern book.

the example is a class duck that a method fly

class duck
{
    virtual void fly(){......}
    void display();

}

class FakeDuck: public duck()
{
    void fly(){ override to nothing...}

}

if I have a FakeDuck class it's need to override the fly method to
nothing because a FakeDuck can't fly.
And so on for every subclass of duck that have no fly feature.

book propose the pattern strategy and to extern the fly behavior as a
external class

class FlyBehavior
{
     virtual void fly()=0;

}

class NoFly: public FlyBehavior
{
    virtual void fly(){ printf("can't fly")};

and the class duck became

class duck
{
    FlyBehavior * bhv;
    void performFly(){ bhv->fly(); }

}

I have doubts about the design proposed because book says
encapsulating the algorithms in a class hierarchy and make them
interchangeable.
But ok if I need the private data about duck class to implement the
fly method of NoFly class how can do it??
What's the best solutions?

I'm not an expert, but it seems to me that the proposed solution is
trying to provide a "library" of different fly behaviours, with each
type of duck able to choose any of them, presumably in a way that
doesn't fit in well with the duck hierarchy. So if you want the fly
behaviour to depend on details of the particular duck you are probably
using the wrong solution.

Of course, if the different fly behaviours get built out of standard
blocks, you could define virtual functions to do these, perhaps
something like:

class Fly: public FlyBehavior
{
virtual void fly(duck *d) { d -> FlapWings(); } };

class duck
{
....
virtual void FlapWings(); }

Hope that helps.
Paul.