Re: design problem with inheritance
On 8/23/2011 4:48 PM, alessio211734 wrote:
First off, what correction? If you post a reply, consider quoting or at
least commenting on what you're referring to.
Second, nobody but Duck (or its friends) can access private members of
Duck. If you need to get to those from derived classes, provide an
interface (possibly 'protected' to limit access to derived classes only)
in the Duck itself.
Ok thanks, you explain me how c++ work with friend and protected
keyword but I ask you how adjust my design.
How do you adjust a radio to give you coffee? You can't. A coffee
machine is not going to play a tune, unless it is *also* a radio. If
it's just a coffee machine, it cannot be "adjusted" to become a radio.
In the initial design I have one class Duck and some derived class
StandardDuck and FakeDuck both derived from Duck class.
So?
the class FakeDuck override the fly method and make it empty. I think
this is a bad design.
You didn't explain why it's bad.
What's a Duck? Something that has the 'fly' method. By your design,
since you derived 'FakeDuck' from 'Duck', publicly, you said,
essentially, that a 'FakeDuck' *is-a* 'Duck', which means 'FakeDuck'
*shall have* a 'fly' method. How is that "bad", if that's your design?
If 'FakeDuck' is NOT a 'Duck', don't derive it from 'Duck'.
Book proposed to use a pattern strategy and use FlyBehav interface to
handle behavior as fly and nofly
class FlyBehav{
public:
virtual void Fly()=0;
};
class NoFly: public FlyBehav{
public:
void Fly(){std::cout<<"Cannot Fly\n";}
};
class CanFly: public FlyBehav{
public:
void Fly(){std::cout<<"Flying across the sky\n";}
};
Rename 'FlyBehav' to 'Duck', 'CanFly' to 'StandardDuck', and 'NoFly' to
'FakeDuck', and you get the same hierarchy as before.
I think this pattern is right if method Fly no needs private data of
Duck class as in this example.
I don't get it.
but in a real case I think that the Fly method needs to data about
Duck class. And so in this case what's the best design to solve my
problem?
I don't know. WHAT PROBLEM?
I would like avoid to create a class FakeDuck with a empty method. I
think Strategy it's not right.
So, don't use the Strategy.
Perhaps you need to start with the PROBLEM you're trying to solve
instead of the NON-WORKING solutions you have come up with.
V
--
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