Re: How to do "derived" type extensions?

From:

LR <lruss@superlink.net>

Newsgroups:

comp.lang.c++

Date:

Wed, 09 Jan 2008 13:46:24 -0500

Message-ID:

<478515ec$0$26882$cc2e38e6@news.uslec.net>

Kira Yamato wrote:

On 2008-01-08 12:17:35 -0500, LR <lruss@superlink.net> said:

Kira Yamato wrote:

On 2008-01-08 09:18:53 -0500, LR <lruss@superlink.net> said:

Kira Yamato wrote:

Suppose class B is a subtype of class A, i.e.,
   class B : public A
   {
       ...
   };

But now how do I do the following kind of morphism? Suppose I have
functions
   void foo(const B &);
and
   void bar(const A &);
I like to be able to declare a function pointer
   void (*fp)(const B&);
and make "polymorphic" assignments like
   p = foo; // this is ok in C++.
   p = bar; // semantically this make sense, but C++ syntax
won't allow this!

Would it be acceptable to create a function,
void bar(const B &);

You mean
void foo(const B &);

No, that's not what I meant. I don't think that I made myself clear,
so I'll try again.

Untested, and not complete...

This is what I think you started with.
class A {
};
class B : public A {
};
void foo(const B &) {
}
void bar(const A &) {
}

now I suggest adding this function
void bar(const B &b) {
     ::bar( reinterpret_cast<const A&>(b) );
}

so you can
     void (*fp)(const B&);
and
     B b;
     fp = bar; // will assign the address of void bar(const B &) to fp
     fp(b);
     fp = foo;
     fp(b);

Yes. This is a nice work-around in the sense that it is direct and
seemingly not too much artifact.

Your solution is similar to the adaptor pattern (or am I misusing the
terminology here?).

I suppose in some ways, maybe not in others. However, consider this
well known, although I don't know the source, quote: "any [programming]
problem can be solved by introducing an extra level of indirection."

Thanks for the suggestion.

You're welcome.

Please don't overlook Pete Becker's correction and advice to use
static_cast instead of reinterpret_cast elsethread.

BTW, I've been thinking about this, and wondering about what you want to
do because I suspect this might be a little bit of a maintenance problem
if you're using this extensively in your code and your class structure
changes.

LR