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Hi,

Please forgive me if this is elementary. I can't seem to find the
solution anywhere.

// foo() is only declared/defined in Derived
Base* base = new Derived();
((Derived*)base)->foo();

Given that I would like to use a Base*; is casting the only solution?

-J

jimmy wrote:

> Hi,
>
> Please forgive me if this is elementary. I can't seem to find the
> solution anywhere.
>
> // foo() is only declared/defined in Derived
> Base* base = new Derived();
> ((Derived*)base)->foo();
>
> Given that I would like to use a Base*; is casting the only solution?

Yes. But don't, I repeat, DON'T use C style casts for it. Use dynamic_cast
or static_cast.

> Yes. But don't, I repeat, DON'T use C style casts for it. Use dynamic_cast
> or static_cast.

dynamic_cast may NOT work if Base has no virtual function at all.
static_cast is a better alternative.

ben

ben wrote:
>
> > Yes. But don't, I repeat, DON'T use C style casts for it. Use dynamic_cast
> > or static_cast.
>
> dynamic_cast may NOT work if Base has no virtual function at all.

But in situations like the one the OP is in, the Base class
usually has at least one virtual function: the destructor.

--
Karl Heinz Buchegger

ben wrote:
>
> > Yes. But don't, I repeat, DON'T use C style casts for it. Use dynamic_cast
> > or static_cast.
>
> dynamic_cast may NOT work if Base has no virtual function at all.

Besides: Can you quote where this is nailed down in the C++ standard?

--
Karl Heinz Buchegger

> But in situations like the one the OP is in, the Base class
> usually has at least one virtual function: the destructor.

How do you know? I've seen plausible designs where no virtual function is
involved in inheritance.

ben

> Besides: Can you quote where this is nailed down in the C++ standard?
>

hmm, I don't have the standard (can't afford it) but if you try to
dynamic_cast a non polymorphic type the compiler's not gonna do it for you.

ben

ben wrote:
>
> > But in situations like the one the OP is in, the Base class
> > usually has at least one virtual function: the destructor.
>
> How do you know? I've seen plausible designs where no virtual function is
> involved in inheritance.

Because the OP posted:

> Base* base = new Derived();
> ((Derived*)base)->foo();

And since I assume that he is a good guy he will eventuall delete
the object. And for that he needs a virtual destructor or he
has undefined behaviour in his program.

--
Karl Heinz Buchegger

> Because the OP posted:
>
> > Base* base = new Derived();
> > ((Derived*)base)->foo();

without virtual destructors:

Derived* ptr_to_derived = reinterpret_cast<Base*>(base);
delete ptr_to_derived;

oops, I wonder why I wrote "reinterpret_cast"...and it is an ugly example
just to show it would work. It is not what I meant "plausible design" though

ben

> Ok, besides the fact that it's wrong (*): Do you really think this is a
> plausible design?
>
> (*) You have to use a static_cast, not a reinterpret_cast.
>