Re: Switching between interfaces without knowing derived object.

From:

"Martin T." <0xCDCDCDCD@gmx.at>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 12 Aug 2009 15:42:54 CST

Message-ID:

<h5tpf6$8oc$1@news.eternal-september.org>

hardy wrote:

Hello

Consider a case in which we have two interface classes I1 and I2 and a
derived Class D implementing both of these interfaces. This is a part
of the library and the library-user does not know the definition of
Class D. It knows only about I1 and I2. API's of this library return
pointers to interfaces, say I1 for this example. So if the user has a
pointer to object of type I1 and wants to convert it to I2*. Is there
a C++ standard way of converting it?
(....)

dynamic_cast works for me.
(I am not sure though, if this can work always, for example if the
library was built with different compiler settings or a different compiler)

Common Header File:
- - -
class Itf1 {
public:
    virtual ~Itf1() {}
    virtual void foo() = 0;
};

class Itf2 {
public:
    virtual ~Itf2() {}
    virtual void bar() = 0;
};

DLL1_API Itf1* GetItf();
- - -
Test Code:
- - -
{
Itf1* p = GetItf();
Itf2* p2 = dynamic_cast<Itf2*>(p);
p->foo();
p2->bar();
}
- - -

br,
Martin

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