Re: compare two objects, if they have the same vtable

From:

Seungbeom Kim <musiphil@bawi.org>

Newsgroups:

comp.lang.c++.moderated

Date:

Sun, 5 Oct 2008 13:56:24 CST

Message-ID:

<gc8bfl$75c$1@news.stanford.edu>

Albert Zeyer wrote:

Bart van Ingen Schenau schrieb:

RTTI and dynamic_cast<> are defined in such a way that a runtime lookup
is only needed if the object can have polymorphic behaviour, as
indicated by the presence of a virtual member function.

Ah, that's interesting. I always thought it adds the vtable-pointer to
everything then.

That would have violated one of the principal design rules of C++,
the zero-overhead rule: "What you don't use, you don't pay for."

If you try to use RTTI with a non-polymorphic object, all the
information will be (assumed to be) present at compile time, so there
is no need to add an otherwise useless vtable/vptr.

So, the following is not possible then?

class Base { public: int data1; };
class Derived : public Base { public: int data2; };

Base* obj = new Derived();
Derived* d = dynamic_cast<Derived*>(obj);

It is not allowed. You can try it yourself at Comeau Online:
<http://www.comeaucomputing.com/tryitout/>.

--
Seungbeom Kim

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