Re: check for polymorphism during runtime

From:

Tom Widmer <tom_usenet@hotmail.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 26 Jun 2006 14:41:19 +0100

Message-ID:

<e7ontr$f04$1@nntp.aioe.org>

Jan Sch4fer wrote:

Hi all,

is there a simple way to find out during runtime if a class of some type
is polymorphic? I am writing some serialization functions and need to
handle polymorphic and non-polymorphic classes differently. I am sure
there are hundreds of workarounds for this, but I am interested if this
can be done by some clever function with the help of casting and/or rtti
which determines if a class is polymorphic or not.
I found a is_polymorphic template in the boost libraries but didn't get
the trick yet!?

Any ideas?

You've answered your own question - boost::is_polymorphic (a.k.a
std::tr1::is_polymorphic). It's part of the new extension to the
standard, TR1, and an implementation is also available from here:
http://www.dinkumware.com/manuals/?manual=compleat&page=typetrait.html#is_polymorphic

There is no portable standard conforming solution AFAIK - boost relies
on a non-portable hack to do with the fact that polymorphic classes have
a vtable pointer, and the standard library version can do it how it likes.

IIRC the boost implementation is based on this non-portable idea:

template <class T>
struct is_polymorphic
{
private:
   struct dummy: T
   {
     virtual void f();
   };

public:
   static bool const value = sizeof(dummy) == sizeof(T);
};

If adding a virtual function doesn't change the size of the class, it
must already have a vtable pointer. Obviously, a real implementation
will handle non-class types too (returning false).

Tom