Re: ((Object *)NULL)->MemberFunction();

From:

Kai-Uwe Bux <jkherciueh@gmx.net>

Newsgroups:

comp.lang.c++

Date:

Sat, 01 Mar 2008 22:15:21 -0500

Message-ID:

<fqd64a$k0n$1@aioe.org>

Ian Collins wrote:

jason.cipriani@gmail.com wrote:

Sorry about the subject but it's the most descriptive thing I came up
with. Is something like this safe to do:

// all objects let you get some string ID
class BaseObject {
public:
virtual const char * GetID (void) const;

Drop the C style void,

virtual const char* GetID() const = 0;

Is correct and idiomatic C++.

Well,

virtual const char * GetID (void) const = 0;

is correct C++, too.

// i can guarantee that no GetID implementations access any
// member data; they all just return a char array, nothing more.

// then, this function gets the ID of an object given it's type,
// without actually instantiating a T:
template <class T> const char * GetObjectID () {
return ((T *)NULL)->GetID();
}

This can not work for polymorphic objects. The object has to be
properly constructed in order to call a virtual method.

The code dereferences a null pointer. It has undefined behavior regardless
of whether the method is virtual or not and of whether the the "object"
(which does not exist) is polymorphic or not.

Best

Kai-Uwe Bux