Re: C++ rtti hack

From:

brangdon@ntlworld.com (Dave Harris)

Newsgroups:

comp.lang.c++.moderated

Date:

Fri, 11 May 2007 14:59:52 CST

Message-ID:

<memo.20070511203724.3228A@brangdon.cix.compulink.co.uk>

bji-ggcpp@ischo.com () wrote (abridged):

Eventually I hit on this:

class Rtti
{
virtual ~Rtti() { }
};

const std::type_info &LookupTypeInfo(void *pObjectWithVtable)
{
return typeid(* (Rtti *) pObjectWithVtable);
}

I would test it with a class like:

    struct Base {
        int baseData;
    };
    struct Derived: Base {
        int moreData;
        virtual ~TestThis() {}
        int evenMoreData;
    }

    Derived testThis;

I would expect the compiler to store the RTTI in the vtable, and the
classes could be laid out like:

     struct _Rtti {
         struct __VTable *__pVTable;
     };

     struct _Base {
        int baseData;
     };

     struct _Derived {
         int baseData;
         int moreData;
         struct __VTable *__pVTable;
         int evenMoreData;
     };

If you are lucky the compiler might move pVTable to in front of moreData,
but it's unlikely to move it in front of baseData. So the offset of the
vtable is different in Derived to Rtti and LookupTypeInfo( &testThis )
will likely fail, when it ought to succeed.

I've not tested whether g++ does actually fail, because even if by some
miracle it works it's too unportable to rely on.

-- Dave Harris, Nottingham, UK.

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