Re: Getting address of enclosing object from address of inner object

From:
courpron@gmail.com
Newsgroups:
comp.lang.c++.moderated
Date:
Sat, 19 Apr 2008 18:15:05 CST
Message-ID:
<98f84ca5-ac0b-4c81-9e65-2f0ad273c6f0@e67g2000hsa.googlegroups.com>
On Apr 19, 1:16 am, xtrigger...@gmail.com wrote:

Thanks a lot to all.

I got Lance point.
But I just want to get the address of the first enclosing object.
I'm not interested in the dynamic type of an object that may have
derived from this "first encloser".
( Hope my english is good enough ).


If you don't care at all about the dynamic type of the object, then it
may work in practice.

PLUS, I do not understand why this technique should not work with non
POD types.


It ultimately depends on the compiler. For example, popular
implementations of multiple and virtual inheritance involve member
reordering. Consider the following example :

#include <cassert>
#include <cstddef>

struct COuter {
    char mPad1[ 5 ];
    int mInner;
    char mPad2[ 5 ];
};

struct Base : public virtual COuter {
    int b;
};

struct Derived :public Base, public virtual COuter {
    int d;
};

void* GetEnclAddrFromInner ( int * inner )
{
    return (void*) ( (char*)inner -
                      offsetof(COuter, mInner) );
}

void CheckPointers ( void* true_ptr, COuter * obj ) {
    assert( true_ptr == GetEnclAddrFromInner( &obj->mInner ) );
}

int main()
{
    COuter a;
    Derived b;

    CheckPointers( (void*)&a, &a ); #1
    CheckPointers( (void*)&b, &b ); #2
}

Line #1 will probably be ok, while line #2 probably won't pass the
assertion, depending on the compiler.

Alexandre Courpron.

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