Re: about pointers on class' members

From:

Michael Doubez <michael.doubez@free.fr>

Newsgroups:

comp.lang.c++

Date:

Tue, 22 Dec 2009 06:18:55 -0800 (PST)

Message-ID:

<513b329a-c751-43de-8c50-a79cddeecf89@s31g2000yqs.googlegroups.com>

On 22 d=E9c, 13:51, Chameleon <gessos.p...@yahoo.gr> wrote:

We have this structure:
-----------------------
struct A {
        struct B {
        ...
        } b;

        ...

        struct C {
                void no_way_dude();
        } c;};

-----------------------

How can I have access on 'b' from inside of 'no_way_dude()'?
I mean without pass parameters to 'C()' or to 'no_way_dude()'
You can use 'friend' as you wish.

Until now I use:
B &b = *(B*)(((char*) this) - sizeof(B));
But it is totally lame.
If I add a definition between B and C, it will screwed.

Basically I need something like:
off_t rel = (char*) A::C - (char*) A::B;

Any workaround without pass pointers to functions?

There is a solution but it is not practical with the current standard,
a little bit better with the next (I will explain latter): the trick
is to use the offsetof() macro.

struct A {
        struct B {
        ...
        } b;

        ...

        struct C {
                void no_way_dude()
                {
                  B& my_b = owner().b;
                }
         private:
            A& owner() {
                return *reinterpret_cast<A*>(
                    reinterpret_cast<char*>(this) -
                    offsetof(A, c));
            }

        } c;
};

It has the drawback that it is not guaranteed by the current standard
if A is not a POD and in the next standard if A is not a /standard
layout/ class.

Which means that
- with the current standard, you have UB as soon as you define a
constructor or private/public areas.
- with the next standard, you have UB if you add virtual functions or
virtual inheritance.

Now in practice, compilers such as gcc and I think VC++ already
support the offsetof() macro with a simple class (without virtual
elements).

It is a risky trick and you may be better without it.

--
Michael