Re: [Solved] sequence of inheritance from virtual base class?

From:

jgk@panix.com (Joe keane)

Newsgroups:

comp.lang.c++.moderated

Date:

Tue, 21 May 2013 14:15:22 CST

Message-ID:

<kngcfn$31d$1@reader1.panix.com>

In article <knan10$67p$1@dont-email.me>, Wil Evers <bouncer@dev.null> wrote:

I think the lesson to learn from this thread is that a void pointer
obtained from the address of an X must not be cast back to a pointer
to some other type, even if that other type is closely related to X.

What we know is that this will work:

    Base *bp1;
    void *vp;
    Base *bp2;

    ...
    vp = (void *) bp1;
    bp2 = (Base *) vp;

also

    Derived *dp1;
    void *vp;
    Derived *dp2;

    ...
    vp = (void *) dp1;
    dp2 = (Derived *) vp;

We also know that this will work:

    Derived *dp;
    Base *bp;

    ...
    bp = (Base *) dp;

And -maybe- there is some way to go backward:

    Base *bp;
    Derived *dp;

    ...
    dp = ???_cast<Derived *> bp;

You just can't mix them up!

So don't lie to the compiler.

There is an exception:

    struct bar
    {
    int bar_a;
    };

    struct foo
    {
    struct bar foo_a;
    int foo_b;
    };

    struct foo *fp;
    struct bar *bp;

    ...
    bp = (struct foo *) fp;
    ...
    fp = (struct bar *) bp;

But it is some C rule...

    1) no base classes, cause that's not C
    2) no virtual methods, cause that's not C
    3) no virtual inheritance, cause that's not C

So don't try it with a 'complicated' class.

It won't work, or if it does, it won't work some time.

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