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Subject: Re: [ub] type punning through congruent base class?
From: Fabio Fracassi (f.fracassi_at_[hidden])
Date: 2014-01-06 09:34:56


> From: ub-bounces_at_[hidden] [mailto:ub-bounces_at_[hidden]] On
> Behalf Of Gabriel Dos Reis
> Sent: Montag, 6. Januar 2014 15:33
>
> "Fabio Fracassi" <f.fracassi_at_[hidden]> writes:
>
> | Hello UB-Experts,
> |
> | as far as I can see the following (static_cast) is UB:
> |
> | struct B {
> | int i;
> | };
> |
> | struct D : B {
> | void foo() { /* access B::i */ }
> | };
> |
> | B b;
> | static_cast<D&>(b).foo();
> |
> | because of [expr.static.cast] clause 11
> |
> | first question: is my assessment of the situation correct or is this
> | use legal?
> | if it is not (legal): could we make it legal or would we run afoul of
> | the aliasing rules?
>
> Yes, it is undefined behavior -- you're referring to an inexistent
> (sub)object of type D.
>
> Why can't you create an object of type D if your program needs it? Why
> is it a hardship?
>

It would enable us to extend or change the interface of a class without
copying or moving the underlying object (think mixin without additional data).
Of course we could make our "extention members" (like foo() in the example above)
free functions which take a B&, but the grouping and the access to protected
members would simplify the design.

But use case for this example aside (for now), I'd still be curious if we were to
allow this, would it interfere with aliasing?

best regards

Fabio

 


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