Re: [boost] [interest] underlying type library

Subject: Re: [boost] [interest] underlying type library
From: TONGARI (tongari95_at_[hidden])
Date: 2011-08-20 13:42:16

2011/8/21 Julian Gonggrijp <j.gonggrijp_at_[hidden]>

> Mathias Gaunard wrote:
>
> > On 08/20/2011 06:09 PM, Julian Gonggrijp wrote:
> >
> >> UNDERLYING_TYPE(T) represents a type which is (conceptually speaking)
> >> a POD-type with the same member data (and hence the same alignment) as
> >> T.
> >
> > You mean like boost::aligned_storage<sizeof(T),
> boost::alignment_of<T>::value> ?
>
> Pretty much. I think it could do the job.
> (Although of course it's not THE underlying type in a very strict
> conceptual sense.)
>
> >> template<class T>
> >> void swap (T&  x, T&  y) {
> >>     UNDERLYING_TYPE(T) temp;
> >>     move_raw (x, temp);
> >>     move_raw (y, x);
> >>     move_raw (temp, y);
> >> }
> >
> > This clearly looks like undefined behaviour.
>
> It isn't. :-)
> The code is guaranteed to swap x and y and leave them in a valid
> state. temp will be discarded like any POD object, without any chance
> for unwanted side effects.
> For clarity, I have already implemented and tested this and everything
> works as expected.
>

Do you have codes? Something that just works may not be guaranteed to
work...

 > You cannot use bitwise copy to implement move semantics.
> > That just doesn't work.
>
> It probably depends on what you mean by 'move semantics' and what you
> mean by 'implement'. What I'm sure of, is that my library can
> (unintrusively) simulate move constructors, move destructors and the
> returning of rvalue references from functions. Of course, it doesn't
> use *only* bitwise copies for that purpose and the type involved needs
> to be default-constructible.
>

What type needs to be default-constructible?
Something like underlying_type_traits<T>::underlying_type or T itself?