Re: [boost] [iterator] Is there general const_iterator adaptor?

Subject: Re: [boost] [iterator] Is there general const_iterator adaptor?
From: TONGARI (tongari95_at_[hidden])
Date: 2013-04-29 13:54:00

2013/4/30 Jeffrey Lee Hellrung, Jr. <jeffrey.hellrung_at_[hidden]>

> On Sun, Apr 28, 2013 at 6:38 AM, Philipp Moeller <
> philipp.moeller_at_[hidden]> wrote:
>
> > TONGARI <tongari95_at_[hidden]> writes:
> >
> > > Hi folks,
> > >
> > > I wonder if there's a general const_iterator adaptor that transforms
> any
> > > iterator to a const one:
> > >
> > >     template<class Iterator>
> > >     struct const_iterator
> > >       : iterator_adaptor
> > >         <
> > >             const_iterator<Iterator>, Iterator
> > >           , typename iterator_value<Iterator>::type const
> > >         >
> > >     {
> > >         const_iterator() {}
> > >
> > >         const_iterator(Iterator const& base)
> > >           : const_iterator::iterator_adaptor_(base)
> > >         {}
> > >     };
> > >
> > > Which is simple but handy for use in that we don't have to make 2
> > versions
> > > for both const & non-const iterators each time we make a container.
> > >
> > > Does Boost offer this which I missed somewhere, or would you think
> it's a
> > > good idea to add?
> >
> > If you look at the documentation of iterator_facade you will see that it
> > already shows how to just write one version of the iterator and get a
> > const version for free: pass a `const T` as the second argument of
> > iterator_face and it should work.
> >
>
> I think this is typically essentially what you want to do. Define a helper
> template like
>
> template< class T > struct iterator_helper { typedef /*...*/ type; };
>
> then
>
> typedef iterator_helper< value_type >::type iterator;
> typedef iterator_helper< value_type const >::type const_iterator;
>
> If that isn't possible or convenient, your iterator and const_iterator are
> probably sufficiently different that defining them separately will be
> acceptable.
>
> The typical problem with your const_iterator adaptor above is that it
> likely can't be constructed in a const context, since you wouldn't be able
> to obtain an (mutating) iterator (unless you do const_cast contortions).
>

True, that's what I did exactly (i.e. const_cast).

> Also, shouldn't the reference type passed to iterator_adaptor also need to
> be "constified"? Which gets complicated when the reference type is a proxy
> reference...
>

Haven't considered that, so the non-intrusive const_iterator may not be
general enough, but  what I looked for was something that works for most
cases.

We could have that for our 'detail' usage, but if it's common enough, then
it might be nice to have it in public.

So would you suggest such an adaptor for those simple cases no more than
adding the const?