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

Subject: Re: [boost] [iterator] Is there general const_iterator adaptor?
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2013-04-29 13:01:02

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).

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...

- Jeff