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

On Sun, Apr 28, 2013 at 6:38 AM, Philipp Moeller < philipp.moeller@geometryfactory.com> wrote:

TONGARI 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