Re: [boost] Is Boost.Range broken?

Subject: Re: [boost] Is Boost.Range broken?
From: Giovanni Piero Deretta (gpderetta_at_[hidden])
Date: 2008-11-26 06:46:53

On Wed, Nov 26, 2008 at 4:46 AM, Scott McMurray <me22.ca+boost_at_[hidden]> wrote:
> On Tue, Nov 25, 2008 at 13:31, David Abrahams <dave_at_[hidden]> wrote:
>>
>> on Mon Nov 24 2008, "Tomas Puverle" <Tomas.Puverle-AT-morganstanley.com> wrote:
>>
>>>> There's no reason that they both have to be in boost. Boost range
>>>> should let you use your own range class.
>>>
>>> And you're absolutely right.  Having said that, it's not inconceivable that
>>> others may have been relying on the same behavour.
>>> That's one of the things this discussion was about.
>>
>> Yes.  The library maintainer ought to do something to help those people,
>> by providing the old behavior in code somewhere (even if that code only
>> appears in docs).
>>
>
> How about providing code something like this (thanks to Boost.Iterator):
>
>    template <class Iter>
>    class nonsingular_default
>      : public boost::iterator_adaptor<
>            node_iter<Value>                // Derived
>          , Iter                            // Base
>        >
>    {
>     private:
>        bool is_default_constructed;
>
>     public:
>        node_iter()
>          : node_iter::iterator_adaptor_(),
>            is_default_constructed(true) {}
>
>        explicit node_iter(Iter i)
>          : node_iter::iterator_adaptor_(i),
>            is_default_constructed(false) {}
>
>     private:
>        friend class boost::iterator_core_access;
>        bool equal(nonsingular_default i) {
>            if (is_default_constructed && i.is_default_constructed) {
>                return true;
>            }
>            if (!is_default_constructed && !i.is_default_constructed) {
>                return base == i.base();
>            }
>            return false;
>        }
>    };
>

[ i think you want to change all node_iter to  nonsingular_default ]

> Allowing recovery of the old semantics using
>
>    iterator_range< nonsingular_default<I> >
>
> (at least for Tomas Puverle's use case.)
>
> This, of course, depends on the asserts getting removed, but that
> behaviour is also replicable using the same technique.

I do not think you can even copy a singular iterator (and at least
libstdc++ will assert in debug mode), so I do not think  that the
above will let you have safe default constructable iterators (in fact
I think this is a problem even with the old range implementation, a
good reason to have changed it).

boost.optional might help, but I have this feeling that it will hinder
optimizations.

-- 
gpd