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