Re: [boost] [iterator] could non-small predicates/functors be stored as boost::optional<T> to avoid double construction?

Subject: Re: [boost] [iterator] could non-small predicates/functors be stored as boost::optional<T> to avoid double construction?
From: David Abrahams (dave_at_[hidden])
Date: 2008-09-04 15:07:45

on Thu Sep 04 2008, "Giovanni Piero Deretta" <gpderetta-AT-gmail.com> wrote:

> On Thu, Sep 4, 2008 at 8:18 PM, Thorsten Ottosen
> <thorsten.ottosen_at_[hidden]> wrote:
>> David Abrahams skrev:
>>>
>>> on Thu Sep 04 2008, Thorsten Ottosen <thorsten.ottosen-AT-dezide.com>
>>> wrote:
>>>
>>>> Arno Schödl skrev:
>>>>>>
>>>>>> Hi,
>>>>>> The headline pretty much says it all. Take transform iterator. When the
>>>>>> predicate is not trivial, we also have a redundant construction of the
>>>>>> object
>>>>>> in the end iterator. Maybe that could be removed?
>>>>>> best regards
>>>>>> -Thorsten
>>>>>
>>>>> I guess that would be restricted to forward iterators.
>>>>
>>>> This has nothing to do with the wrapped iterator's type. A transform
>>>> iterator
>>>> currently stores a functor by value in addition to the wrapped iterator.
>>>
>>> Have you forgotten
>>>
>>>  *--end
>>
>> Yes. Sorry for the noise.
>>
>
> There is still some value in storing functors in optional<T>:
> boost.lambda expressions are not asignable, thus rendering any
> iterator that contains a lambda does no longer meet the requirements
> of  Assignable of which TrivialIterator is a refinement of.
>
> It is easy to add a wrapper around optional<T> that makes it
> Assignable as long as T is copy constructible (which lambdas are), so
> it can be used as a quick workaround for this problem.

But for that you wouldn't need the bool in optional.

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com