[boost] Re: making bind and mem_fn results default constructible?

From: Fredrik Blomqvist (fredrik_blomqvist_at_[hidden])
Date: 2004-09-16 14:09:32

Peter Dimov wrote:
> Fredrik Blomqvist wrote:
>> Hi
>>
>> Consider this example of mem_fn usage:
>>
>> struct X { int bar; };
>> std::vector<X> vec;
>> ....
>> boost::make_transform_iterator(vec.begin(), boost::mem_fn(&X::bar));
>>
>> The above is, strictly speaking, _not_ a Random Access Traversal
>> Iterator,
>> or even a Forward Traversal Iterator, and would fail in otherwise
>> concept conforming code (or in a potential concept-check).
>>
>> The problems lies in the function objects returned by boost::mem_fn
>> and boost::bind not being default constructible, thus making the
>> resulting iterator not default constructible which is required by the
>> Forward Traversal concept.
>
> No, the problem lies in (the current implementation of)
> transform_iterator. It is not reasonable to require function objects
> to be default
> constructible, because few of them are, and in fact, the documentation
> places no such requirement on the function object.
>

Ah, well, my intention wasn't really to focus on transform_iterator, I just
used it as an example. Although I can see that you're right about it not
being in sync with it's documentation.
My point was that default-constructability would make creating _any_ kind of
adaptor based on bind/mem_fn easier and more flexible. As long as the bound
expression is default constructible a default constructor in bind/mem_fn
would simply expose that and improve implementation convenience. The
potential 'no default constructor' error would simply move to its "proper
place" IMO.
Am I missing something?

// Fredrik Blomqvist