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

From: Peter Dimov (pdimov_at_[hidden])
Date: 2004-09-16 12:29:48

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.