[boost] Re: Proposal for New Template Metafunction 'is_iterator'

From: Jonathan Turkanis (technews_at_[hidden])
Date: 2004-08-22 13:51:04

"Alex Chovanec" <achovane_at_[hidden]> wrote in message
news:cganiq$q27$1_at_sea.gmane.org...
>
> "Jonathan Turkanis" <technews_at_[hidden]> wrote in message
> news:cgal34$l63$1_at_sea.gmane.org...
>
> > Currently there is detail::is_incrementable, which works well if
you
> > only need to distinguish between iterators and very different
types.
> > For instance, I would guess that very few if any containers are
> > incrementable (I'd be happy to be proved wrong here), so that if
you
> > want define a function which behaves differently for container and
> > iterators you can use is_incrementable.
>
> In this case, I need to know whether or not type T is
dereferenceable

I've attached a naively modified version of David Abraham's
is_incrementable as a first attempt at is_dereferenceable. There may
be all sorts of subtle issues I haven't though of.

> (and
> more generally, whether or not it's an iterator).

Why? When you write a template, you always have some idea of the range
of types for which the template should be instantiated. It's rarely
instantiable for all types. Given this, all you need is a way to
separate iterators from the other candidate template arguments.

> I have a small template
> metaprogram which determines how an instance of type T should be
treated. If
> it's an iterator, I want to dereference it. Otherwise, I don't.
>
> > Don't forget pointers. Also these operators can also be defined as
> > non-member functions.
>
> Pointers are fairly easy to handle.

True.

> You bring up an important point about operators defined as
non-member
> functions. My current implementation doesn't handle this case, but I
may yet
> be able to fix that. :-)

Try the attachment.

> Thanks for the feedback,
>
> Alex

begin 666 is_dereferenceable.hpp
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`
end