[boost] Finding algorithms returning ranges?

From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2008-07-04 12:09:57

Finding algorithms (find, find_if, max_element, and others) usually
return an iterator to the found position, or the past-the-end iterator.

While enough, it can be quite annoying when used with range adaptors and
the like.

Imagine something like this:

auto r = some_range | some adapters...;
auto it = max_element(r, comparison_function);
if(it != end(r))
{
    do_something;
}

Without auto, it is simply unusable.

If such algorithms returned the range [found, end), we would be able to
write:

if(!empty(max_element(some_range | some adapters..., comparison_function))
{
    do_something;
}

Particularly, I find it practical to use it with a function like that:
optional<typename range_value<Range>::type>
deref_opt(const Range& r)
{
    typedef typename range_value<Range>::type T;
    return empty(r) ? optional<T>() : optional<T>(*begin(r));
}

or like that:
typename range_value<Range>::type
deref_def(const Range& r)
{
    typedef typename range_value<Range>::type T;
    return empty(r) ? T() : *begin(r);
}

For compatibility, maybe returning a range type that implicitly converts
to the beginning iterator could work.

Do people think this is a good idea?