[LLVMdev] [RFC] Add empty() method to iterator_range.

[Alexey Samsonov]

On Wed, Mar 19, 2014 at 10:57 PM, Duncan P. N. Exon Smith <
dexonsmith at apple.com> wrote:

>
> On Mar 19, 2014, at 11:32 AM, Aaron Ballman <aaron at aaronballman.com>
> wrote:
>
> > On Wed, Mar 19, 2014 at 2:13 PM, Lang Hames <lhames at gmail.com> wrote:
> >> Hi all,
> >>
> >> As RFCs go this is short and sweet - I think it would be nice to add an
> >> empty method to iterator_range. Something like:
> >>
> >> bool empty() const { return begin() != end(); }
> >>
> >> My motivation is to make the 'if' test at the start of this common
> pattern
> >> tidier:
> >>
> >> if (!collection.empty())
> >>  // initialization...
> >> for (auto& c : collection)
> >>  // do something for each c.
> >>
> >> which I think that is preferable to:
> >>
> >> if (collection.begin() != collection.end())
> >>  // initialization...
> >> // for each...
> >>
> >> The empty method is just a single iterator comparison, so it should be
> valid
> >> and cheap for any underlying iterator type.
> >>
> >> Pros:
> >> - Enables small aesthetic improvements.
> >> - Would make iterator_range look slightly more "container-like", so it
> could
> >> substitute into more template code, though I'd expect templates that
> take
> >> read-only containers rather than iterators to be very rare in C++, and
> >> templates that take collections and use only begin, end and empty to be
> >> rarer still.
> >>
> >> Cons: ?
>
> I’m in favour.
>

+1. I also ran into this pattern, and suggested to add empty() method in
r203354 review thread.


>
> > This is a pattern I ran into a handful of times when range-ifying
> > parts of clang (but not so frequently that I felt it was a major win),
> > so I'm roughly in favor of the API. However, if there's been
> > standardization efforts for range, we should be sure that we're doing
> > something compatible there.
>
> There are a couple of proposals [1][2] that I can find.  It sounds like the
> exact set of methods to provide is somewhat contentious [3].  empty() seems
> pretty innocuous though.  Note that the standard proposals use std::range
> instead of std::iterator_range.
>
> In my opinion, other methods (e.g., front(), back(), and (for random access
> iterators) operator[]) would also be useful.
>
> [1]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1871.html
> [2]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3350.html
> [3]:
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3513.html#range-arg
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-- 
Alexey Samsonov, MSK
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