Re: [boost] [range] [general] making member functions SFINAE-friendly

Subject: Re: [boost] [range] [general] making member functions SFINAE-friendly
From: Jonathan Wakely (jwakely.boost_at_[hidden])
Date: 2013-02-18 05:50:49

On 18 February 2013 10:19, Nathan Ridge wrote:
>
>> Could you share a use case that requires to depend on size()
>> specifically rather than the iterator category?
>
> Given an arbitrary range r, determine its size by the fasest
> means possible.
>
> If we use std::distance, we can do it in O(1) for std::vector,
> O(n) for std::map, and O(n) for std::forward_list.
>
> If we can detect whether "r.size()" is a valid expression,
> and use that if available, and std::distance otherwise, then
> we have O(1) for std::vector, O(1) for std::map, and O(n) for
> std::forward_list. Notice how that's an improvement for
> std::map.
>
> However, if detecting whether "r.size()" is a valid expression
> cannot be done reliably (for example, if we determine that for
> iterator_range<I> where I is not random-access it is a valid
> expression, but then that static-asserts on us), then we
> can't use this approach.

Yes, that perfectly sums up my use case, thanks.

I'm not "Depending on members of a third party component", I'm writing
a generic component and someone tried to use it with
boost::iterator_range and asked me why it didn't work.

The reason iterator_range::size() doesn't use std::distance is given
at http://permalink.gmane.org/gmane.comp.lib.boost.devel/193055
I think it's a valid choice to only allow size() when its O(1), c.f.
std::forward_list, but I think it would be even better to not even
define it when it can't be used, c.f. std::forward_list.