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

Subject: Re: [boost] [range] [general] making member functions SFINAE-friendly
From: Dave Abrahams (dave_at_[hidden])
Date: 2013-02-18 13:04:17

on Mon Feb 18 2013, Nathan Ridge <zeratul976-AT-hotmail.com> 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.

But you can't detect that in general.  If you're trying to make this
work in generic code, simply fixing it for iterator_range doesn't fix
anything.

> 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.

Then you can't use this approach (in general).

-- 
Dave Abrahams