on Mon Feb 18 2013, Jonathan Wakely <jwakely.boost-AT-kayari.org> wrote:
On 18 February 2013 18:04, Dave Abrahams wrote:
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.
It fixes my generic template so it can be used with iterator_range.
What's the general case you're referring to?
Do you mean because there are other cases (apart from iterator_range) where a size() member function exists, but must not be called?
Yes.
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).
Why? What other range-like types provide an size() member that appears to be callable but must not be called?
Anyone else's personal version of iterator_range, for example, that hasn't yet had this process applied to it. Also there are some types that might provide a size() with different meaning (or efficiency) than you expect. -- Dave Abrahams