Den 27-10-2017 kl. 14:47 skrev Ion Gaztañaga via Boost:
On 26/10/2017 20:10, Thorsten Ottosen via Boost wrote:
But I think it's the best definition we have seen.
Yes, not only that. These days I was thinking about a modified capacity() definition.
If size() == capacity() an insertion can provoke more than optimal data movement (iterator invalidation) and (optionally) reallocation. That could be implemented if capacity() is defined as:
max(size(), min(back_capacity(), front_capacity());
Is this not just equivalent to min(back_capacity(), front_capacity()) given that back/front_capacity >= size()?
If an insertion is done in the side that has free capacity, no data movement and memory allocation is done, but for the user is similar to a in-place buffer expansion. Capacity has grown to accommodate the insertion without moving anything more that needed.
If a user want's to be sure if an insertion in one side will be supported without more than optimal data movement or iterator invalidation, then he/she can check back/front_capacity() against size().
Sure, we can make new semantics. I think Joaquin's concern was that if we provide both capacity() and reserve(), we better do it 100% compatible with vector. In our source code, I found just one mention of capacity(). In the dependencies, less than fourty. And only one involved a comparison with size(). That one in 800-900k lines of code. Other code bases may be different. And given that we do provide means for exact queries (this code will reallocate unless you call reserve_xxx), I don't know if it makes any sense to use time on capacity. My only point was: Nevin's definition would solve Joaquin's concern if reserve() was an alias for reserve_back(), but since we may not want to do that (e.g., depending on policy, reserve() could allocate space in both ends), I think the best option is to drop capacity() for devector and dream about a standard that had used the names back_capacity()/reserve_back()/resize_back() for what it now calls capacity()/reserve()/resize(). kind regards Thorsten