Andrey Semashev skrev:
Thorsten Ottosen wrote:
Andrey Semashev skrev:
Thorsten Ottosen wrote:
Comments? Your approach requires the T to be default constructible which is not always the case. Why is this better than the far more expensive copy-construction?
1. It adds a new requirement to T.
But only for swap().
I'd rather not have it since, as other noted, optional is frequently used with non-default-constructible types. You'll simply restrict swap to be available only for default-constructible types, which is not the case ATM.
True.
2. I'd rather not assume what is more or less expensive. The opposite case is quite common too (shared pimpl object is an example).
In that case they are about the same complexity. But for any container or aggregation of containers, there is a huge difference. Just consider std::vector<std::map<int,int>>, for example.
Besides, to my mind, the current implementation is more obvious for the user. No user expects swap to throw or invalidate iterators or references like the current implementation does.
Default constructor may throw as well, you won't gain anything here.
Default constructors are much less likely to throw.
And I'm not sure I understand what you meant with iterators or references invalidation.
If I hold a reference or an iterator into, say a vector<T> object, that reference or iterator is not invalidated by swapping that object with another. For optional<vector<T>> this is no longer true. Anyway, I'll like to hear Fernando's take on it. -Thorsten