On Wed, Feb 14, 2018 at 10:48 PM, Nevin Liber via Boost <boost@lists.boost.org> wrote:
On Wed, Feb 14, 2018 at 3:18 PM, Miguel Ojeda via Boost < boost@lists.boost.org> wrote:
IMO, such a class is definitely useful.
Thanks *a lot* for the almost instantaneous review! :)
Design concerns:
- unique_val should be copyable if T is copyable. Whether or not a type which aggregates this is copyable is an orthogonal, separate concern from what the moved-from state should be. If you go this route, unique_val is the wrong name.
As Steven mentions, it cannot be copyable since it is meant to hold values where you need to perform an action on deletion once per value, i.e. I meant "unique" here as the same as "unique_ptr" means: only one "owner" is supposed to hold a given value.
- Instead of providing operator bool(), why not provide operator T&() and operator T const&()?
Hum, that could be interesting, not sure of all the implications, though.
- What about comparisons of the form: unique_val<T, a> op unique_val<T, b>? - What about comparisons of the form: unique_val<T> op T (as well as T op unique_val<T>)?
Indeed, I suggest the second ones in the Rationale as possible extensions, I think they make quite a lot of sense. I thought about the first ones, but I don't think they are that useful in practise, since when you are using the unique_val, you already know what your "invalid" or "moved-from" or "default" value has to be, so you will not be using different ones (i.e. I don't think in practise unique_val<T, x> and unique_val<T, y> where x != y is a common case).
- Propagate noexcept correctly, as it is dependent on the underlying operations.
I thought that, given that T has to be a type suitable for the default (which itself is a non-type template parameter), it couldn't be anything throwing exceptions -- but I may be wrong here; please let me know!
- Propagate constexpr correctly (for instance, your default constructor should be constexpr).
Indeed, probably a few constexpr's are missing. I only added the obvious get_default() one since I am not sure what are the best practices for writing them for C++11, C++14 and C++17 at the same time without breaking something. Since it was optional, I left it at that for the moment. Thanks a lot again! Miguel
-- Nevin ":-)" Liber <mailto:nevin@cplusplusguy.com <nevin@eviloverlord.com>> +1-847-691-1404 <(847)%20691-1404>
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