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### 951. Various threading bugs #1

Section: 27.4.1 [time.traits.is.fp] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

Related to 953.

[time.traits.is_fp] says that the type `Rep` "is assumed to be ... a class emulating an integral type." What are the requirements for such a type?

[ 2009-05-10 Howard adds: ]

> `IntegralLike`.

[ Batavia (2009-05): ]

> As with issue 953, we recommend this issue be addressed in the context of providing concepts for the entire `thread` header.

> We look forward to proposed wording.

> Move to Open.

[ 2009-08-01 Howard adds: ]

> I have surveyed all clauses of [time.traits.duration_values], 27.4.3 [time.traits.specializations] and 27.5 [time.duration]. I can not find any clause which involves the use of a `duration::rep` type where the requirements on the `rep` type are not clearly spelled out. These requirements were carefully crafted to allow any arithmetic type, or any user-defined type emulating an arithmetic type.

> Indeed, `treat_as_floating_point` becomes completely superfluous if `duration::rep` can never be a class type.

> There will be some `Rep` types which will not meet the requirements of every `duration` operation. This is no different than the fact that `vector<T>` can easily be used for types `T` which are not `DefaultConstructible`, even though some members of `vector<T>` require `T` to be `DefaultConstructible`. This is why the requirements on `Rep` are specified for each operation individually.

> In [time.traits.is_fp] p1:

> >     template <class Rep> struct treat_as_floating_point
> >       : is_floating_point<Rep> { };

> > > The `duration` template uses the `treat_as_floating_point` trait to help determine if a `duration` object can be converted to another `duration` with a different tick period. If `treat_as_floating_point<Rep>::value` is `true`, then `Rep` is a floating-point type and implicit conversions are allowed among `duration`s. Otherwise, the implicit convertibility depends on the tick periods of the `duration`s. If `Rep` is a class type which emulates a floating-point type, the author of `Rep` can specialize `treat_as_floating_point` so that `duration` will treat this `Rep` as if it were a floating-point type. Otherwise `Rep` is assumed to be an integral type or a class emulating an integral type.

> The phrases "a class type which emulates a floating-point type" and "a class emulating an integral type" are clarifying phrases which refer to the summation of all the requirements on the `Rep` type specified in detail elsewhere (and should not be repeated here).

> This specification has been implemented, now multiple times, and the experience has been favorable. The current specification clearly specifies the requirements at each point of use (though I'd be happy to fix any place I may have missed, but none has been pointed out).

> I am amenable to improved wording of this paragraph (and any others), but do not have any suggestions for improved wording at this time. I am strongly opposed to changes which would significantly alter the semantics of the specification under 27 [time] without firmly grounded and documented rationale, example implementation, testing, and user experience which relates a positive experience.

> I recommend NAD unless someone wants to produce some clarifying wording.

[ 2009-10 Santa Cruz: ]

> Stefanus to provide wording to turn this into a note.

[ 2010-02-11 Stefanus provided wording. ]

[ 2010 Rapperswil: ]

> Move to Ready.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change [time.traits.is_fp]/1:

> 1 The `duration` template uses the `treat_as_floating_point` trait to help determine if a `duration` object can be converted to another `duration` with a different tick period. If `treat_as_floating_point<Rep>::value` is `true`, then `Rep` is a floating-point type and implicit conversions are allowed among `duration`s. Otherwise, the implicit convertibility depends on the tick periods of the `duration`s. If `Rep` is a class type which emulates a floating-point type, the author of `Rep` can specialize `treat_as_floating_point` so that duration will treat this `Rep` as if it were a floating-point type. Otherwise `Rep` is assumed to be an integral type or a class emulating an integral type. [Note: The intention of this trait is to indicate whether a given class behaves like a floating point type, and thus allows division of one value by another with acceptable loss of precision. If `treat_as_floating_point<Rep>::value` is `false`, `Rep` will be treated as if it behaved like an integral type for the purpose of these conversions. — end note]