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### 3123. `duration` constructor from representation shouldn't be effectively non-throwing

Section: 27.5 [time.duration] Status: WP Submitter: Johel Ernesto Guerrero Peña Opened: 2018-06-22 Last modified: 2021-10-14

Priority: 3

View all other issues in [time.duration].

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Discussion:

[time.duration]/4 states:

> Members of duration shall not throw exceptions other than those thrown by the indicated operations on their representations.

Where representation is defined in the non-normative, brief description at [time.duration]/1:

> […] A duration has a representation which holds a count of ticks and a tick period. […]

[time.duration.cons]/2 doesn't indicate the operation undergone by its representation, merely stating a postcondition in [time.duration.cons]/3:

> Effects: Constructs an object of type `duration`.

> Postconditions: `count() == static_cast<rep>(r)`.

I suggest this reformulation that follows the format of [time.duration.cons]/5.

> Effects: Constructs an object of type duration, constructing `rep_` from `r`.

Now it is clear why the constructor would throw.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4750.

> Change 27.5.2 [time.duration.cons] as indicated:

> >     template<class Rep2>
> >       constexpr explicit duration(const Rep2& r);

> > > -1- Remarks: This constructor shall not participate in overload resolution unless […]

> > > -2- Effects: Constructs an object of type `duration`, constructing `rep_` from `r`.

> > > -3- Postconditions: `count() == static_cast<rep>(r)`.

[2018-06-27 after reflector discussion]

Priority set to 3. Improved wording as result of that discussion.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4750.

> Change 27.5.2 [time.duration.cons] as indicated:

> >     template<class Rep2>
> >       constexpr explicit duration(const Rep2& r);

> > > -1- Remarks: This constructor shall not participate in overload resolution unless […]

> > > -2- Effects: Constructs an object of type `duration` Initializes `rep_` with `r`.

> > > -3- Postconditions: `count() == static_cast<rep>(r)`.

[2020-05-02; Daniel resyncs wording with recent working draft]

[2021-09-20; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4861.

Change 27.5.2 [time.duration.cons] as indicated:

>     template<class Rep2>
>       constexpr explicit duration(const Rep2& r);

> > -1- Constraints: `is_convertible_v<const Rep2&, rep>` is `true` and

> > 1. (1.1) — `treat_as_floating_point_v<rep>` is `true` or

> > 2. (1.2) — `treat_as_floating_point_v<Rep2>` is `false`.

> > [Example: […] end example]

> > -?- Effects: Initializes `rep_` with `r`.

> > -2- Postconditions: `count() == static_cast<rep>(r)`.