947. duration arithmetic: contradictory requirements

Section: 30.5.6 [time.duration.nonmember] Status: Resolved Submitter: Pete Becker Opened: 2008-12-20 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

In 30.5.6 [time.duration.nonmember], paragraph 8 says that calling dur / rep when rep is an instantiation of duration requires a diagnostic. That's followed by an operator/ that takes two durations. So dur1 / dur2 is legal under the second version, but requires a diagnostic under the first.

[ Howard adds: ]

Please see the thread starting with c++std-lib-22980 for more information.

[ Batavia (2009-05): ]

Move to Open, pending proposed wording (and preferably an implementation).

[ 2009-07-27 Howard adds: ]

I've addressed this issue under the proposed wording for 1177 which cleans up several places under 30.5 [time.duration] which used the phrase "diagnostic required".

For clarity's sake, here is an example implementation of the constrained operator/:

template <class _Duration, class _Rep, bool = __is_duration<_Rep>::value>
struct __duration_divide_result
{
};

template <class _Duration, class _Rep2,
    bool = is_convertible<_Rep2,
                          typename common_type<typename _Duration::rep, _Rep2>::type>::value>
struct __duration_divide_imp
{
};

template <class _Rep1, class _Period, class _Rep2>
struct __duration_divide_imp<duration<_Rep1, _Period>, _Rep2, true>
{
    typedef duration<typename common_type<_Rep1, _Rep2>::type, _Period> type;
};

template <class _Rep1, class _Period, class _Rep2>
struct __duration_divide_result<duration<_Rep1, _Period>, _Rep2, false>
    : __duration_divide_imp<duration<_Rep1, _Period>, _Rep2>
{
};

template <class _Rep1, class _Period, class _Rep2>
inline
typename __duration_divide_result<duration<_Rep1, _Period>, _Rep2>::type
operator/(const duration<_Rep1, _Period>& __d, const _Rep2& __s)
{
    typedef typename common_type<_Rep1, _Rep2>::type _Cr;
    duration<_Cr, _Period> __r = __d;
    __r /= static_cast<_Cr>(__s);
    return __r;
}

__duration_divide_result is basically a custom-built enable_if that will contain type only if Rep2 is not a duration and if Rep2 is implicitly convertible to common_type<typename Duration::rep, Rep2>::type. __is_duration is simply a private trait that answers false, but is specialized for duration to answer true.

The constrained operator% works identically.

[ 2009-10 Santa Cruz: ]

Mark NAD EditorialResolved, fixed by 1177.

Proposed resolution: