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duration
arithmetic: contradictory requirementsSection: 29.5.6 [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 29.5.6 [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(i) which cleans up several places under 29.5 [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-builtenable_if
that will containtype
only ifRep2
is not aduration
and ifRep2
is implicitly convertible tocommon_type<typename Duration::rep, Rep2>::type
.__is_duration
is simply a private trait that answersfalse
, but is specialized forduration
to answertrue
.The constrained
operator%
works identically.
[ 2009-10 Santa Cruz: ]
Proposed resolution: