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974. duration<double> should not implicitly convert to duration<int>

Section: 30.5.2 [time.duration.cons] Status: C++11 Submitter: Howard Hinnant Opened: 2009-01-21 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

The following code should not compile because it involves implicit truncation errors (against the design philosophy of the duration library).

duration<double> d(3.5);
duration<int> i = d;  // implicit truncation, should not compile

This intent was codified in the example implementation which drove this proposal but I failed to accurately translate the code into the specification in this regard.

[ Batavia (2009-05): ]

We agree with the proposed resolution.

Move to Tentatively Ready.

[ 2009-07 Frankfurt ]

Moved from Tentatively Ready to Open only because the wording needs to be improved for enable_if type constraining, possibly following Robert's formula.

[ 2009-08-01 Howard adds: ]

Addressed by 1177⁽ⁱ⁾.

[ 2009-10 Santa Cruz: ]

Not completely addressed by 1177⁽ⁱ⁾. Move to Ready.

Proposed resolution:

Change 30.5.2 [time.duration.cons], p4:

template <class Rep2, class Period2> 
  duration(const duration<Rep2, Period2>& d);

-4- Requires: treat_as_floating_point<rep>::value shall be true or both ratio_divide<Period2, period>::type::den shall be 1 and treat_as_floating_point<Rep2>::value shall be false. Diagnostic required. [Note: This requirement prevents implicit truncation error when converting between integral-based duration types. Such a construction could easily lead to confusion about the value of the duration. — end note]