Re: [boost] [chrono/date] Unit specifiers arithmetic

6 May 2013


      On May 6, 2013, at 9:55 AM, "Vicente J. Botet Escriba"  wrote:
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Le 06/05/13 15:34, Howard Hinnant a écrit :
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On May 6, 2013, at 6:31 AM, "Vicente J. Botet Escriba"  wrote:
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Hi,
Moving from checked dates to unchecked ones by default and forcing to user the Unit specifiers year/month/day in any date constructor (even the unchecked ones) has some ugly consequences. Note that I was able to build an unchecked date as
date d(2013,5,6, no_check);
Given
year y;
month m;
 day d;
the following will not compile now as the constructors expects a year but y+1 is an int.
date d(y+1, m, d);
The user needs to type
date d(year(y+1), m, d);
Is this what we want to provide to the user or should we add basic arithmetic on these unit specifiers?
This is an example of where flexible ordering and implicit last unit starts to shine.  Examples from my 2011 paper:
start = mon <= jan4/(d.year()-1);
date next_start = mon <= jan4/(start.year()+1);
Or translated to your example and syntax:
date d(d, m, y+1);  // works because last unit can be int
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Last unit or any unit but only one?
Any two explicit units should disambiguate.  We could even take it more lax than that and still avoid ambiguity if desired.

A warning though:  We want to make the checked syntax more attractive than the unchecked syntax, else all dates will be unnecessarily unchecked in practice.
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If we start adding arithmetic to the unit specifier day, it becomes indistinguishable from a duration.  Now it is possible that we could use durations as unit specifiers.
-1
Agreed.

Howard