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Subject: Re: [boost] [chrono/date] conversion between concrete dates
From: Rob Stewart (robertstewart_at_[hidden])
Date: 2013-05-11 09:22:58

On May 11, 2013, at 5:54 AM, Roland Bock <rbock_at_[hidden]> wrote:

> On 2013-05-08 18:08, Vicente J. Botet Escriba wrote:
>> as the conversion between concrete dates could be expensive I guess that these conversion must be explicit.
>> But this has some consequences when used the implicit conversion was hidden a not efficient implementation, e.g.
>> date ISO_week_start = mon <= jan/day(4)/y;
>> jan/day(4)/y should be ymd_date as is the efficient representation.
>> The date generator was declared as
>> date operator<=(weekday wd, date x);
> Do you really want to allow/document/support/advocate the American date format in C++? Why not restrict the format to something close to ISO date format for ISO C++?

If the types are unambiguous, such as with day, month, and year types, then any order can be supported. Those comfortable with a non-ISO order can do what they prefer with type safety.

> That way you could introduce a new type, say ym_type and
> ym_type operator/(year y, month m);
> date operator/(ym_type ym, day d);
> Neither explicit nor implicit conversion necessary :-)

You misunderstood the conversion context. There are several formats in contemplation: serial, days since an epoch; ymd; day of year; week of year; etc. conversions among them are sometimes costly, so the notion is to make them explicit.

> Another question I have about this is: Is is really a good idea to use operator/()? It does prevent you from calculating ratios between periods, e.g.
> int ratio = year(3)/month(4); // ratio = 9

I've thought that using the operator was a little too cute, but this is an interesting argument against it.

> I'd therefore prefer another operator, e.g. operator <<


That operator is for I/O, even if unsupported. (Someone might want to add I/O, even if not standardized.)

> But to be completely honest: I don't see the benefit of constructing dates this way. Why not simply use a constructor?
> date(year, month, day);

The issue is that there are other argument lists to support, so the combinations get complicated. We're exploring lots of options.

> With C++11 you could then write:
> date ISO_week_start = mon <= { y, jan, day(4) };

Using an initializer list is yet another option to consider.


(Sent from my portable computation engine)

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