Re: [boost] [chrono/date] conversion between concrete dates

Subject: Re: [boost] [chrono/date] conversion between concrete dates
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2013-05-11 09:13:47

Le 11/05/13 11:54, Roland Bock a écrit :
> On 2013-05-08 18:08, Vicente J. Botet Escriba wrote:
>> Hi,
>>
>> 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);
> Hi,
>
> 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++? 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 :-)
We have suggested a more neutral factory

auto dt = make_date(year(2013), may, 11);
>
> 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'd therefore prefer another operator, e.g.  operator <<
>
year,month and day are unit specifiers not periods (years, months, days).
The ratio is obtained using

int ratio = years(3)/months(4);  // ratio = 9

> 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);
>
> With C++11 you could then write:
>
> date ISO_week_start = mon <= { y, jan, day(4) };
I guess this depends on the type of {y, jan, day(4)} and the expected 
type of the rhs of the operator<= ?
???  operator <=(month, ???)

Note that we are talking of several dates types.

{ y, jan, day(4) } would be accepted where a ymd_date is expected.

The operator <= is defined on serial_date as the algorithm is efficient only with this representation.

> That's the way I'd prefer :-)
>
In order to support

ymd_date ISO_week_start = mon <= { y, jan, day(4) };

we need to define operator <= for ymd_date or as a template,

template <typename Date>
Date operator <=(month,Date);

but the implementation would do a conversion from Date to serial_date 
and return the conversion of the calculated serial_date to a Date.

Best,
Vicente