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 09:19:16

Le 11/05/13 15:13, Vicente J. Botet Escriba a écrit :
> 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) };
>
>
An alternative would be to define the constructor
   serail_date(year, month, day);

and define the operator
   serial_date operator <=(month, serial_date);

As for the time been ymd_date is implicit convertible, the preceding 
constructor goes in the same direction. So yes, you should be able to

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

but not

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

Best,
Vicente
> 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.
>