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Subject: Re: [boost] [gsoc 2013] draft proposal for chrono::date
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2013-05-02 18:19:25

Le 02/05/13 23:35, Anurag Kalia a écrit :
> Rob Stewart-6 wrote
>> On May 1, 2013, at 4:33 PM, Anurag Kalia <
>> anurag.kalia@
>> > wrote:
>>> Vicente Botet wrote
>>>> Anurag Kalia wrote
>>> As I have already mentioned, an application requiring absolute
>>> performance from date class would use its operations. And nothing can
>>> beat a serial representation at comparisons, assignments and day/week
>>> arithmetic.
>>> ymd represntations, OTOH, optimize I/O. And an application requiring such
>>> optimization can simply employ a representation like:
>>> struct date
>>> {
>>> int y, m, d;
>>> };
>>> which serves their purpose well enough.
>> Optimizing I/O, which is a relatively slow process anyway, is possibly
>> misguided. The YMD format could be extracted from the serial format to
>> reuse, but I imagine a given object being formatted for I/O in a single
>> function call, à la strftime(), rather than in a series of formatting
>> calls. Still, even the latter is possible with a formatter object which
>> extracts the YMD values once and applies various formatting operations, as
>> desired.
> So, you are trying to say that optimizing I/O is not really rewarding since
> I/O by itself is already orders of magnitude slower? It makes sense to me.
> But might there not be cases where we have to store the output not to some
> output device, but to a string in memory only?
I don't see how the format at construction could improve the
performances of I/O? COul dyou elaborate?

>>> For the vast majority however,
>>> serial representation is good enough. After caching as you know,
>>> performance nears ymd in these scenarios too.
>> Caching suggests larger objects and, possibly lower locality of reference.
> By caching, I meant to say retrieve y,m,d values explicitly as well as some
> other properties of the date.
Please, could you elaborate on the cache mechanism you have in mind?
Could you tell us from where would you retrieve these informations?
Would you use memoization? Or is the cache static and build at startup?
Would the user be able to configure how many mappings could be stored?
What would be the keys of the catch?

>>>>> I genuinely am curious why adding that fourth parameter might be a bad
>>>>> design.
>>>> Well, this is a taste question. I will comeback to your functional
>>>> format later.
>>> I myself have dropped this constructor. After all, it is not difficult
>>> for users to check documentation for the order of the date parameters. It
>>> just
>>> eats up the performance anyway, even if it is just a switch statement.
>> I'm not sure of the context here, but relying on documentation to ensure
>> correct argument order will only frustrate users, because it is error
>> prone. The Named Constructor Idiom is beneficial in such cases.
> To recap, the constructor under discussion looks like:
> date(int a, int b, int c, date_format fmt);
> where date_format is an enum type:
> enum date_format { ymd, dmy, mdy };
> so that following construction is possible:
> date dt = date(2013, 6, 5, ymd);
> All information required is directly in front of the eyes. But thinking more
> about it, y-m-d should be the only case possible in the constructor. It is
> because date object is streamable. Thus, such construction is possible:
> std::cout << date(4, 5, 2013, dmy);
> where output comes out to be "2013-05-04". This seems a little clumsy to me.
> Having never seen a precedent like this, my mind is against it a little.
As I have said several times already the date shouldn't include any I/O
formatting information.
> That is the only reason I am antagonist to otherwise completely natural
> construction of:
> date natural = day(7) / month(12) / year(2013);
> Now, back to your named constructor idiom. Thanks! How true that you learn
> something new everyday! I liked it. It actually seems very suitable to my
> date class since we are constructing dates also in ordinal date notation
> (year, day_of_year) as well as week date notation (year, week_no, weekday).
What do you see wrong with

     date d(day(7),month(12),year(2013));

respect to

     date d = day(7) / month(12) / year(2013);

Take in account that this is not just about naming parameters but about defining the design space. Note that


will fail and throw an exception. If you don't want the check to be done you have the possibility to use a no_check tag

day(7, no_check)

In the same way the library provided some constant object for month we could add constant objects days for days, e.g. d_07 which is correct by definition. So the preceding examples could become

     date d(d_7,dec,year(2013,no_check));
     date d = d_7 / december / year(2013);

If no check is desired the following could construct a date efficiently.

     date d(d_7,dec,2013,no_check);
Note that the operator/() factory don't allows you to avoid the validity check.

> Again, I have doubts still. First that I have never encountered it in
> standard library till now - the library that I use the most. If it exists
> somewhere in it or Boost, that would hearten me. Moreover, named constructor
> idiom allows construction of objects by the objects themselves. It has
> possibility to look ugly. Though, as I write, I think one could always make
> them first-class functions which are friends of date class.
> Is the latter ok approach? My concerns also extend to performance of
> resulting code. But people on internet say that the call is almost always
> optimized away.
The best you can do is to test it, and share your experience on internet ;-)


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