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Subject: Re: [boost] [gsoc 2013] draft proposal for chrono::date
From: Rob Stewart (robertstewart_at_[hidden])
Date: 2013-05-02 23:46:00

On May 2, 2013, at 5:35 PM, Anurag Kalia <anurag.kalia_at_[hidden]> wrote:

> Rob Stewart-6 wrote
>> On May 1, 2013, at 4:33 PM, Anurag Kalia wrote:
>> 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 imagine there would be much call for doing formatted output of many strings to a buffer. Even then, each date of interest must be cast into the I/O friendly form, so there's no room for optimizing that case, is there?

>>> 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.

That sounds like conversion to another date type.

>>> 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.

I comment on this further down, but that does address the documentation dependency problem.

> 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.

If there is I/O support, I'd hope for other formats. Regardless, those in different regions will want the arguments ordered in their locale's order. You could require them in magnitude order -- YMD -- but that's less friendly and isn't checked by the compiler.

> That is the only reason I am antagonist to otherwise completely natural
> construction of:
> date natural = day(7) / month(12) / year(2013);

That will be harder to optimize. I'm not saying that it wouldn't be optimized, just harder, so less likely. It's also unlike anything in the standard.

I don't think that's any clearer than this:

date d(7, dec, year(2013))

(The months can be from an enumerated type.)

One can specify a day or year object and avoid ambiguity:

date(unsigned, month, year);
date(day, month, unsigned);

You can even handle other orders that way:

date(month, unsigned, year);
date(month, day, unsigned);
date(unsigned, day, month);
date(year, unsigned, month);

One can also be explicit for both day and year:

date(year, month, day);
date(month, day, year);
date(day, month, year);
date(day, year, month);

(That last seems particularly odd, but could be supported.)

I'm assuming explicit constructors for day and year, of course.

> 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).
> 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.

make_pair, make_shared are something like it. However, the reason you've not seen it used is that no standardized type has needed it.

> 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.

date::ymd(2013, 5, 3)
make_ymd(2013, 5, 3)

I don't think this is right, though:

ymd(2013, 5, 3)

> 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.

RVO should take care of things.


(Sent from my portable computation engine)

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