[boost] Some points concerning the boost.date_time lib

From: Daniel Spangenberg (dsp_at_[hidden])
Date: 2003-11-03 04:39:55

Hello boosters, Hello Jeff Garland,

After working somewhat with the 1.30 boost version of the date time
library during this weekend I stumbled about the following points:

1)  I was really missing the usual assignment operators += and -= for
the
arithmetic of ptime and date_duration/time_duration, like e.g.

ptime& ptime::operator+=(date_duration);
ptime& ptime::operator-=(time_duration);
time_duration& time_duration::operator+=(time_duration);
date_duration& date_duration::operator-=(date_duration);
date& date::operator+=(date_duration);

Is this just an oversight? Although it would require a bunch of further
operators, their existence would seem natural. Even std::complex
provides
these (I take this as an example, because std::complex provides mixed
arithmetic with both its template argument and itself)

2) I really think, that accessor methods for milliseconds, microseconds
and
nanoseconds should be made available for the time_duration class. See
also point (3)
which is related to this one.

3) To my opinion the name fractional_seconds is misleading, even worse
that
the usually found symbol fs in the source code can be misread as
femtoseconds. Do
I correctly understand, that the actual meaning of fractional_seconds
does depend on
the current resolution?
If so, this means, that I cannot see on the first sight, how much time
the fourth argument
of the c'tor

time_duration(hour_type hours, min_type minutes, sec_type seconds=0,
   fractional_seconds_type frac_sec = 0);

actually means. The same problem arises for the member function
fractional_seconds(),
from whichs result I cannot conclude from the code how much time it
stands for.
I think the main problem arises from the "backward" reading effect of
fractional_seconds.
This has the effect that a value of 3 fractional_seconds() does not show
the distance
of this 3 from the decimal point in expressions like

00:00:00.300000

or

00:00:00.000003

Normal users are known of the concept of milliseconds which have a fixed
relation to
seconds and other date/time components, but fractional_seconds(),
although parallelly
providable in the above mentioned c'tor, have not.

4) As far as I know of, the formula used inside the header
c_local_time_adjuster.hpp

std::time_t t2 = dd.days()*86400 + td.hours()*3600 + td.minutes()*60 +
td.seconds();

bases on one **possible** implementation of time_t and is not guaranteed
to
work generally (also it should work for those implementations I know
of).
Since the C standard does **explicitely** not specify the enconding of
time_t, we cannot exclude for example, that an implementation uses an
arbitrary
offset or scaling which would make the above quoted formula meaningless;

otherwise the need for the function difftime() would not exist.
The C standard only demands, that time_t  must be of some arithmetic
type
capable to represent times and that (time_t)(-1) is an valid expression.

5) I think the documentation misses the creator microsec in
..date_time/doc/class_time_duration.html

Besides these critics I would like to say that I think that
boost.datetime
library is a very nice and valuable tool to work with and I want to
thank you,
Jeff Garland specifically, for this contribution!

Greetings from Bremen,

Daniel