[boost] [date_time] local_date_time noticeably faster than ptime

Subject: [boost] [date_time] local_date_time noticeably faster than ptime
From: oneill1979 (oneill1979_at_[hidden])
Date: 2009-01-30 07:21:00

We noticed recently that using a local_date_time in tight loops was several
times faster than using a ptime object. As these are both based largerly on
the same template code this was a surprise. It appears this is caused by the
implementation of the operater+=/-= and was wondering if there was any
justification for the difference or if this was just an oversight.

.\boost\date_time\local_time\local_date_time.hpp
   //! Local_date_time += time_duration
   local_date_time_base operator+=(const time_duration_type& td)
   {
     this->time_ = time_system_type::add_time_duration(this->time_,td);
     return *this;
   }

.\boost\date_time\time.hpp
   time_type operator+=(const time_duration_type& td)
   {
     time_ = (time_system::get_time_rep(date(), time_of_day() + td));
     return time_type(time_);
   }

It looks like the ptime implementation is doing extra work to split the
ptime into date and time_duration components and if it is changed to follow
a similar pattern to that used in the local time the performance is then
indistinguishable.
   time_type operator+=(const time_duration_type& td)
   {
     this->time_ = time_system::add_time_duration(this->time_, td);
     return time_type(time_);
   }

I can only think that maybe this was done to work around an issue and in
which case should the same then be applied to the local time implementation?

Paul.
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