nobody — Sat, 17 Aug 2002 17:54:51 GMT

Logged In: NO
ops... i asked the same to Dr. John Maddock, directly...
here it is his answer:
>> Second, I found a "leak" in your documentation: you often use
>>   ...
>>   std::ostreamstring t;
>>   std::ostream_iterator<char> oi(t);
>>   boost::regex_merge(oi, in.begin(), in,end(), expr, format);
>>   ...
>> I implemented a tiny sed(1) facility, but using
ostream_iterator slows my
>> program down terribly while executing the sed
substitution command
>> (s/regex/replacement/) many times on a 14Kb-sized html
file (it takes some
>> seconds to accomplish the tasks, while sed(1) takes less
than one second).
>
>> I found the problem is in re_details::re_copy_out
function, which seems fast
>> but it is not fast at all with ostream_iterators.
> Try using a ostreambuf iterator instead (my docs should be
changed to do the
> same): its a lot quicker.
> Make sure you turn on all optimisations before making
comparisons: the
> stream iterators are excrusiatingly slow until
optimisatioms are turned on
> (at which point they can actually be pretty fast).
>> So why don't make re_details::string_out_iterator public (in
documentation)?
> There shouldn't be any need for that: it's a workaround
for broken std
> libraries, you should really be able to use
std::back_inserter with strings.
> regards,
> John Maddock
since i am a beginner in c++, i am still using
re_details::string_out_iterator that is faster than
ostream_iterator (but not fast enough): now my own sed
searches&replaces a >30Kb-sized html file more than 25 times
in 0.63 secs on a PIII/500 under Linux 2.4.18 + gcc 2.95.3.
bye
Claudio

status changed

John Maddock — Fri, 30 May 2003 11:21:04 GMT

status assigned → closed

Logged In: YES
user_id=14804
A couple of points:
* The current cvs code has just been updated with a much
faster version, however the new version still won't improve
performance for long searches, which was already
substantially faster than the GNU regex library for example.
* Using C++ iostreams can be rather slow, unless you are
very careful: check that synch_with_stdio is false for example
(otherwise the performance hit can be *huge*).
* outputting the result to a string, and then copying to stdout
is going to be a heck of a lot slower than just copying to
output (due to memory allocation requirements)
John Maddock.

Boost C++ Libraries: Ticket #101: regex performance issue

status changed