[boost] Re: [Q]: What is the recommended logging facility for boost: log4cpp or ??

From: Rene Rivera (grafik.list_at_[hidden])
Date: 2004-10-22 09:28:07

Caleb Epstein wrote:

> Anyone interested in working on such a beast?  

There's been interest and discussion in the past. Search the archives 
and you should find some discussion and if I remember correctly some 
previous work on the subject.

>It should be reasonably
> simple to write something with the new Boost.Iostreams facilities that
> have been reviewed recently.  The must-have features for me are:
> 
> * Uses iostreams

I disagree. Like most things STL it should be independent of a stream 
back end. And ideally independent of a formatting front end. It should 
only be a tagging, filtering, and multiplexing interface.

> * Thread-safe (e.g. two threads can't have their output intermixed in
> the output file/stream/whatever like they would with simple ostream<<
> operators)

Yes, but only if needed. It might not be appropriate depending on the 
front end or back end. For example you might be dumping the "output" to 
a database which would create individual debug records. No need for 
thread safety at the log level in that case.

> * Multiple possible output media (files, sockets, syslog, Windows
> event log, etc)

Multiplexing.

> * Multiple named channels each with configurable "threshold" (e.g.
> error, warning, debug, etc)

And configurable, so that you can program what each one does on both the 
front end and to which back end they go to.

> * Messages which won't be logged shouldn't even go through the
> formatting step.  I'm pretty sure this cannot be done in a single call
> w/o macros though.  For example:
> 
>     BOOST_LOG (boost::log::debug, "This is a debug message: " <<
> some_expensive_to_stream_class);
> 
> Which would expand to something like:
> 
>     if (boost::log::is_enabled_for (boost::log::debug)) {
>         ostringstream __os; os << "This is a debug message: " <<
> some_expensive_to_stream_class;
>         boost::log::force_write (boost::log::debug, os.str ());
>     }

It's possible without macros for some compilers using careful template 
instaciations.

For example the very simple logger I wrote uses syntax like so (and yes 
I prefer printf formatting :-)...

log<RevisionLog>::trace(
        "RevisionControl::commit_; wait for ticket #%d",ticket);

Because the formatting is done by the trace call, and the trace call is 
empty (and inlined) when the log is disabled, no formatting takes place. 
Something similar can be done regardless of what the formatting object 
is by having a specialization that does nothing.

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