[Boost-users] [log] cost of record instantiation

Subject: [Boost-users] [log] cost of record instantiation
From: Matthias Vallentin (vallentin_at_[hidden])
Date: 2010-08-30 16:26:30

Hello,

I am trying to understand the cost of log record instantiation. Say I
have a global severity logger with several levels, of which the debug
level produces a lot of output and record creation is expensive. Because
the overhead should only incur when actually logging in debug mode, I
would like to know how costly the level check is. That is, in the
following code

    // The custom logger class here accommodates multiple severity
    // loggers for different facilities.
    #define LOG(level, facility)                                    \
        BOOST_LOG_SEV(global::logger->get(facility), level)

    LOG(debug, core) << ... some expensive construction

would the stream expression be instantiated regardless of the logging
level, yet simple be discarded when being in level lower than debug? (I
am asking because I recall they are not streamed in a lazy fashion.) If
this is the case, preventing the construction of the stream expression
would need a different logging mechanism, e.g.,

    /// Expensive logging when in Debug mode.
    #ifdef DEBUG
    #define LOGD(facility)                                          \
        LOG(log::debug, facility)
    #endif

    // Not being compiled.
    LOGD(core) << ... some expensive constrution

However, this design does not have a uniform approach to logging, which
makes it less attractive.

Any thoughts?

   Matthias