I've had success with log4cxx at http://logging.apache.org/log4cxx/index.html. It's a C++ version of the popular Log4j logger, is easy to configure either via a conf file or in the code. The overhead when it is disabled is minimal (method call and integer compare).
The pattern for the output to the log is defined by a conversion pattern that can be as simple as the date/time and a message. It also handles file size limitation, rollover, etc. You can also configure different patterns for various errors and sources.
