Apache Log4cxx is a logging framework for C++ patterned after Apache log4j, which uses Apache Portable Runtime for most platform-specific code and should be usable on any platform supported by APR. Apache Log4cxx is licensed under the Apache License, an open source license certified by the Open Source Initiative.

Experience indicates that almost every large application needs runtime configurable logging. In the usual configuration, informational, warning and error log messages are saved to a persistent medium so they are available for study at a later time. With Log4cxx it is possible to activate additional logging output or disable some output for one or more modules of the running application by changing the configuration file.

In the development cycle, logging can be an auditing tool. Debugging log output can be activated for an aspect of the application simply by modifying a configuration file. The correctness of a function should be verified by viewing logged calculated values. A faulty piece of code can be isolated by viewing logged function input values and the corresponding logged result values. These debugging log statements can be removed from the shipped application using a compiler build directive. Logging statements do increase the (code) size of the application, but with Log4cxx the speed of the application is not generally affected (see Performance).

Logging is useful where debuggers are not, for example:

distributed applications
multithreaded applications
scientific applications (with vector and matrix valued variables)
real-time applications
event centered (e.g. GUI) applications

For persisted messages, the storage location and the quantity retained can be specified and changed using a configuration file. The configuration also allows you to control the destination of log messages. They can be sent to a file, a remote socket server, event loggers as well as the console, with support for directing particular messages to one or more destinations.

The example programs page will let you to quickly start using Log4cxx. The components of Log4cxx are described on the concepts page. For a more general overview, the logging services committee provides a logging overview page that explains more of what logging is and when to use it.