Each log message is logged to a specific log category.

Log categories have a level setting that controls what log messages should be enabled for this category, as well as a list of log handlers that control what should be done with enabled log messages.

Log Category Hierarchy

Log categories are arranged in a hierarchy. Each log category except for the root has a parent category, and they may have zero or more children categories.

The log category hierarchy is determined by category names: the . character acts as a separator in category names. For example, the category spacesim is the parent of the category spacesim.ships. The root category can be referred to either as . or as the empty string.

One recommended option for choosing log category names is to follow the source code directory structure. For example, a partial view of the log category hierarchy for a space simulator project might look something like:

 . --- spacesim --- ships --- corvette -- cpp
                \         \            \- h
                |          \- cruiser -- cpp
                |                     \- h
                |
                 \- actors --- player -- cpp
                           \          \- h
                            \- ai --- enemy -- cpp
                                            \- h

The XLOG() macro automatically selects the log category to use based on the source file path.

Log Level Propagation

Log level settings automatically propagates downward from a particular log category to its children.

If the log verbosity is increased on a particular log category (by lowering the minimum enabled log level) , all of its children also inherit that increased log verbosity by default. For instance, setting the log level to INFO on spacesim.ships will automatically enable INFO and higher log messages on the spacesim.ships category as well as children categories such as spacesim.ships.corvette, spacesim.ships.fighter, etc. This makes it easily possible to control the log verbosity of entire sections of the code base at once.

Log level propagation can be disabled on specific categories by turning off the inherit setting for that category. For instance, disabling the inherit setting on the spacesim.ships.cruiser category will prevent it form inheriting increased log level verbosity from its parent spacesim.ships category (or indirectly inheriting settings from spacesim or the root category). This makes it possible to turn down the verbosity for specific categories even if when a larger category they belong to does have a higher verbosity setting.

Log Message Propagation

Logged messages propagate upwards through the log category hierarchy.

For instance, a message logged to spacesim.ships.corvette.cpp will first be sent to any LogHandler objects configured on spacesim.ship.corvette.cpp, then to the handlers for spacesim.ships.corvette, then spacesim.ships, then to spacesim, and finally to the handlers for the root log category.

Due to this behavior, if you install a LogHandler on the root log category it will automatically receive all messages logged to any category. Installing LogHandler objects on sub-categories allows you to perform handling only for specific category messages. LogHandler objects receive the full LogMessage object, and can perform further filtering based on the log level or other message properties if desired.

The Log Handler documentation provides additional details about log handler behavior.