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Why not using hierarchy of loggers, like in log4j? Why not Boost.Log4j? Etc.

There are enough log4j-like libraries available for C++ already (see here, here and here), so there is no point in implementing yet another one. Instead, this library was aimed to solve more complex tasks, including ones that do not directly fall under the common definition of "logging" term as a debugging tool. Additionally, as Boost.Log was to be a generic library, it had to provide more ways of extending itself, while keeping performance as high as possible. Log4j concept seemed too limiting and inappropriate for these tasks and therefore was rejected.

As for hierarchical loggers, there is no need for this feature in the current library design. One of the main benefits it provides in log4j is determining the appenders (sinks, in terms of this library) in which a log record will end up. This library achieves the same result by filtering. The other application of this feature in Boost.Log could be that the loggers in the hierarchy could combine their sets of attributes for each log record, but there was no demand in real world applications for such a feature. It can be added though, if it proves useful.