easyloggingpp
Single header C++ logging library. It is extremely powerful
...It provides the ability to write your own sinks (via featured referred as LogDispatchCallback). This library is currently used by hundreds of open-source projects on GitHub and other open-source source control management sites. If you are working on a small utility or large project in C++, this library can be handy. Its based on single header and only requires to link to single source file.