easyloggingpp
Single header C++ logging library. It is extremely powerful
...It also provides support for logging your classes, third-party libraries, STL and third-party containers, etc. Easylogging++ is a single header efficient logging library for C++ applications. It is extremely powerful, highly extendable, and configurable to users' requirements. 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.