C++ coding style clean up
Thing that slipped by clang-format for C++ code is underscore prefixes of the identifiers for private members and methods declarations, sorry
Member variable names should be prefixed with m_ to make it easier to distinguish them from function parameters and local variable names
https://community.kde.org/Policies/Frameworks_Coding_Style
Also, using just an underscore as a prefix is a bad idea since it's possible to confuse it with reserved forms (see also: https://en.cppreference.com/w/cpp/language/identifiers)