This is a C++ refactoring:
You have a class that is implemented completely within its own header and you want to hide its implementation from its clients.
Move all the method bodies out of the header file and into a new implementation file. Move as many #includes out of the header file into the implementation file as possible, replacing them in the header with forward declarations.
[ToDo: provide a summary of the mechanics]
You probably want to do this to reduce dependencies either to decrease compile times or to remove a circular dependency.
Copy the entire contents of the .h file to a new .cpp file