The Clipboard
Simple text copy-paste functionality is provided for free by widgets such as Gtk::Entry and Gtk::TextView, but you might need special code to deal with your own data formats. For instance, a drawing program would need special code to allow copy and paste within a view, or between documents.
Gtk::Clipboard is a singleton. You can get the one and only instance with Gtk::Clipboard::get().
So your application doesn't need to wait for clipboard operations, particularly between the time when the user chooses Copy and then later chooses Paste, most Gtk::Clipboard methods take sigc::slots which specify callback methods. When Gtk::Clipboard is ready, it will call these methods, either providing the requested data, or asking for data.