A single radio button performs the same basic function as a Gtk::CheckButton, as its position in the object hierarchy reflects.
It is only when multiple radio buttons are grouped together that they become a different user interface component in their own right. Every radio button is a member of some group of radio buttons. When one is selected, all other radio buttons in the same group are deselected. A Gtk::RadioButton gives the user a choice from many options. After constructing the first RadioButton in a group, use get_group() and provide this Group to the constructors ot the other RadioButtons in the same group. To remove a Gtk::RadioButton from one group and make it part of a new one, use set_group().
Create an empty RadioButton belonging to the specified group, into which you can add() a widget.
If you wish to add a Gtk::Label, you may want to use the Gtk::RadioButton(const Group& group, const Glib::ustring& label, bool mnemonic) constructor directly instead.