class HeaderBar{
public:
    GtkHeaderBar *HeaderBarWidget;
    GtkMenuButton *FileBut;
    GtkButton *IdeBut;
    // Other buttons is not funtioning yet
    GtkButton *CompileBut;
    GtkSearchEntry *SearchBar;
};

HeaderBar class is not a gtk widget. The headerbar you see is GtkHeaderBar *HeaderBarWidget under the class.

Only one HeaderBar is needed so call HeaderBar* LoadHeaderBar(GtkBuilder *builder, GtkApplication *app) to create a HeaderBar

Currently, only FileBut and IdeBut are working.

Click the IDE button will call the IdeButtonClicked() which shows the [[SettingPanel]]

If you read the UI file, you will see that buttons and search entry are placed inside a GtkBox.