gtkmm applications consist of windows containing widgets, such as buttons and text boxes. In some other systems, widgets are called "controls". For each widget in your application's windows, there is a C++ object in your application's code.

Widgets are arranged inside container widgets such as frames and notebooks. Some widgets, such as Gtk::VBox, are not visible - they exist only to arrange other widgets.

Although gtkmm widget instances have lifetimes and scopes just like those of other C++ classes, gtkmm has an optional time-saving feature. Gtk::manage() allows you to say that a child widget is owned by the container into which you place it.

gtkmm, like most GUI toolkits, is event-driven. When an event occurs, such as the press of a mouse button, the appropriate signal will be emitted by the Widget that was pressed. To handle a signal we connect a signal handler to catch the signal. For instance:

m_button1.signal_clicked().connect( sigc::mem_fun(*this, &HelloWorld::on_button_clicked) );

gtkmm uses Glib::ustring instead of std::string, to support Unicode in UTF-8 encoding. It provides almost exactly the same interface as std::string, along with automatic conversions to and from std::string.

Some parts of the gtkmm API use intermediate data containers, such as Glib::StringArrayHandle instead of a specific Standard C++ container such as std::vector or std::list. You should not declare these types yourself -- you should use whatever Standard C++ container you prefer instead. gtkmm will do the conversion for you.

You may sometimes need to use C APIs which do not yet have convenient C++ interfaces. It is generally not a problem to use C APIs from C++, and gtkmm helps by providing access to the underlying C object, and providing an easy way to create a C++ wrapper object from a C object, provided that the C API is also based on the GObject system.

To use a gtkmm instance with a C function that requires a C GObject instance, use the gobj() function to obtain a pointer to the underlying GObject instance. For instance:

Gtk::Button* button = new Gtk::Button("example");
gtk_button_do_something_new(button->gobj());

To obtain a gtkmm instance from a C GObject instance, use the Glib::wrap() function. For instance:

GtkButton* cbutton = get_a_button();
GtkButton* button = Glib::wrap(cbutton);