The GtkContainer Instance Struct

Here is GtkContainer:

typedef struct _GtkContainer       GtkContainer;

struct _GtkContainer
{
  GtkWidget widget;
  
  GtkWidget *focus_child;
  
  guint border_width : 16;
  guint need_resize : 1;
  guint resize_mode : 2;
  
  GSList *resize_widgets;
};
    

The focus_child member is the child in the container with the keyboard focus; it can be NULL if no child has the focus. The GtkContainer implementation handles setting and unsetting this field. The border_width member is a width in pixels to add to the container's size request on all sides; the container will also subtract this value from its allocation. (In other words, the border_width is a blank space around the container.) Library users set the border_width field with gtk_container_set_border_width(); GtkContainer subclasses must honor its value in their size_request and size_allocate implementations.

The need_resize, resize_mode, and resize_widgets fields are implementation details that subclasses should not have to read or modify. These fields are used to implement gtk_widget_queue_resize(); when a resize is queued for a widget, the GtkContainer implementation ensures that the size negotiation process (requisition/allocation) will take place a in a one-shot idle handler. Subclasses of GtkContainer simply implement size_request and size_allocate, and everything works.