( Object Implementation

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 Object internals
    Objects (or the `GtkObject' type) and the class hierarchy in general
 is implemented via a hierarchy of structs and type casting. Be aware
 that when classes are mentioned it is the conceptual idea of classes
 that is being referred to. GTK is written entirely in C which provides
 no direct support for classes.
    The first part to the class mechanism is the object fields. These are
 fields that will be used on a per object basis. For example, the widget
 type contains a field for the widgets parent. Every derived type needs a
 reference to its parent type. A descendant class of `GtkObject' would
 define itself like:
      struct Descendant
        GtkObject object;
    It is important to note that the `GtkObject' field needs to appear
 first in the descendant type structure. This allows pointers to objects
 of type `Descendant' to be cast to pointers to `GtkObject''s and
    The second part to the class mechanism is the class fields. These
 fields are defined on a per class basis. In the case of widgets, the
 class fields are all the "virtual" functions for widgets. The
 `GtkObject' class defines the `destroy' virtual function and the
 necessary fields for the signal mechanism as well as a field for
 determining the runtime type of an object. A virtual function is
 semantically the same as it is in C++. That is, the actual function that
 is called is determined based on the type of the object. Or, more
 specifically, the actual function call depends on the class structure
 that is pointed to by the `klass' field of the `GtkObject' structure.
    To see how the class fields work it is necessary to see the object
 fields for a `GtkObject'. The `GtkObject' type is defined as follows:
      typedef struct _GtkObject GtkObject;
      struct _GtkObject
        guint32 flags;
        GtkObjectClass *klass;
        gpointer object_data;
    The `class' field actually points to a class structure derived from
 `GtkObjectClass'. By convention, each new type defines its own class
 structure even if it is unnecessary. As an example, the hypothetical
 `Descendant' class would define its class structure as:
      struct DescendantClass
        GtkObjectClass parent_class;
    It is convention to name the parent class field (`GtkObjectClass' in
 this case), `parent_class'. For the same reason as stated above for the
 object structure, the parent class field must be the first field in the
 class structure.
       This is certainly
 true for gcc, however, from my precursory reading of the C standard I
 was unable to come to a definite conclusion as to whether this was
 required or simply done for simplicity. I'm not too worried about this
 assumption, though, as every C compiler I've ever encountered would work
 with GTK.
    The `flags' field of the `GtkObject' structure is used to keep track
 of a relatively few object flags and is also used by the `GtkWidget'
 type to store additional flags. At this time, the upper 16 bits of the
 flags field are reserved but unused.
    The `object_data' field of the `GtkObject' structure is an opaque
 pointer used by the object data mechanism. In truth, it is a pointer to
 the beginning of the data list which is composed of the following
      typedef struct _GtkObjectData GtkObjectData;
      struct _GtkObjectData
        guint id;
        gpointer data;
        GtkObjectData *next;
    The data mechanism allows arbitrary data to be associated with a
 character string key in any object. A hash table is used to transform
 the character string key into the data id and then a search through the
 list is made to see if the data exists. The assumption being that the
 data list will usually be short and therefore a linear search is OK.
 Future work on the data mechanism might make use of a resizable array
 instead of a linked list. This would shrink the overhead of the
 `GtkObjectData' structure by 4 bytes on 32 bit architectures.
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