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1.1.1 The GLib Dynamic Type System

Information on the project’s data types are stored in The GLib Dynamic Type System¹. The GLib Dynamic Type System’s model for a data type is that you create a class and then make instances of that class. For it, each type can be described with the following information

• a GType - a unique integer ID for the type
• a type name
• a class data structure
• class initialization and destruction functions
• an instance data structure
• instance initialization and copy functions

This type system has inheritance: each type may be a specialization of a parent type. Note that the class-and-instance capability of The GLib Dynamic Type System is its own unique beast: not identical to C++ classes or to GOOPS.

When the introspection layer describes constants, variables, and function arguments, each of these must have a type that is registered by The GLib Dynamic Type System. There are a set of fundamental GType IDs, denoted with the form G_TYPE_XXX, and the type of each variable or argument traces to a fundamental GType ID or one that is subclassed from a fundamental type.

To allow introspection, there are GType IDs for all the standard native types: char has G_TYPE_CHAR, int has G_TYPE_INT, etc. Each enumerated type or collection of flags has its own type, which is a subclass of G_TYPE_FLAGS or G_TYPE_ENUM. Types that describe information stored in a typed pointer, such as native struct, each have their own type that is a subclass of G_TYPE_BOXED.

And, most importantly for GTK, there are types that descend from G_TYPE_OBJECT. As we’ll discuss in the next section, GObject is the fundamental type providing the common attributes and methods for all object types in GTK+, Pango, and other libraries based on GObject.