## Instantiable classed types: objects

This section covers the theory behind objects. See How to define and implement a new GObject for the recommended way to define a GObject.

Types which are registered with a class and are declared instantiable are what most closely resembles an object. Although GObjects (detailed in The GObject base class) are the most well known type of instantiable classed types, other kinds of similar objects used as the base of an inheritance hierarchy have been externally developed and they are all built on the fundamental features described below.

For example, the code below shows how you could register such a fundamental object type in the type system (using none of the GObject convenience API):

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    typedef struct {
      GObject parent;

      /* instance members */
      gchar *filename;
    } ViewerFile;

    typedef struct {
      GObjectClass parent;

      /* class members */
      /* the first is public, pure and virtual */
      void (*open)  (ViewerFile  *self,
                     GError     **error);

      /* the second is public and virtual */
      void (*close) (ViewerFile  *self,
                     GError     **error);
    } ViewerFileClass;

    #define VIEWER_TYPE_FILE (viewer_file_get_type ())

    GType
    viewer_file_get_type (void)
    {
      static GType type = 0;
      if (type == 0) {
        const GTypeInfo info = {
          sizeof (ViewerFileClass),
          NULL,           /* base_init */
          NULL,           /* base_finalize */
          (GClassInitFunc) viewer_file_class_init,
          NULL,           /* class_finalize */
          NULL,           /* class_data */
          sizeof (ViewerFile),
          0,              /* n_preallocs */
          (GInstanceInitFunc) NULL /* instance_init */
        };
        type = g_type_register_static (G_TYPE_OBJECT,
                                       "ViewerFile",
                                       &info, 0);
      }
      return type;
    }

Upon the first call to `viewer_file_get_type`, the type named ViewerFile will be registered in the type system as inheriting from the type G_TYPE_OBJECT.

Every object must define two structures: its class structure and its instance structure. All class structures must contain as first member a GTypeClass structure. All instance structures must contain as first member a GTypeInstance structure. The declaration of these C types, coming from `gtype.h` is shown below:

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    struct _GTypeClass
    {
      GType g_type;
    };
    struct _GTypeInstance
    {
      GTypeClass *g_class;
    };

These constraints allow the type system to make sure that every object instance (identified by a pointer to the object's instance structure) contains in its first bytes a pointer to the object's class structure.

This relationship is best explained by an example: let's take object B which inherits from object A:

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    /* A definitions */
    typedef struct {
      GTypeInstance parent;
      int field_a;
      int field_b;
    } A;
    typedef struct {
      GTypeClass parent_class;
      void (*method_a) (void);
      void (*method_b) (void);
    } AClass;

    /* B definitions. */
    typedef struct {
      A parent;
      int field_c;
      int field_d;
    } B;
    typedef struct {
      AClass parent_class;
      void (*method_c) (void);
      void (*method_d) (void);
    } BClass;

The C standard mandates that the first field of a C structure is stored starting in the first byte of the buffer used to hold the structure's fields in memory. This means that the first field of an instance of an object B is A's first field which in turn is GTypeInstance's first field which in turn is `g_class`, a pointer to B's class structure.

Thanks to these simple conditions, it is possible to detect the type of every object instance by doing:

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    B *b;
    b->parent.parent.g_class->g_type

or, more quickly:

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    B *b;
    ((GTypeInstance *) b)->g_class->g_type

### Initialization and Destruction

instantiation of these types can be done with `g_type_create_instance`, which will look up the type information structure associated with the type requested. Then, the instance size and instantiation policy (if the `n_preallocs` field is set to a non-zero value, the type system allocates the object's instance structures in chunks rather than mallocing for every instance) declared by the user are used to get a buffer to hold the object's instance structure.

If this is the first instance of the object ever created, the type system must create a class structure. It allocates a buffer to hold the object's class structure and initializes it. The first part of the class structure (ie: the embedded parent class structure) is initialized by copying the contents from the class structure of the parent class. The rest of class structure is initialized to zero. If there is no parent, the entire class structure is initialized to zero. The type system then invokes the `base_class_initialization` functions (GBaseInitFunc) from topmost fundamental object to bottom-most most derived object. The object's `class_init` (GClassInitFunc) function is invoked afterwards to complete initialization of the class structure. Finally, the object's interfaces are initialized (we will discuss interface initialization in more detail later).

Once the type system has a pointer to an initialized class structure, it sets the object's instance class pointer to the object's class structure and invokes the object's `instance_init` (GInstanceInitFunc) functions, from top-most fundamental type to bottom-most most-derived type.

Object instance destruction through `g_type_free_instance` is very simple: the instance structure is returned to the instance pool if there is one and if this was the last living instance of the object, the class is destroyed.

Class destruction (the concept of destruction is sometimes partly referred to as finalization in GType) is the symmetric process of the initialization: interfaces are destroyed first. Then, the most derived class_finalize (GClassFinalizeFunc) function is invoked. Finally, the base_class_finalize (GBaseFinalizeFunc) functions are invoked from bottom-most most-derived type to top-most fundamental type and the class structure is freed.

The base initialization/finalization process is very similar to the C++ constructor/destructor paradigm. The practical details are different though and it is important not to get confused by superficial similarities. GTypes have no instance destruction mechanism. It is the user's responsibility to implement correct destruction semantics on top of the existing GType code. (This is what GObject does: see The GObject base class.) Furthermore, C++ code equivalent to the `base_init` and `class_init` callbacks of GType is usually not needed because C++ cannot really create object types at runtime.

The instantiation/finalization process can be summarized as follows:

Table 1. GType Instantiation/Finalization

Invocation time | Function invoked | Function's parameters
--- | --- | ---
First call to `g_type_create_instance` for target type | type's `base_init` function | On the inheritance tree of classes from fundamental type to target type. `base_init` is invoked once for each class structure.
target type's `class_init` function | On target type's class structure
interface initialization, see the section called “Interface Initialization” |
Each call to `g_type_create_instance` for target type | target type's `instance_init` function | On object's instance
Last call to `g_type_free_instance` for target type | interface destruction, see the section called “Interface Destruction” |
target type's `class_finalize` function | On target type's class structure
type's `base_finalize` function | On the inheritance tree of classes from fundamental type to target type. `base_finalize` is invoked once for each class structure.