# The GLib Dynamic Type System

A type, as manipulated by the GLib type system, is much more generic than what is usually understood as an Object type. It is best explained by looking at the structure and the functions used to register new types in the type system.

    typedef struct _GTypeInfo               GTypeInfo;
    struct _GTypeInfo
    {
      /* interface types, classed types, instantiated types */
      guint16                class_size;

      GBaseInitFunc          base_init;
      GBaseFinalizeFunc      base_finalize;

      /* classed types, instantiated types */
      GClassInitFunc         class_init;
      GClassFinalizeFunc     class_finalize;
      gconstpointer          class_data;

      /* instantiated types */
      guint16                instance_size;
      guint16                n_preallocs;
      GInstanceInitFunc      instance_init;

      /* value handling */
      const GTypeValueTable *value_table;
    };
    GType g_type_register_static (GType             parent_type,
                                  const gchar      *type_name,
                                  const GTypeInfo  *info,
                                  GTypeFlags        flags);
    GType g_type_register_fundamental (GType                       type_id,
                                       const gchar                *type_name,
                                       const GTypeInfo            *info,
                                       const GTypeFundamentalInfo *finfo,
                                       GTypeFlags                  flags);

`g_type_register_static` and `g_type_register_fundamental` are the C functions, defined in `gtype.h` and implemented in `gtype.c` which you should use to register a new GType in the program's type system. It is not likely you will ever need to use `g_type_register_fundamental` (you have to be Tim Janik to do that) but in case you want to, the last chapter explains how to create new fundamental types. Please note that there exists another registration function: the `g_type_register_dynamic`. We will not discuss this function here since its use is very similar to the `_static` version.

Fundamental types are top-level types which do not derive from any other type while other non-fundamental types derive from other types. Upon initialization by `g_type_init`, the type system not only initializes its internal data structures but it also registers a number of core types: some of these are fundamental types. Others are types derived from these fundamental types.

Fundamental and non-fundamental types are defined by:

- class size: the class_size field in GTypeInfo.

- class initialization functions (C++ constructor): the base_init and class_init fields in GTypeInfo.

- class destruction functions (C++ destructor): the base_finalize and class_finalize fields in GTypeInfo.

- instance size (C++ parameter to new): the instance_size field in GTypeInfo.

- instantiation policy (C++ type of new operator): the n_preallocs field in GTypeInfo.

- copy functions (C++ copy operators): the value_table field in GTypeInfo.

- type characteristic flags: GTypeFlags.

Fundamental types are also defined by a set of GTypeFundamentalFlags which are stored in a GTypeFundamentalInfo. Non-fundamental types are furthermore defined by the type of their parent which is passed as the parent_type parameter to `g_type_register_static` and `g_type_register_dynamic`.

## Copy functions

The major common point between all GLib types (fundamental and non-fundamental, classed and non-classed, instantiable and non-instantiable) is that they can all be manipulated through a single API to copy/assign them.

The GValue structure is used as an abstract container for all of these types. Its simplistic API (defined in `gobject/gvalue.h`) can be used to invoke the value_table functions registered during type registration: for example `g_value_copy` copies the content of a GValue to another GValue. This is similar to a C++ assignment which invokes the C++ copy operator to modify the default bit-by-bit copy semantics of C++/C structures/classes.

The following code shows how you can copy around a 64 bit integer, as well as a GObject instance pointer (sample code for this is located in the source tarball for this document in `sample/gtype/test.c`):

    static void test_int (void)
    {
      GValue a_value = {0, };
      GValue b_value = {0, };
      guint64 a, b;

      a = 0xdeadbeaf;

      g_value_init (&a_value, G_TYPE_UINT64);
      g_value_set_uint64 (&a_value, a);

      g_value_init (&b_value, G_TYPE_UINT64);
      g_value_copy (&a_value, &b_value);

      b = g_value_get_uint64 (&b_value);

      if (a == b) {
        g_print ("Yay !! 10 lines of code to copy around a uint64.\n");
      } else {
        g_print ("Are you sure this is not a Z80 ?\n");
      }
    }

    static void test_object (void)
    {
      GObject *obj;
      GValue obj_vala = {0, };
      GValue obj_valb = {0, };
      obj = g_object_new (MAMAN_TYPE_BAR, NULL);

      g_value_init (&obj_vala, MAMAN_TYPE_BAR);
      g_value_set_object (&obj_vala, obj);

      g_value_init (&obj_valb, G_TYPE_OBJECT);

      /* g_value_copy's semantics for G_TYPE_OBJECT types is to copy the reference.
         This function thus calls g_object_ref.
         It is interesting to note that the assignment works here because
         MAMAN_TYPE_BAR is a G_TYPE_OBJECT.
       */
      g_value_copy (&obj_vala, &obj_valb);

      g_object_unref (G_OBJECT (obj));
      g_object_unref (G_OBJECT (obj));
    }

The important point about the above code is that the exact semantics of the copy calls is undefined since they depend on the implementation of the copy function. Certain copy functions might decide to allocate a new chunk of memory and then to copy the data from the source to the destination. Others might want to simply increment the reference count of the instance and copy the reference to the new GValue.

The value_table used to specify these assignment functions is defined in `gtype.h` and is thoroughly described in the API documentation provided with GObject (for once ;-) which is why we will not detail its exact semantics.

    typedef struct _GTypeValueTable         GTypeValueTable;
    struct _GTypeValueTable
    {
      void     (*value_init)         (GValue       *value);
      void     (*value_free)         (GValue       *value);
      void     (*value_copy)         (const GValue *src_value,
                                      GValue       *dest_value);
      /* varargs functionality (optional) */
      gpointer (*value_peek_pointer) (const GValue *value);
      gchar            *collect_format;
      gchar*   (*collect_value)      (GValue       *value,
                                      guint         n_collect_values,
                                      GTypeCValue  *collect_values,
                                      guint                collect_flags);
      gchar            *lcopy_format;
      gchar*   (*lcopy_value)        (const GValue *value,
                                      guint         n_collect_values,
                                      GTypeCValue  *collect_values,
                                      guint                collect_flags);
    };

Interestingly, it is also very unlikely you will ever need to specify a value_table during type registration because these value_tables are inherited from the parent types for non-fundamental types which means that unless you want to write a fundamental type (not a great idea!), you will not need to provide a new value_table since you will inherit the value_table structure from your parent type.

## Conventions

There are a number of conventions users are expected to follow when creating new types which are to be exported in a header file:

- Use the `object_method` pattern for function names: to invoke the method named foo on an instance of object type bar, call `bar_foo`.

- Use prefixing to avoid namespace conflicts with other projects. If your library (or application) is named Maman, Maman is the French word for mum or mother - nothing more and nothing less.

  prefix all your function names with maman_. For example: `maman_object_method`.

- Create a macro named `PREFIX_TYPE_OBJECT` which always returns the GType for the associated object type. For an object of type Bar in a libray prefixed by maman, use: `MAMAN_TYPE_BAR`. It is common although not a convention to implement this macro using either a global static variable or a function named `prefix_object_get_type`. We will follow the function pattern wherever possible in this document.

- Create a macro named `PREFIX_OBJECT (obj)` which returns a pointer of type PrefixObject. This macro is used to enforce static type safety by doing explicit casts wherever needed. It also enforces dynamic type safety by doing runtime checks. It is possible to disable the dynamic type checks in production builds (see building glib). For example, we would create `MAMAN_BAR (obj)` to keep the previous example.

- If the type is classed, create a macro named `PREFIX_OBJECT_CLASS (klass)`. This macro is strictly equivalent to the previous casting macro: it does static casting with dynamic type checking of class structures. It is expected to return a pointer to a class structure of type PrefixObjectClass. Again, an example is: `MAMAN_BAR_CLASS`.

- Create a macro named `PREFIX_IS_BAR (obj)`: this macro is expected to return a gboolean which indicates whether or not the input object instance pointer of type BAR.

- If the type is classed, create a macro named `PREFIX_IS_OBJECT_CLASS (klass)` which, as above, returns a boolean if the input class pointer is a pointer to a class of type OBJECT.

- If the type is classed, create a macro named `PREFIX_OBJECT_GET_CLASS (obj)` which returns the class pointer associated to an instance of a given type. This macro is used for static and dynamic type safety purposes (just like the previous casting macros).

The implementation of these macros is pretty straightforward: a number of simple-to-use macros are provided in `gtype.h`. For the example we used above, we would write the following trivial code to declare the macros:

    #define MAMAN_TYPE_BAR                  (maman_bar_get_type ())
    #define MAMAN_BAR(obj)                  (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_BAR, MamanBar))
    #define MAMAN_BAR_CLASS(klass)          (G_TYPE_CHECK_CLASS_CAST ((klass), MAMAN_TYPE_BAR, MamanBarClass))
    #define MAMAN_IS_BAR(obj)          (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_BAR))
    #define MAMAN_IS_BAR_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), MAMAN_TYPE_BAR))
    #define MAMAN_BAR_GET_CLASS(obj)  (G_TYPE_INSTANCE_GET_CLASS ((obj), MAMAN_TYPE_BAR, MamanBarClass))

Stick to the naming `klass` as `class` is a registered c++ keyword.

The following code shows how to implement the `maman_bar_get_type` function:

    GType maman_bar_get_type (void)
    {
      static GType type = 0;
      if (type == 0) {
        static const GTypeInfo info = {
          /* You fill this structure. */
        };
        type = g_type_register_static (G_TYPE_OBJECT,
                                       "MamanBarType",
                                       &info, 0);
      }
      return type;
    }

When having no special requirements you also can use the `G_DEFINE_TYPE` macro:

    G_DEFINE_TYPE (MamanBar, maman_bar, G_TYPE_OBJECT)

## Non-instantiable non-classed fundamental types

A lot of types are not instantiable by the type system and do not have a class. Most of these types are fundamental trivial types such as gchar, registered in `g_value_types_init` (in `gvaluetypes.c`).

To register such a type in the type system, you just need to fill the GTypeInfo structure with zeros since these types are also most of the time fundamental:

      GTypeInfo info = {
        0,                                /* class_size */
        NULL,                        /* base_init */
        NULL,                        /* base_destroy */
        NULL,                        /* class_init */
        NULL,                        /* class_destroy */
        NULL,                        /* class_data */
        0,                                /* instance_size */
        0,                                /* n_preallocs */
        NULL,                        /* instance_init */
        NULL,                        /* value_table */
      };
      static const GTypeValueTable value_table = {
        value_init_long0,                /* value_init */
        NULL,                        /* value_free */
        value_copy_long0,                /* value_copy */
        NULL,                        /* value_peek_pointer */
        "i",                        /* collect_format */
        value_collect_int,        /* collect_value */
        "p",                        /* lcopy_format */
        value_lcopy_char,                /* lcopy_value */
      };
      info.value_table = &value_table;
      type = g_type_register_fundamental (G_TYPE_CHAR, "gchar", &info, &finfo, 0);

Having non-instantiable types might seem a bit useless: what good is a type if you cannot instantiate an instance of that type ? Most of these types are used in conjunction with GValues: a GValue is initialized with an integer or a string and it is passed around by using the registered type's value_table. GValues (and by extension these trivial fundamental types) are most useful when used in conjunction with object properties and signals.

## Instantiable classed types: objects

Types which are registered with a class and are declared instantiable are what most closely resembles an object. Although GObjects (detailed in ) 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:

    typedef struct {
      GObject parent;
      /* instance members */
      int field_a;
    } MamanBar;

    typedef struct {
      GObjectClass parent;
      /* class members */
      void (*do_action_public_virtual) (MamanBar *self, guint8 i);

      void (*do_action_public_pure_virtual) (MamanBar *self, guint8 i);
    } MamanBarClass;

    #define MAMAN_TYPE_BAR (maman_bar_get_type ())

    GType
    maman_bar_get_type (void)
    {
      static GType type = 0;
      if (type == 0) {
        static const GTypeInfo info = {
          sizeof (MamanBarClass),
          NULL,           /* base_init */
          NULL,           /* base_finalize */
          (GClassInitFunc) foo_class_init,
          NULL,           /* class_finalize */
          NULL,           /* class_data */
          sizeof (MamanBar),
          0,              /* n_preallocs */
          (GInstanceInitFunc) NULL /* instance_init */
        };
        type = g_type_register_static (G_TYPE_OBJECT,
                                       "BarType",
                                       &info, 0);
      }
      return type;
    }

Upon the first call to `maman_bar_get_type`, the type named BarType 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:

    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:

    /* 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:

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

or, more quickly:

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

### Initialization and Destruction

instantiation of these types can be done with `g_type_create_instance`:

    GTypeInstance* g_type_create_instance (GType          type);
    void           g_type_free_instance   (GTypeInstance *instance);

`g_type_create_instance` will look up the type information structure associated to 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. It first copies the parent's class structure over this structure (if there is no parent, it initializes it to zero). It 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 (ClassFinalizeFunc) function is invoked. The base_class_finalize (GBaseFinalizeFunc) functions are Finally invoked from bottom-most most-derived type to top-most fundamental type and the class structure is freed.

As many readers have now understood it, 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 ) 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:

#### 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 |
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 |
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.

## Non-instantiable classed types: interfaces

GType's interfaces are very similar to Java's interfaces. They allow to describe a common API that several classes will adhere to. Imagine the play, pause and stop buttons on hi-fi equipment - those can be seen as a playback interface. Once you know what they do, you can control your CD player, MP3 player or anything that uses these symbols. To declare an interface you have to register a non-instantiable classed type which derives from GTypeInterface. The following piece of code declares such an interface.

    #define MAMAN_IBAZ_TYPE                (maman_ibaz_get_type ())
    #define MAMAN_IBAZ(obj)                (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_IBAZ_TYPE, MamanIbaz))
    #define MAMAN_IS_IBAZ(obj)             (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_IBAZ_TYPE))
    #define MAMAN_IBAZ_GET_INTERFACE(inst) (G_TYPE_INSTANCE_GET_INTERFACE ((inst), MAMAN_IBAZ_TYPE, MamanIbazInterface))

    typedef struct _MamanIbaz MamanIbaz; /* dummy object */
    typedef struct _MamanIbazInterface MamanIbazInterface;

    struct _MamanIbazInterface {
      GTypeInterface parent;

      void (*do_action) (MamanIbaz *self);
    };

    GType maman_ibaz_get_type (void);

    void maman_ibaz_do_action (MamanIbaz *self);

The interface function, `maman_ibaz_do_action` is implemented in a pretty simple way:

    void maman_ibaz_do_action (MamanIbaz *self)
    {
      MAMAN_IBAZ_GET_INTERFACE (self)->do_action (self);
    }

`maman_ibaz_get_type` registers a type named MamanIBaz which inherits from G_TYPE_INTERFACE. All interfaces must be children of G_TYPE_INTERFACE in the inheritance tree.

An interface is defined by only one structure which must contain as first member a GTypeInterface structure. The interface structure is expected to contain the function pointers of the interface methods. It is good style to define helper functions for each of the interface methods which simply call the interface' method directly: `maman_ibaz_do_action` is one of these.

Once an interface type is registered, you must register implementations for these interfaces. The function named `maman_baz_get_type` registers a new GType named MamanBaz which inherits from GObject and which implements the interface MamanIBaz.

    static void maman_baz_do_action (MamanIbaz *self)
    {
      g_print ("Baz implementation of IBaz interface Action.\n");
    }

    static void
    baz_interface_init (gpointer         g_iface,
                        gpointer         iface_data)
    {
      MamanIbazInterface *iface = (MamanIbazInterface *)g_iface;
      iface->do_action = maman_baz_do_action;
    }

    GType
    maman_baz_get_type (void)
    {
      static GType type = 0;
      if (type == 0) {
        static const GTypeInfo info = {
          sizeof (MamanBazInterface),
          NULL,   /* base_init */
          NULL,   /* base_finalize */
          NULL,   /* class_init */
          NULL,   /* class_finalize */
          NULL,   /* class_data */
          sizeof (MamanBaz),
          0,      /* n_preallocs */
          NULL    /* instance_init */
        };
        static const GInterfaceInfo ibaz_info = {
          (GInterfaceInitFunc) baz_interface_init,    /* interface_init */
          NULL,               /* interface_finalize */
          NULL          /* interface_data */
        };
        type = g_type_register_static (G_TYPE_OBJECT,
                                       "MamanBazType",
                                       &info, 0);
        g_type_add_interface_static (type,
                                     MAMAN_IBAZ_TYPE,
                                     &ibaz_info);
      }
      return type;
    }

`g_type_add_interface_static` records in the type system that a given type implements also FooInterface (`foo_interface_get_type` returns the type of FooInterface). The GInterfaceInfo structure holds information about the implementation of the interface:

    struct _GInterfaceInfo
    {
      GInterfaceInitFunc     interface_init;
      GInterfaceFinalizeFunc interface_finalize;
      gpointer               interface_data;
    };

### Interface Initialization

When an instantiable classed type which registered an interface implementation is created for the first time, its class structure is initialized following the process described in . After that, the interface implementations associated with the type are initialized.

First a memory buffer is allocated to hold the interface structure. The parent's interface structure is then copied over to the new interface structure (the parent interface is already initialized at that point). If there is no parent interface, the interface structure is initialized with zeros. The g_type and the g_instance_type fields are then initialized: g_type is set to the type of the most-derived interface and g_instance_type is set to the type of the most derived type which implements this interface.

Finally, the interface' most-derived `base_init` function and then the implementation's `interface_init` function are invoked. It is important to understand that if there are multiple implementations of an interface the `base_init` and `interface_init` functions will be invoked once for each implementation initialized.

It is thus common for base_init functions to hold a local static boolean variable which makes sure that the interface type is initialized only once even if there are multiple implementations of the interface:

    static void
    maman_ibaz_base_init (gpointer g_iface)
    {
      static gboolean initialized = FALSE;

      if (!initialized) {
        /* create interface signals here. */
        initialized = TRUE;
      }
    }

If you have found the stuff about interface hairy, you are right: it is hairy but there is not much I can do about it. What I can do is summarize what you need to know about interfaces:

The above process can be summarized as follows:

#### Interface Initialization

Invocation time | Function Invoked | Function's parameters | Remark
--- | --- | --- | ---
First call to `g_type_create_instance` for type implementing interface | interface' base_init function | On interface' vtable | Register interface' signals here (use a local static boolean variable as described above to make sure not to register them twice.).
interface' interface_init function | On interface' vtable | Initialize interface' implementation. That is, initialize the interface method pointers in the interface structure to the function's implementation.

It is highly unlikely (i.e. I do not know of anyone who actually used it) you will ever need other more fancy things such as the ones described in the following section ().

### Interface Destruction

When the last instance of an instantiable type which registered an interface implementation is destroyed, the interface's implementations associated to the type are destroyed.

To destroy an interface implementation, GType first calls the implementation's `interface_finalize` function and then the interface's most-derived `base_finalize` function.

Again, it is important to understand, as in , that both `interface_finalize` and `base_finalize` are invoked exactly once for the destruction of each implementation of an interface. Thus, if you were to use one of these functions, you would need to use a static integer variable which would hold the number of instances of implementations of an interface such that the interface's class is destroyed only once (when the integer variable reaches zero).

The above process can be summarized as follows:

#### Interface Finalization

Invocation time | Function Invoked | Function's parameters
--- | --- | ---
Last call to `g_type_free_instance` for type implementing interface | interface' interface_finalize function | On interface' vtable
interface' base_finalize function | On interface' vtable