path: root/docs/reference/gobject/tut_gtype.xml

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]>
  <chapter id="chapter-gtype">
    <title>The GLib Dynamic Type System</title>

      <para>
        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.
        <informalexample><programlisting>
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);
        </programlisting></informalexample>
      </para>

      <para>
        <function><link linkend="g-type-register-static">g_type_register_static</link></function>,
        <function><link linkend="g-type-register-dynamic">g_type_register_dynamic</link></function> and 
        <function><link linkend="g-type-register-fundamental">g_type_register_fundamental</link></function>
        are the C functions, defined in
        <filename>gtype.h</filename> and implemented in <filename>gtype.c</filename>
        which you should use to register a new <link linkend="GType"><type>GType</type></link> in the program's type system.
        It is not likely you will ever need to use 
        <function><link linkend="g-type-register-fundamental">g_type_register_fundamental</link></function>
        but in case you want to, the last chapter explains how to create
        new fundamental types.
      </para>

      <para>
        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, 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.
      </para>

      <para>
        Fundamental and non-fundamental types are defined by:
        <itemizedlist>
          <listitem><para>
            class size: the class_size field in <link linkend="GTypeInfo"><type>GTypeInfo</type></link>.
          </para></listitem>
          <listitem><para>
            class initialization functions (C++ constructor): the <function>base_init</function> and 
            <function>class_init</function> fields in <link linkend="GTypeInfo"><type>GTypeInfo</type></link>.
          </para></listitem>
          <listitem><para>
            class destruction functions (C++ destructor): the base_finalize and 
            class_finalize fields in <link linkend="GTypeInfo"><type>GTypeInfo</type></link>.
          </para></listitem>
          <listitem><para>
            instance size (C++ parameter to new): the instance_size field in 
            <link linkend="GTypeInfo"><type>GTypeInfo</type></link>.
          </para></listitem>
          <listitem><para>
            instantiation policy (C++ type of new operator): the n_preallocs
            field in <link linkend="GTypeInfo"><type>GTypeInfo</type></link>.
          </para></listitem>
          <listitem><para>
            copy functions (C++ copy operators): the value_table field in 
            <link linkend="GTypeInfo"><type>GTypeInfo</type></link>.
          </para></listitem>
          <listitem><para>
            type characteristic flags: <link linkend="GTypeFlags"><type>GTypeFlags</type></link>.
          </para></listitem>
        </itemizedlist>
        Fundamental types are also defined by a set of <link linkend="GTypeFundamentalFlags"><type>GTypeFundamentalFlags</type></link> 
        which are stored in a <link linkend="GTypeFundamentalInfo"><type>GTypeFundamentalInfo</type></link>.
        Non-fundamental types are furthermore defined by the type of their parent which is
        passed as the parent_type parameter to <function><link linkend="g-type-register-static">g_type_register_static</link></function>
        and <function><link linkend="g-type-register-dynamic">g_type_register_dynamic</link></function>.
      </para>
      
      <sect1 id="gtype-copy">
        <title>Copy functions</title>

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

        <para>
          The <link linkend="GValue"><type>GValue</type></link> structure is used as an abstract container for all of these 
          types. Its simplistic API (defined in <filename>gobject/gvalue.h</filename>) can be 
          used to invoke the value_table functions registered
          during type registration: for example <function><link linkend="g-value-copy">g_value_copy</link></function> copies the 
          content of a <link linkend="GValue"><type>GValue</type></link> to another <link linkend="GValue"><type>GValue</type></link>. 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.
        </para>

        <para>
          The following code shows how you can copy around a 64 bit integer, as well as a <link linkend="GObject"><type>GObject</type></link>
          instance pointer:
<informalexample><programlisting>
static void test_int (void)
{
  GValue a_value = G_VALUE_INIT;
  GValue b_value = G_VALUE_INIT;
  guint64 a, b;

  a = 0xdeadbeef;

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

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

  b = g_value_get_uint64 (&amp;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 = G_VALUE_INIT;
  GValue obj_valb = G_VALUE_INIT;
  obj = g_object_new (VIEWER_TYPE_FILE, NULL);

  g_value_init (&amp;obj_vala, VIEWER_TYPE_FILE);
  g_value_set_object (&amp;obj_vala, obj);

  g_value_init (&amp;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
   * VIEWER_TYPE_FILE is a G_TYPE_OBJECT.
   */
  g_value_copy (&amp;obj_vala, &amp;obj_valb);

  g_object_unref (G_OBJECT (obj));
  g_object_unref (G_OBJECT (obj));
}
</programlisting></informalexample>
          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.
        </para>
        
        <para>
          The value table used to specify these assignment functions is
          documented in
          <link linkend="GTypeValueTable"><type>GTypeValueTable</type></link>.
        </para>
        <para>
          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.
        </para>
      </sect1>

      <sect1 id="gtype-conventions">
        <title>Conventions</title>


      <para>
        There are a number of conventions users are expected to follow when creating new types
        which are to be exported in a header file:
        <itemizedlist>
          <listitem><para>
            Type names (including object names) must be at least three
            characters long and start with ‘a–z’, ‘A–Z’ or ‘_’.
          </para></listitem>
          <listitem><para>
            Use the <function>object_method</function> pattern for function names: to invoke
            the method named <function>save</function> on an instance of object type <type>file</type>, call 
            <function>file_save</function>.
          </para></listitem>
          <listitem><para>Use prefixing to avoid namespace conflicts with other projects.
            If your library (or application) is named <emphasis>Viewer</emphasis>,
            prefix all your function names with <emphasis>viewer_</emphasis>.
            For example: <function>viewer_object_method</function>.
          </para></listitem>
          <listitem><para>Create a macro named <function>PREFIX_TYPE_OBJECT</function> which always 
            returns the GType for the associated object type. For an object of type 
            <emphasis>File</emphasis> in the <emphasis>Viewer</emphasis> namespace,
            use: <function>VIEWER_TYPE_FILE</function>.
            This macro is implemented using a function named
            <function>prefix_object_get_type</function>; for example, <function>viewer_file_get_type</function>.
          </para></listitem>
          <listitem>
            <para>
              Use <link linkend="G-DECLARE-FINAL-TYPE:CAPS"><function>G_DECLARE_FINAL_TYPE</function></link>
              or <link linkend="G-DECLARE-DERIVABLE-TYPE:CAPS"><function>G_DECLARE_DERIVABLE_TYPE</function></link>
              to define various other conventional macros for your object:
            </para>
            <itemizedlist>
              <listitem><para><function>PREFIX_OBJECT (obj)</function>, which 
                returns a pointer of type <type>PrefixObject</type>. 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 <link linkend="glib-building">building GLib</link>).
                For example, we would create 
                <function>VIEWER_FILE (obj)</function> to keep the previous example.
              </para></listitem>
              <listitem><para><function>PREFIX_OBJECT_CLASS (klass)</function>, which
                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 <type>PrefixObjectClass</type>. An example is:
                <function>VIEWER_FILE_CLASS</function>.
              </para></listitem>
              <listitem><para><function>PREFIX_IS_OBJECT (obj)</function>, which
                returns a <type>gboolean</type> which indicates whether the input
                object instance pointer is non-<type>NULL</type> and of type <type>OBJECT</type>.
                For example, <function>VIEWER_IS_FILE</function>.
              </para></listitem>
              <listitem><para><function>PREFIX_IS_OBJECT_CLASS (klass)</function>, which returns a boolean
                if the input class pointer is a pointer to a class of type OBJECT.
                For example, <function>VIEWER_IS_FILE_CLASS</function>.
              </para></listitem>
              <listitem><para><function>PREFIX_OBJECT_GET_CLASS (obj)</function>,
                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).
                For example, <function>VIEWER_FILE_GET_CLASS</function>.
              </para></listitem>
            </itemizedlist>
          </listitem>
        </itemizedlist>
        The implementation of these macros is pretty straightforward: a number of simple-to-use 
        macros are provided in <filename>gtype.h</filename>. For the example we used above, we would 
        write the following trivial code to declare the macros:
<informalexample><programlisting>
#define VIEWER_TYPE_FILE viewer_file_get_type ()
G_DECLARE_FINAL_TYPE (ViewerFile, viewer_file, VIEWER, FILE, GObject)
</programlisting></informalexample>
      </para>

      <para>
        Unless your code has special requirements, you can use the
        <function><link linkend="G-DEFINE-TYPE:CAPS">G_DEFINE_TYPE</link></function>
	macro to define a class:
<informalexample><programlisting>
G_DEFINE_TYPE (ViewerFile, viewer_file, G_TYPE_OBJECT)
</programlisting></informalexample>
      </para>

      <para>
        Otherwise, the <function>viewer_file_get_type</function> function must be
        implemented manually:
<informalexample><programlisting>
GType viewer_file_get_type (void)
{
  static GType type = 0;
  if (type == 0) {
    const GTypeInfo info = {
      /* You fill this structure. */
    };
    type = g_type_register_static (G_TYPE_OBJECT,
                                   "ViewerFile",
                                   &amp;info, 0);
  }
  return type;
}
</programlisting></informalexample>
      </para>

      </sect1>

      <sect1 id="gtype-non-instantiatable">
        <title>Non-instantiatable non-classed fundamental types</title>

        <para>
          A lot of types are not instantiatable by the type system and do not have
          a class. Most of these types are fundamental trivial types such as <emphasis>gchar</emphasis>, 
          and are already registered by GLib.
        </para>

        <para>
          In the rare case of needing to register such a type in the type
          system, fill a
          <link linkend="GTypeInfo"><type>GTypeInfo</type></link> structure with zeros since these types are also most of the time
          fundamental:
          <informalexample><programlisting>
  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 = &amp;value_table;
  type = g_type_register_fundamental (G_TYPE_CHAR, "gchar", &amp;info, &amp;finfo, 0);
          </programlisting></informalexample>
        </para>


        <para>
          Having non-instantiatable 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 <link linkend="GValue"><type>GValue</type></link>s: a GValue is initialized
          with an integer or a string and it is passed around by using the registered 
          type's value_table. <link linkend="GValue"><type>GValue</type></link>s (and by extension these trivial fundamental
          types) are most useful when used in conjunction with object properties and signals.
        </para>

      </sect1>

      <sect1 id="gtype-instantiatable-classed">
        <title>Instantiatable classed types: objects</title>

        <para>
          This section covers the theory behind objects. See
          <xref linkend="howto-gobject"/> for the recommended way to define a
          GObject.
        </para>

        <para>
          Types which are registered with a class and are declared instantiatable are
          what most closely resembles an <emphasis>object</emphasis>. 
          Although <link linkend="GObject"><type>GObject</type></link>s (detailed in <xref linkend="chapter-gobject"/>) 
          are the most well known type of instantiatable
          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.
        </para>

        <para>
          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):
<informalexample><programlisting>
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",
                                   &amp;info, 0);
  }
  return type;
}
</programlisting></informalexample>
          Upon the first call to <function>viewer_file_get_type</function>, the type named
          <emphasis>ViewerFile</emphasis> will be registered in the type system as inheriting
          from the type <emphasis>G_TYPE_OBJECT</emphasis>.
        </para>

        <para>
          Every object must define two structures: its class structure and its 
          instance structure. All class structures must contain as first member
          a <link linkend="GTypeClass"><type>GTypeClass</type></link> structure. All instance structures must contain as first
          member a <link linkend="GTypeInstance"><type>GTypeInstance</type></link> structure. The declaration of these C types,
          coming from <filename>gtype.h</filename> is shown below:
<informalexample><programlisting>
struct _GTypeClass
{
  GType g_type;
};
struct _GTypeInstance
{
  GTypeClass *g_class;
};
</programlisting></informalexample>
          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.
        </para>
        <para>
          This relationship is best explained by an example: let's take object B which
          inherits from object A:
<informalexample><programlisting>
/* 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;
</programlisting></informalexample>          
          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 <type>GTypeInstance</type>'s first field which in
          turn is <structfield>g_class</structfield>, a pointer
          to B's class structure.
        </para>

        <para>
          Thanks to these simple conditions, it is possible to detect the type of every
          object instance by doing: 
<informalexample><programlisting>
B *b;
b->parent.parent.g_class->g_type
</programlisting></informalexample>
          or, more quickly:
<informalexample><programlisting>
B *b;
((GTypeInstance *) b)->g_class->g_type
</programlisting></informalexample>
        </para>

        <sect2 id="gtype-instantiatable-classed-init-done">
          <title>Initialization and Destruction</title>

          <para>
            instantiation of these types can be done with 
            <function><link linkend="g-type-create-instance">g_type_create_instance</link></function>,
            which will look up the type information
            structure associated with the type requested. Then, the instance size and instantiation
            policy (if the <structfield>n_preallocs</structfield> 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.
          </para>

          <para>
            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
            <function>base_class_initialization</function> functions
            (<link linkend="GBaseInitFunc"><type>GBaseInitFunc</type></link>) from topmost 
            fundamental object to bottom-most most derived object. The object's <function>class_init</function>
            (<link linkend="GClassInitFunc"><type>GClassInitFunc</type></link>) 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).
          </para>

          <para>
            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
            <function>instance_init</function>
            (<link linkend="GInstanceInitFunc"><type>GInstanceInitFunc</type></link>)
            functions, from top-most fundamental 
            type to bottom-most most-derived type.
          </para>

          <para>
            Object instance destruction through <function><link linkend="g-type-free-instance">g_type_free_instance</link></function> 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.
          </para>


          <para>
            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 (<link linkend="GClassFinalizeFunc"><type>GClassFinalizeFunc</type></link>) function is invoked. Finally, the
            base_class_finalize (<link linkend="GBaseFinalizeFunc"><type>GBaseFinalizeFunc</type></link>) functions are 
            invoked from bottom-most most-derived type to top-most fundamental type and
            the class structure is freed.
          </para>

          <para>
            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 
            <xref linkend="chapter-gobject"/>.)
           Furthermore, C++ code equivalent to the <function>base_init</function>
           and <function>class_init</function> callbacks of GType is usually not needed because C++ cannot really create object 
           types at runtime.
          </para>

          <para>
            The instantiation/finalization process can be summarized as follows:
            <table id="gtype-init-fini-table">
              <title>GType Instantiation/Finalization</title>
              <tgroup cols="3">
                <colspec colwidth="*" colnum="1" align="left"/>
                <colspec colwidth="*" colnum="2" align="left"/>
                <colspec colwidth="8*" colnum="3" align="left"/>
    
                <thead>
                  <row>
                    <entry>Invocation time</entry>
                    <entry>Function invoked</entry>
                    <entry>Function's parameters</entry>
                  </row>
                </thead>
                <tbody>
                  <row>
                    <entry morerows="2">First call to <function><link linkend="g-type-create-instance">g_type_create_instance</link></function> for target type</entry>
                    <entry>type's <function>base_init</function> function</entry>
                    <entry>On the inheritance tree of classes from fundamental type to target type. 
                      <function>base_init</function> is invoked once for each class structure.</entry>
                  </row>
                  <row>
                    <!--entry>First call to <function><link linkend="g-type-create-instance">g_type_create_instance</link></function> for target type</entry-->
                    <entry>target type's <function>class_init</function> function</entry>
                    <entry>On target type's class structure</entry>
                  </row>
                  <row>
                    <!--entry>First call to <function><link linkend="g-type-create-instance">g_type_create_instance</link></function> for target type</entry-->
                    <entry>interface initialization, see 
                      <xref linkend="gtype-non-instantiatable-classed-init"/></entry>
                    <entry></entry>
                  </row>
                  <row>
                    <entry>Each call to <function><link linkend="g-type-create-instance">g_type_create_instance</link></function> for target type</entry>
                    <entry>target type's <function>instance_init</function> function</entry>
                    <entry>On object's instance</entry>
                  </row>
                  <row>
                    <entry morerows="2">Last call to <function><link linkend="g-type-free-instance">g_type_free_instance</link></function> for target type</entry>
                    <entry>interface destruction, see
                      <xref linkend="gtype-non-instantiatable-classed-dest"/></entry>
                    <entry></entry>
                  </row>
                  <row>
                    <!--entry>Last call to <function><link linkend="g-type-free-instance">g_type_free_instance</link></function> for target type</entry-->
                    <entry>target type's <function>class_finalize</function> function</entry>
                    <entry>On target type's class structure</entry>
                  </row>
                  <row>
                    <!--entry>Last call to <function><link linkend="g-type-free-instance">g_type_free_instance</link></function> for target type</entry-->
                    <entry>type's <function>base_finalize</function> function</entry>
                    <entry>On the inheritance tree of classes from fundamental type to target type. 
                      <function>base_finalize</function> is invoked once for each class structure.</entry>
                  </row>
                </tbody>
              </tgroup>
            </table>
          </para>
          
        </sect2>

      </sect1>

      <sect1 id="gtype-non-instantiatable-classed">
        <title>Non-instantiatable classed types: interfaces</title>

        <para>
          This section covers the theory behind interfaces. See
          <xref linkend="howto-interface"/> for the recommended way to define an
          interface.
        </para>

        <para>
          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-instantiatable
          classed type which derives from 
          <link linkend="GTypeInterface"><type>GTypeInterface</type></link>. The following piece of code declares such an interface.
<informalexample><programlisting>
#define VIEWER_TYPE_EDITABLE viewer_editable_get_type ()
G_DECLARE_INTERFACE (ViewerEditable, viewer_editable, VIEWER, EDITABLE, GObject)

struct _ViewerEditableInterface {
  GTypeInterface parent;

  void (*save) (ViewerEditable  *self,
                GError         **error);
};

void viewer_editable_save (ViewerEditable  *self,
                           GError         **error);
</programlisting></informalexample>
          The interface function, <function>viewer_editable_save</function> is implemented
          in a pretty simple way:
<informalexample><programlisting>
void
viewer_editable_save (ViewerEditable  *self,
                      GError         **error)
{
  ViewerEditableinterface *iface;

  g_return_if_fail (VIEWER_IS_EDITABLE (self));
  g_return_if_fail (error == NULL || *error == NULL);

  iface = VIEWER_EDITABLE_GET_IFACE (self);
  g_return_if_fail (iface->save != NULL);
  iface->save (self);
}
</programlisting></informalexample>
         <function>viewer_editable_get_type</function> registers a type named <emphasis>ViewerEditable</emphasis>
         which inherits from <type>G_TYPE_INTERFACE</type>. All interfaces must
         be children of <type>G_TYPE_INTERFACE</type> in the  inheritance tree.
        </para>

        <para>
          An interface is defined by only one structure which must contain as first member
          a <link linkend="GTypeInterface"><type>GTypeInterface</type></link> 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's method directly: <function>viewer_editable_save</function>
          is one of these.
        </para>

        <para>
        If you have no special requirements you can use the
        <link linkend="G-IMPLEMENT-INTERFACE:CAPS">G_IMPLEMENT_INTERFACE</link> macro
        to implement an interface:
<informalexample><programlisting>
static void
viewer_file_save (ViewerEditable *self)
{
  g_print ("File implementation of editable interface save method.\n");
}

static void
viewer_file_editable_interface_init (ViewerEditableInterface *iface)
{
  iface->save = viewer_file_save;
}

G_DEFINE_TYPE_WITH_CODE (ViewerFile, viewer_file, VIEWER_TYPE_FILE,
                         G_IMPLEMENT_INTERFACE (VIEWER_TYPE_EDITABLE,
                                                viewer_file_editable_interface_init))
</programlisting></informalexample>
        </para>

        <para>
          If your code does have special requirements, you must write a custom
          <function>get_type</function> function to register your GType which
          inherits from some <link linkend="GObject"><type>GObject</type></link>
          and which implements the interface <type>ViewerEditable</type>. For
          example, this code registers a new <type>ViewerFile</type> class which
          implements <type>ViewerEditable</type>:
<informalexample><programlisting>
static void
viewer_file_save (ViewerEditable *editable)
{
  g_print ("File implementation of editable interface save method.\n");
}

static void
viewer_file_editable_interface_init (gpointer g_iface,
                                     gpointer iface_data)
{
  ViewerEditableInterface *iface = g_iface;

  iface->save = viewer_file_save;
}

GType 
viewer_file_get_type (void)
{
  static GType type = 0;
  if (type == 0) {
    const GTypeInfo info = {
      sizeof (ViewerFileClass),
      NULL,   /* base_init */
      NULL,   /* base_finalize */
      NULL,   /* class_init */
      NULL,   /* class_finalize */
      NULL,   /* class_data */
      sizeof (ViewerFile),
      0,      /* n_preallocs */
      NULL    /* instance_init */
    };
    const GInterfaceInfo editable_info = {
      (GInterfaceInitFunc) viewer_file_editable_interface_init,  /* interface_init */
      NULL,   /* interface_finalize */
      NULL    /* interface_data */
    };
    type = g_type_register_static (VIEWER_TYPE_FILE,
                                   "ViewerFile",
                                   &amp;info, 0);
    g_type_add_interface_static (type,
                                 VIEWER_TYPE_EDITABLE,
                                 &amp;editable_info);
  }
  return type;
}
</programlisting></informalexample>
        </para>

        <para>
          <function><link linkend="g-type-add-interface-static">g_type_add_interface_static</link></function> records in the type system that
          a given type implements also <type>FooInterface</type> 
          (<function>foo_interface_get_type</function> returns the type of 
          <type>FooInterface</type>).
                The <link linkend="GInterfaceInfo"><type>GInterfaceInfo</type></link> structure holds
          information about the implementation of the interface:
<informalexample><programlisting>
struct _GInterfaceInfo
{
  GInterfaceInitFunc     interface_init;
  GInterfaceFinalizeFunc interface_finalize;
  gpointer               interface_data;
};
</programlisting></informalexample>
        </para>

        <sect2 id="gtype-non-instantiatable-classed-init">
          <title>Interface Initialization</title>

          <para>
            When an instantiatable classed type which implements an interface
            (either directly or by inheriting an implementation from a superclass)
            is created for the first time, its class structure is initialized
            following the process described in <xref linkend="gtype-instantiatable-classed"/>.
            After that, the interface implementations associated with
            the type are initialized.
          </para>

          <para>
            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
            <structfield>g_type</structfield> and the
            <structfield>g_instance_type</structfield> fields are then
            initialized: <structfield>g_type</structfield> is set to the type of
            the most-derived interface and
            <structfield>g_instance_type</structfield> is set to the type of the
            most derived type which implements  this interface.
          </para>

          <para>
            The interface's <function>base_init</function> function is called,
            and then the interface's <function>default_init</function> is invoked.
            Finally if the type has registered an implementation of the interface,
            the implementation's <function>interface_init</function>
            function is invoked. If there are multiple implementations of an
            interface the <function>base_init</function> and
            <function>interface_init</function> functions will be invoked once
            for each implementation initialized.
          </para>

          <para>
            It is thus recommended to use a <function>default_init</function> function to
            initialize an interface. This function is called only once for the interface no
            matter how many implementations there are. The
            <function>default_init</function> function is declared by
            <link linkend="G-DEFINE-INTERFACE:CAPS">G_DEFINE_INTERFACE</link>
            which can be used to define the interface:
<informalexample><programlisting>
G_DEFINE_INTERFACE (ViewerEditable, viewer_editable, G_TYPE_OBJECT)

static void
viewer_editable_default_init (ViewerEditableInterface *iface)
{
  /* add properties and signals here, will only be called once */
}
</programlisting></informalexample>
          </para>

          <para>
            Or you can do that yourself in a GType function for your interface:
<informalexample><programlisting>
GType
viewer_editable_get_type (void)
{
  static gsize type_id = 0;
  if (g_once_init_enter (&amp;type_id)) {
    const GTypeInfo info = {
      sizeof (ViewerEditableInterface),
      NULL,   /* base_init */
      NULL,   /* base_finalize */
      viewer_editable_default_init, /* class_init */
      NULL,   /* class_finalize */
      NULL,   /* class_data */
      0,      /* instance_size */
      0,      /* n_preallocs */
      NULL    /* instance_init */
    };
    GType type = g_type_register_static (G_TYPE_INTERFACE,
                                         "ViewerEditable",
                                         &amp;info, 0);
    g_once_init_leave (&amp;type_id, type);
  }
  return type_id;
}

static void
viewer_editable_default_init (ViewerEditableInterface *iface)
{
  /* add properties and signals here, will only called once */
}
</programlisting></informalexample>
          </para>

        <para>
          In summary, interface initialization uses the following functions:
        </para>

          <para>
          <table id="ginterface-init-table">
            <title>Interface Initialization</title>
            <tgroup cols="3">
              <colspec colwidth="*" colnum="1" align="left"/>
              <colspec colwidth="*" colnum="2" align="left"/>
              <colspec colwidth="8*" colnum="3" align="left"/>
              
              <thead>
                <row>
                  <entry>Invocation time</entry>
                  <entry>Function Invoked</entry>
                  <entry>Function's parameters</entry>
                  <entry>Remark</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>First call to <function><link linkend="g-type-create-instance">g_type_create_instance</link></function>
                    for <emphasis>any</emphasis> type implementing interface
                   </entry>
                  <entry>interface's <function>base_init</function> function</entry>
                  <entry>On interface's vtable</entry>
                  <entry>Rarely necessary to use this. Called once per instantiated classed type implementing the interface.</entry>
                </row>
                <row>
                  <entry>First call to <function><link linkend="g-type-create-instance">g_type_create_instance</link></function>
                    for <emphasis>each</emphasis> type implementing interface
                   </entry>
                  <entry>interface's <function>default_init</function> function</entry>
                  <entry>On interface's vtable</entry>
                  <entry>Register interface's signals, properties, etc. here. Will be called once.</entry>
                </row>
                <row>
                  <entry>First call to <function><link linkend="g-type-create-instance">g_type_create_instance</link></function>
                    for <emphasis>any</emphasis> type implementing interface
                   </entry>
                  <entry>implementation's <function>interface_init</function> function</entry>
                  <entry>On interface's vtable</entry>
                  <entry>
                    Initialize interface implementation. Called for each class that that
                    implements the interface. Initialize the interface method pointers
                    in the interface structure to the implementing class's implementation.
                  </entry>
                </row>
              </tbody>
            </tgroup>
          </table>
        </para>
        
        </sect2>
        
        <sect2 id="gtype-non-instantiatable-classed-dest">
          <title>Interface Destruction</title>

          <para>
            When the last instance of an instantiatable type which registered
            an interface implementation is destroyed, the interface's 
            implementations associated to the type are destroyed.
          </para>

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

          <para>
            Again, it is important to understand, as in 
            <xref linkend="gtype-non-instantiatable-classed-init"/>,
              that both <function>interface_finalize</function> and <function>base_finalize</function>
              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).
          </para>
          
        <para>
          The above process can be summarized as follows:
          <table id="ginterface-fini-table">
            <title>Interface Finalization</title>
            <tgroup cols="3">
              <colspec colwidth="*" colnum="1" align="left"/>
              <colspec colwidth="*" colnum="2" align="left"/>
              <colspec colwidth="8*" colnum="3" align="left"/>
              
              <thead>
                <row>
                  <entry>Invocation time</entry>
                  <entry>Function Invoked</entry>
                  <entry>Function's parameters</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry morerows="1">Last call to <function><link linkend="g-type-free-instance">g_type_free_instance</link></function> for type
                    implementing interface
                   </entry>
                  <entry>interface's <function>interface_finalize</function> function</entry>
                  <entry>On interface's vtable</entry>
                </row>
                <row>
                  <!--entry>Last call to <function><link linkend="g-type-free-instance">g_type_free_instance</link></function>for type
                    implementing interface
                   </entry-->
                  <entry>interface's <function>base_finalize</function> function</entry>
                  <entry>On interface's vtable</entry>
                </row>
              </tbody>
            </tgroup>
          </table>
        </para>
      </sect2>
    </sect1>
  </chapter>