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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, instantiable and non-instantiable) 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 (&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 = G_VALUE_INIT;
GValue obj_valb = G_VALUE_INIT;
obj = g_object_new (VIEWER_TYPE_FILE, NULL);
g_value_init (&obj_vala, VIEWER_TYPE_FILE);
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
* VIEWER_TYPE_FILE is a G_TYPE_OBJECT.
*/
g_value_copy (&obj_vala, &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",
&info, 0);
}
return type;
}
</programlisting></informalexample>
</para>
</sect1>
<sect1 id="gtype-non-instantiable">
<title>Non-instantiable non-classed fundamental types</title>
<para>
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 <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 = &value_table;
type = g_type_register_fundamental (G_TYPE_CHAR, "gchar", &info, &finfo, 0);
</programlisting></informalexample>
</para>
<para>
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 <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-instantiable-classed">
<title>Instantiable 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 instantiable 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 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.
</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",
&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-instantiable-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-instantiable-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-instantiable-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-instantiable-classed">
<title>Non-instantiable 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-instantiable
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",
&info, 0);
g_type_add_interface_static (type,
VIEWER_TYPE_EDITABLE,
&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-instantiable-classed-init">
<title>Interface Initialization</title>
<para>
When an instantiable 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-instantiable-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 volatile gsize type_id = 0;
if (g_once_init_enter (&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",
&info, 0);
g_once_init_leave (&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-instantiable-classed-dest">
<title>Interface Destruction</title>
<para>
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.
</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-instantiable-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>