objects.sgml

This GLib version 2.16.1. GLib is the low-level core library that forms the basis for projects such

<!-- ##### SECTION Title ##### -->
GObject

<!-- ##### SECTION Short_Description ##### -->
The base object type

<!-- ##### SECTION Long_Description ##### -->
<para>
GObject is the fundamental type providing the common attributes and methods 
for all object types in GTK+, Pango and other libraries based on GObject. 
The GObject class provides methods for object construction and destruction, 
property access methods, and signal support. 
Signals are described in detail in <xref linkend="gobject-Signals"/>.
</para>
<para id="floating-ref">
#GInitiallyUnowned is derived from #GObject. The only difference between
the two is that the initial reference of a #GInitiallyUnowned is flagged 
as a <firstterm>floating</firstterm> reference.
This means that it is not specifically claimed to be "owned" by
any code portion. The main motivation for providing floating references is
C convenience. In particular, it allows code to be written as:
<example><programlisting>
  container = create_container();
  container_add_child (container, create_child());
</programlisting></example>
If <function>container_add_child()</function> will g_object_ref_sink() the
passed in child, no reference of the newly created child is leaked.
Without floating references, <function>container_add_child()</function>
can only g_object_ref() the new child, so to implement this code without
reference leaks, it would have to be written as:
<example><programlisting>
  Child *child;
  container = create_container();
  child = create_child();
  container_add_child (container, child);
  g_object_unref (child);
</programlisting></example>
The floating reference can be converted into 
an ordinary reference by calling g_object_ref_sink().
For already sunken objects (objects that don't have a floating reference
anymore), g_object_ref_sink() is equivalent to g_object_ref() and returns
a new reference.
Since floating references are useful almost exclusively for C convenience,
language bindings that provide automated reference and memory ownership
maintenance (such as smart pointers or garbage collection) therefore don't
need to expose floating references in their API.
</para>
<para>
Some object implementations may need to save an objects floating state
across certain code portions (an example is #GtkMenu), to achive this, the
following sequence can be used:
</para>

<example><programlisting>
  /* save floating state */
  gboolean was_floating = g_object_is_floating (object);
  g_object_ref_sink (object);
  /* protected code portion */
  ...;
  /* restore floating state */
  if (was_floating)
    g_object_force_floating (object);
  g_obejct_unref (object); /* release previously acquired reference */
</programlisting></example>

<!-- ##### SECTION See_Also ##### -->
<para>
#GParamSpecObject, g_param_spec_object()
</para>

<!-- ##### SECTION Stability_Level ##### -->


<!-- ##### STRUCT GObject ##### -->
<para>
All the fields in the <structname>GObject</structname> structure are private 
to the #GObject implementation and should never be accessed directly.
</para>


<!-- ##### SIGNAL GObject::notify ##### -->
<para>
The notify signal is emitted on an object when one of its properties 
has been changed. Note that getting this signal doesn't guarantee that the
value of the property has actually changed, it may also be emitted when
the setter for the property is called to reinstate the previous value.
</para>
<para>
This signal is typically used to obtain change notification for a 
single property, by specifying the property name as a detail in the
g_signal_connect() call, like this:
<informalexample><programlisting>
g_signal_connect (text_view->buffer, "notify::paste-target-list",
                  G_CALLBACK (gtk_text_view_target_list_notify),
                  text_view)
</programlisting></informalexample>
It is important to note that you must use 
<link linkend="canonical-parameter-name">canonical</link> parameter names as
detail strings for the notify signal.
</para>

@pspec: the #GParamSpec of the property which changed
@gobject: the object which received the signal.

<!-- ##### STRUCT GObjectClass ##### -->
<para>
The class structure for the <structname>GObject</structname> type.
</para>
<example>
<title>Implementing singletons using a constructor</title>
<programlisting>
static MySingleton *the_singleton = NULL;

static GObject*
my_singleton_constructor (GType                  type,
                          guint                  n_construct_params,
                          GObjectConstructParam *construct_params)
{
  GObject *object;
  
  if (!the_singleton)
    {
      object = G_OBJECT_CLASS (parent_class)->constructor (type,
                                                           n_construct_params,
                                                           construct_params);
      the_singleton = MY_SINGLETON (object);
    }
  else
    object = g_object_ref (G_OBJECT (the_singleton));

  return object;
}
</programlisting></example>

@g_type_class: the parent class
@constructor:  the @constructor function is called by g_object_new () to 
  complete the object initialization after all the construction properties are
  set. The first thing a @constructor implementation must do is chain up to the
  @constructor of the parent class. Overriding @constructor should be rarely 
  needed, e.g. to handle construct properties, or to implement singletons.
@set_property: the generic setter for all properties of this type. Should be
  overridden for every type with properties. Implementations of @set_property
  don't need to emit property change notification explicitly, this is handled
  by the type system.
@get_property: the generic getter for all properties of this type. Should be
  overridden for every type with properties.
@dispose: the @dispose function is supposed to drop all references to other 
  objects, but keep the instance otherwise intact, so that client method 
  invocations still work. It may be run multiple times (due to reference 
  loops). Before returning, @dispose should chain up to the @dispose method 
  of the parent class.
@finalize: instance finalization function, should finish the finalization of 
  the instance begun in @dispose and chain up to the @finalize method of the 
  parent class.
@dispatch_properties_changed: emits property change notification for a bunch
  of properties. Overriding @dispatch_properties_changed should be rarely 
  needed.
@notify: the class closure for the notify signal
@constructed: the @constructed function is called by g_object_new() as the
  final step of the object creation process.  At the point of the call, all
  construction properties have been set on the object.  The purpose of this
  call is to allow for object initialisation steps that can only be performed
  after construction properties have been set.  @constructed implementors
  should chain up to the @constructed call of their parent class to allow it
  to complete its initialisation.

<!-- ##### STRUCT GObjectConstructParam ##### -->
<para>
The <structname>GObjectConstructParam</structname> struct is an auxiliary 
structure used to hand #GParamSpec/#GValue pairs to the @constructor of
a #GObjectClass.
</para>

@pspec: the #GParamSpec of the construct parameter
@value: the value to set the parameter to

<!-- ##### USER_FUNCTION GObjectGetPropertyFunc ##### -->
<para>
The type of the @get_property function of #GObjectClass. 
</para>

@object: a #GObject
@property_id: the numeric id under which the property was registered with
   g_object_class_install_property().
@value: a #GValue to return the property value in
@pspec: the #GParamSpec describing the property


<!-- ##### USER_FUNCTION GObjectSetPropertyFunc ##### -->
<para>
The type of the @set_property function of #GObjectClass. 
</para>

@object: a #GObject
@property_id: the numeric id under which the property was registered with
   g_object_class_install_property().
@value: the new value for the property
@pspec: the #GParamSpec describing the property


<!-- ##### USER_FUNCTION GObjectFinalizeFunc ##### -->
<para>
The type of the @finalize function of #GObjectClass.
</para>

@object: the #GObject being finalized


<!-- ##### MACRO G_TYPE_IS_OBJECT ##### -->
<para>
Returns a boolean value of %FALSE or %TRUE indicating whether
the passed in type id is a %G_TYPE_OBJECT or derived from it.
</para>

@type: Type id to check for is a %G_TYPE_OBJECT relationship.
@Returns: %FALSE or %TRUE, indicating whether @type is a %G_TYPE_OBJECT.


<!-- ##### MACRO G_OBJECT ##### -->
<para>
Casts a #GObject or derived pointer into a (GObject*) pointer.
Depending on the current debugging level, this function may invoke
certain runtime checks to identify invalid casts.
</para>

@object: Object which is subject to casting.


<!-- ##### MACRO G_IS_OBJECT ##### -->
<para>
Checks whether a valid #GTypeInstance pointer is of type %G_TYPE_OBJECT.
</para>

@object: Instance to check for being a %G_TYPE_OBJECT.


<!-- ##### MACRO G_OBJECT_CLASS ##### -->
<para>
Casts a derived #GObjectClass structure into a #GObjectClass structure.
</para>

@class: a valid #GObjectClass


<!-- ##### MACRO G_IS_OBJECT_CLASS ##### -->
<para>
Checks whether @class "is a" valid #GObjectClass structure of type
%G_TYPE_OBJECT or derived.
</para>

@class: a #GObjectClass


<!-- ##### MACRO G_OBJECT_GET_CLASS ##### -->
<para>
Returns the class structure associated to a #GObject instance.
</para>

@object: a #GObject instance.


<!-- ##### MACRO G_OBJECT_TYPE ##### -->
<para>
Return the type id of an object.
</para>

@object: Object to return the type id for.
@Returns: Type id of @object.


<!-- ##### MACRO G_OBJECT_TYPE_NAME ##### -->
<para>
Returns the name of an object's type.
</para>

@object: Object to return the type name for.
@Returns: Type name of @object. The string is owned by the type system and 
should not be freed.


<!-- ##### MACRO G_OBJECT_CLASS_TYPE ##### -->
<para>
Return the type id of a class structure.
</para>

@class: a valid #GObjectClass
@Returns: Type id of @class.


<!-- ##### MACRO G_OBJECT_CLASS_NAME ##### -->
<para>
Return the name of a class structure's type.
</para>

@class: a valid #GObjectClass
@Returns: Type name of @class. The string is owned by the type system and 
should not be freed.


<!-- ##### FUNCTION g_object_class_install_property ##### -->
<para>
Installs a new property. This is usually done in the class initializer.
</para>

@oclass: a #GObjectClass
@property_id: the id for the new property
@pspec: the #GParamSpec for the new property


<!-- ##### FUNCTION g_object_class_find_property ##### -->
<para>
Looks up the #GParamSpec for a property of a class.
</para>

@oclass: a #GObjectClass
@property_name: the name of the property to look up
@Returns: the #GParamSpec for the property, or %NULL if the class doesn't have
a property of that name


<!-- ##### FUNCTION g_object_class_list_properties ##### -->
<para>
Returns an array of #GParamSpec* for all properties of a class.
</para>

@oclass: a #GObjectClass
@n_properties: return location for the length of the returned array
@Returns: an array of #GParamSpec* which should be freed after use


<!-- ##### FUNCTION g_object_class_override_property ##### -->
<para>
Registers @property_id as referring to a property with the
name @name in a parent class or in an interface implemented
by @oclass. This allows this class to <firstterm>override</firstterm>
a property implementation in a parent class or to provide
the implementation of a property from an interface.
</para>
<note>
<para>
Internally, overriding is implemented by creating a property of type
#GParamSpecOverride; generally operations that query the properties of
the object class, such as g_object_class_find_property() or
g_object_class_list_properties() will return the overridden
property. However, in one case, the @construct_properties argument of
the @constructor virtual function, the #GParamSpecOverride is passed
instead, so that the @param_id field of the #GParamSpec will be
correct.  For virtually all uses, this makes no difference. If you
need to get the overridden property, you can call
g_param_spec_get_redirect_target().
</para>
</note>

@oclass: a #GObjectClass
@property_id: the new property ID
@name: the name of a property registered in a parent class or
       in an interface of this class.
@Since: 2.4


<!-- ##### FUNCTION g_object_interface_install_property ##### -->
<para>
Add a property to an interface; this is only useful for interfaces
that are added to GObject-derived types. Adding a property to an
interface forces all objects classes with that interface to have a
compatible property. The compatible property could be a newly
created #GParamSpec, but normally
g_object_class_override_property() will be used so that the object
class only needs to provide an implementation and inherits the
property description, default value, bounds, and so forth from the
interface property.