Floating references

**Note**: Floating references are a C convenience API and should not be used in modern GObject code. Language bindings in particular find the concept highly problematic, as floating references are not identifiable through annotations, and neither are deviations from the floating reference behavior, like types that inherit from GInitiallyUnowned and still return a full reference from g_object_new().

GInitiallyUnowned is derived from GObject. The only difference between the two is that the initial reference of a GInitiallyUnowned is flagged as a "floating" 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:

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container = create_container ();
container_add_child (container, create_child());

If container_add_child() calls g_object_ref_sink() on the passed-in child, no reference of the newly created child is leaked. Without floating references, container_add_child() can only g_object_ref() the new child, so to implement this code without reference leaks, it would have to be written as:

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Child *child;
container = create_container ();
child = create_child ();
container_add_child (container, child);
g_object_unref (child);

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) should not expose floating references in their API. The best practice for handling types that have initially floating references is to immediately sink those references after g_object_new() returns, by checking if the GType inherits from GInitiallyUnowned. For instance:

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GObject *res = g_object_new_with_properties (gtype,
                                             n_props,
                                             prop_names,
                                             prop_values);

// or: if (g_type_is_a (gtype, G_TYPE_INITIALLY_UNOWNED))
if (G_IS_INITIALLY_UNOWNED (res))
  g_object_ref_sink (res);

return res;

Some object implementations may need to save an objects floating state across certain code portions (an example is GtkMenu), to achieve this, the following sequence can be used:

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// 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);
else
  g_object_unref (object); // release previously acquired reference

GObjectGetPropertyFunc ()

void
(*GObjectGetPropertyFunc) (GObject *object,
                           guint property_id,
                           GValue *value,
                           GParamSpec *pspec);

The type of the get_property function of GObjectClass.

GObjectSetPropertyFunc ()

void
(*GObjectSetPropertyFunc) (GObject *object,
                           guint property_id,
                           const GValue *value,
                           GParamSpec *pspec);

The type of the set_property function of GObjectClass.

GObjectFinalizeFunc ()

void
(*GObjectFinalizeFunc) (GObject *object);

The type of the finalize function of GObjectClass.

G_TYPE_IS_OBJECT()

#define G_TYPE_IS_OBJECT(type)      (G_TYPE_FUNDAMENTAL (type) == G_TYPE_OBJECT)

Check if the passed in type id is a G_TYPE_OBJECT or derived from it.

G_OBJECT()

#define G_OBJECT(object)            (G_TYPE_CHECK_INSTANCE_CAST ((object), G_TYPE_OBJECT, GObject))

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.

G_IS_OBJECT()

#define G_IS_OBJECT(object)         (G_TYPE_CHECK_INSTANCE_FUNDAMENTAL_TYPE ((object), G_TYPE_OBJECT))

Checks whether a valid GTypeInstance pointer is of type G_TYPE_OBJECT.

G_OBJECT_CLASS()

#define G_OBJECT_CLASS(class)       (G_TYPE_CHECK_CLASS_CAST ((class), G_TYPE_OBJECT, GObjectClass))

Casts a derived GObjectClass structure into a GObjectClass structure.

G_IS_OBJECT_CLASS()

#define G_IS_OBJECT_CLASS(class)    (G_TYPE_CHECK_CLASS_TYPE ((class), G_TYPE_OBJECT))

Checks whether class "is a" valid GObjectClass structure of type G_TYPE_OBJECT or derived.

G_OBJECT_GET_CLASS()

#define G_OBJECT_GET_CLASS(object)  (G_TYPE_INSTANCE_GET_CLASS ((object), G_TYPE_OBJECT, GObjectClass))

Get the class structure associated to a GObject instance.

G_OBJECT_TYPE()

#define G_OBJECT_TYPE(object)       (G_TYPE_FROM_INSTANCE (object))

Get the type id of an object.

G_OBJECT_TYPE_NAME()

#define G_OBJECT_TYPE_NAME(object)  (g_type_name (G_OBJECT_TYPE (object)))

Get the name of an object's type.

G_OBJECT_CLASS_TYPE()

#define G_OBJECT_CLASS_TYPE(class)  (G_TYPE_FROM_CLASS (class))

Get the type id of a class structure.

G_OBJECT_CLASS_NAME()

#define G_OBJECT_CLASS_NAME(class)  (g_type_name (G_OBJECT_CLASS_TYPE (class)))

Return the name of a class structure's type.

g_object_class_install_property ()

void
g_object_class_install_property (GObjectClass *oclass,
                                 guint property_id,
                                 GParamSpec *pspec);

Installs a new property.

All properties should be installed during the class initializer. It is possible to install properties after that, but doing so is not recommend, and specifically, is not guaranteed to be thread-safe vs. use of properties on the same type on other threads.

Note that it is possible to redefine a property in a derived class, by installing a property with the same name. This can be useful at times, e.g. to change the range of allowed values or the default value.

g_object_class_install_properties ()

void
g_object_class_install_properties (GObjectClass *oclass,
                                   guint n_pspecs,
                                   GParamSpec **pspecs);

Installs new properties from an array of GParamSpecs.

All properties should be installed during the class initializer. It is possible to install properties after that, but doing so is not recommend, and specifically, is not guaranteed to be thread-safe vs. use of properties on the same type on other threads.

The property id of each property is the index of each GParamSpec in the pspecs array.

The property id of 0 is treated specially by GObject and it should not be used to store a GParamSpec.

This function should be used if you plan to use a static array of GParamSpecs and g_object_notify_by_pspec(). For instance, this class initialization:

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typedef enum {
  PROP_FOO = 1,
  PROP_BAR,
  N_PROPERTIES
} MyObjectProperty;

static GParamSpec *obj_properties[N_PROPERTIES] = { NULL, };

static void
my_object_class_init (MyObjectClass *klass)
{
  GObjectClass *gobject_class = G_OBJECT_CLASS (klass);

  obj_properties[PROP_FOO] =
    g_param_spec_int ("foo", "Foo", "Foo",
                      -1, G_MAXINT,
                      0,
                      G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS);

  obj_properties[PROP_BAR] =
    g_param_spec_string ("bar", "Bar", "Bar",
                         NULL,
                         G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS);

  gobject_class->set_property = my_object_set_property;
  gobject_class->get_property = my_object_get_property;
  g_object_class_install_properties (gobject_class,
                                     G_N_ELEMENTS (obj_properties),
                                     obj_properties);
}

allows calling g_object_notify_by_pspec() to notify of property changes:

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void
my_object_set_foo (MyObject *self, gint foo)
{
  if (self->foo != foo)
    {
      self->foo = foo;
      g_object_notify_by_pspec (G_OBJECT (self), obj_properties[PROP_FOO]);
    }
 }

Since: 2.26

g_object_class_find_property ()

GParamSpec *
g_object_class_find_property (GObjectClass *oclass,
                              const gchar *property_name);

Looks up the GParamSpec for a property of a class.

g_object_class_list_properties ()

GParamSpec **
g_object_class_list_properties (GObjectClass *oclass,
                                guint *n_properties);

Get an array of GParamSpec* for all properties of a class.

g_object_class_override_property ()

void
g_object_class_override_property (GObjectClass *oclass,
                                  guint property_id,
                                  const gchar *name);

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 "override" a property implementation in a parent class or to provide the implementation of a property from an interface.

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().

Since: 2.4

g_object_interface_install_property ()

void
g_object_interface_install_property (gpointer g_iface,
                                     GParamSpec *pspec);

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.

This function is meant to be called from the interface's default vtable initialization function (the class_init member of GTypeInfo.) It must not be called after after class_init has been called for any object types implementing this interface.

If pspec is a floating reference, it will be consumed.

Since: 2.4

g_object_interface_find_property ()

GParamSpec *
g_object_interface_find_property (gpointer g_iface,
                                  const gchar *property_name);

Find the GParamSpec with the given name for an interface. Generally, the interface vtable passed in as g_iface will be the default vtable from g_type_default_interface_ref(), or, if you know the interface has already been loaded, g_type_default_interface_peek().

Since: 2.4

g_object_interface_list_properties ()

GParamSpec **
g_object_interface_list_properties (gpointer g_iface,
                                    guint *n_properties_p);

Lists the properties of an interface.Generally, the interface vtable passed in as g_iface will be the default vtable from g_type_default_interface_ref(), or, if you know the interface has already been loaded, g_type_default_interface_peek().

Since: 2.4

g_object_new ()

gpointer
g_object_new (GType object_type,
              const gchar *first_property_name,
              ...);

Creates a new instance of a GObject subtype and sets its properties.

Construction parameters (see G_PARAM_CONSTRUCT, G_PARAM_CONSTRUCT_ONLY) which are not explicitly specified are set to their default values. Any private data for the object is guaranteed to be initialized with zeros, as per g_type_create_instance().

Note that in C, small integer types in variable argument lists are promoted up to gint or guint as appropriate, and read back accordingly. gint is 32 bits on every platform on which GLib is currently supported. This means that you can use C expressions of type gint with g_object_new() and properties of type gint or guint or smaller. Specifically, you can use integer literals with these property types.

When using property types of gint64 or guint64, you must ensure that the value that you provide is 64 bit. This means that you should use a cast or make use of the G_GINT64_CONSTANT or G_GUINT64_CONSTANT macros.

Similarly, gfloat is promoted to gdouble, so you must ensure that the value you provide is a gdouble, even for a property of type gfloat.

Since GLib 2.72, all GObjects are guaranteed to be aligned to at least the alignment of the largest basic GLib type (typically this is guint64 or gdouble). If you need larger alignment for an element in a GObject, you should allocate it on the heap (aligned), or arrange for your GObject to be appropriately padded.

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g_object_new_with_properties ()

GObject *
g_object_new_with_properties (GType object_type,
                              guint n_properties,
                              const char *names[],
                              const GValue values[]);

Creates a new instance of a GObject subtype and sets its properties using the provided arrays. Both arrays must have exactly n_properties elements, and the names and values correspond by index.

Construction parameters (see G_PARAM_CONSTRUCT, G_PARAM_CONSTRUCT_ONLY) which are not explicitly specified are set to their default values.

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Since: 2.54

g_object_newv ()

gpointer
g_object_newv (GType object_type,
               guint n_parameters,
               GParameter *parameters);

Creates a new instance of a GObject subtype and sets its properties.

Construction parameters (see G_PARAM_CONSTRUCT, G_PARAM_CONSTRUCT_ONLY) which are not explicitly specified are set to their default values.

g_object_ref ()

gpointer
g_object_ref (gpointer object);

Increases the reference count of object .

Since GLib 2.56, if GLIB_VERSION_MAX_ALLOWED is 2.56 or greater, the type of object will be propagated to the return type (using the GCC typeof() extension), so any casting the caller needs to do on the return type must be explicit.

g_object_unref ()

void
g_object_unref (gpointer object);

Decreases the reference count of object . When its reference count drops to 0, the object is finalized (i.e. its memory is freed).

If the pointer to the GObject may be reused in future (for example, if it is an instance variable of another object), it is recommended to clear the pointer to NULL rather than retain a dangling pointer to a potentially invalid GObject instance. Use g_clear_object() for this.

g_object_ref_sink ()

gpointer
g_object_ref_sink (gpointer object);

Increase the reference count of object , and possibly remove the floating reference, if object has a floating reference.

In other words, if the object is floating, then this call "assumes ownership" of the floating reference, converting it to a normal reference by clearing the floating flag while leaving the reference count unchanged. If the object is not floating, then this call adds a new normal reference increasing the reference count by one.

Since GLib 2.56, the type of object will be propagated to the return type under the same conditions as for g_object_ref().

Since: 2.10

g_object_take_ref ()

gpointer
g_object_take_ref (gpointer object);

If object is floating, sink it. Otherwise, do nothing.

In other words, this function will convert a floating reference (if present) into a full reference.

Typically you want to use g_object_ref_sink() in order to automatically do the correct thing with respect to floating or non-floating references, but there is one specific scenario where this function is helpful.

The situation where this function is helpful is when creating an API that allows the user to provide a callback function that returns a GObject. We certainly want to allow the user the flexibility to return a non-floating reference from this callback (for the case where the object that is being returned already exists).

At the same time, the API style of some popular GObject-based libraries (such as Gtk) make it likely that for newly-created GObject instances, the user can be saved some typing if they are allowed to return a floating reference.

Using this function on the return value of the user's callback allows the user to do whichever is more convenient for them. The caller will alway receives exactly one full reference to the value: either the one that was returned in the first place, or a floating reference that has been converted to a full reference.

This function has an odd interaction when combined with g_object_ref_sink() running at the same time in another thread on the same GObject instance. If g_object_ref_sink() runs first then the result will be that the floating reference is converted to a hard reference. If g_object_take_ref() runs first then the result will be that the floating reference is converted to a hard reference and an additional reference on top of that one is added. It is best to avoid this situation.

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Since: 2.70

g_set_object ()

gboolean
g_set_object (GObject **object_ptr,
              GObject *new_object);

Updates a GObject pointer to refer to new_object .

It increments the reference count of new_object (if non-NULL), decrements the reference count of the current value of object_ptr (if non-NULL), and assigns new_object to object_ptr . The assignment is not atomic.

object_ptr must not be NULL, but can point to a NULL value.

A macro is also included that allows this function to be used without pointer casts. The function itself is static inline, so its address may vary between compilation units.

One convenient usage of this function is in implementing property setters:

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void
foo_set_bar (Foo *foo,
             Bar *new_bar)
{
  g_return_if_fail (IS_FOO (foo));
  g_return_if_fail (new_bar == NULL || IS_BAR (new_bar));

  if (g_set_object (&foo->bar, new_bar))
    g_object_notify (foo, "bar");
}

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Since: 2.44

g_clear_object ()

void
g_clear_object (GObject **object_ptr);

Clears a reference to a GObject.

object_ptr must not be NULL.

If the reference is NULL then this function does nothing. Otherwise, the reference count of the object is decreased and the pointer is set to NULL.

A macro is also included that allows this function to be used without pointer casts.

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Since: 2.28

g_object_is_floating ()

gboolean
g_object_is_floating (gpointer object);

Checks whether object has a floating reference.

Since: 2.10

g_object_force_floating ()

void
g_object_force_floating (GObject *object);

This function is intended for GObject implementations to re-enforce a floating object reference. Doing this is seldom required: all GInitiallyUnowneds are created with a floating reference which usually just needs to be sunken by calling g_object_ref_sink().

Since: 2.10

GWeakNotify ()

void
(*GWeakNotify) (gpointer data,
                GObject *where_the_object_was);

A GWeakNotify function can be added to an object as a callback that gets triggered when the object is finalized.

Since the object is already being disposed when the GWeakNotify is called, there's not much you could do with the object, apart from e.g. using its address as hash-index or the like.

In particular, this means it’s invalid to call g_object_ref(), g_weak_ref_init(), g_weak_ref_set(), g_object_add_toggle_ref(), g_object_weak_ref(), g_object_add_weak_pointer() or any function which calls them on the object from this callback.

g_object_weak_ref ()

void
g_object_weak_ref (GObject *object,
                   GWeakNotify notify,
                   gpointer data);

Adds a weak reference callback to an object. Weak references are used for notification when an object is disposed. They are called "weak references" because they allow you to safely hold a pointer to an object without calling g_object_ref() (g_object_ref() adds a strong reference, that is, forces the object to stay alive).

Note that the weak references created by this method are not thread-safe: they cannot safely be used in one thread if the object's last g_object_unref() might happen in another thread. Use GWeakRef if thread-safety is required.

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g_object_weak_unref ()

void
g_object_weak_unref (GObject *object,
                     GWeakNotify notify,
                     gpointer data);

Removes a weak reference callback to an object.

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g_object_add_weak_pointer ()

void
g_object_add_weak_pointer (GObject *object,
                           gpointer *weak_pointer_location);

Adds a weak reference from weak_pointer to object to indicate that the pointer located at weak_pointer_location is only valid during the lifetime of object . When the object is finalized, weak_pointer will be set to NULL.

Note that as with g_object_weak_ref(), the weak references created by this method are not thread-safe: they cannot safely be used in one thread if the object's last g_object_unref() might happen in another thread. Use GWeakRef if thread-safety is required.

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g_object_remove_weak_pointer ()

void
g_object_remove_weak_pointer (GObject *object,
                              gpointer *weak_pointer_location);

Removes a weak reference from object that was previously added using g_object_add_weak_pointer(). The weak_pointer_location has to match the one used with g_object_add_weak_pointer().

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g_set_weak_pointer ()

gboolean
g_set_weak_pointer (gpointer *weak_pointer_location,
                    GObject *new_object);

Updates a pointer to weakly refer to new_object .

It assigns new_object to weak_pointer_location and ensures that weak_pointer_location will automatically be set to NULL if new_object gets destroyed. The assignment is not atomic. The weak reference is not thread-safe, see g_object_add_weak_pointer() for details.

The weak_pointer_location argument must not be NULL.

A macro is also included that allows this function to be used without pointer casts. The function itself is static inline, so its address may vary between compilation units.

One convenient usage of this function is in implementing property setters:

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void
foo_set_bar (Foo *foo,
             Bar *new_bar)
{
  g_return_if_fail (IS_FOO (foo));
  g_return_if_fail (new_bar == NULL || IS_BAR (new_bar));

  if (g_set_weak_pointer (&foo->bar, new_bar))
    g_object_notify (foo, "bar");
}

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Since: 2.56

g_clear_weak_pointer ()

void
g_clear_weak_pointer (gpointer *weak_pointer_location);

Clears a weak reference to a GObject.

weak_pointer_location must not be NULL.

If the weak reference is NULL then this function does nothing. Otherwise, the weak reference to the object is removed for that location and the pointer is set to NULL.

A macro is also included that allows this function to be used without pointer casts. The function itself is static inline, so its address may vary between compilation units.

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Since: 2.56

GToggleNotify ()

void
(*GToggleNotify) (gpointer data,
                  GObject *object,
                  gboolean is_last_ref);

A callback function used for notification when the state of a toggle reference changes.

See also: g_object_add_toggle_ref()

g_object_add_toggle_ref ()

void
g_object_add_toggle_ref (GObject *object,
                         GToggleNotify notify,
                         gpointer data);

Increases the reference count of the object by one and sets a callback to be called when all other references to the object are dropped, or when this is already the last reference to the object and another reference is established.

This functionality is intended for binding object to a proxy object managed by another memory manager. This is done with two paired references: the strong reference added by g_object_add_toggle_ref() and a reverse reference to the proxy object which is either a strong reference or weak reference.

The setup is that when there are no other references to object , only a weak reference is held in the reverse direction from object to the proxy object, but when there are other references held to object , a strong reference is held. The notify callback is called when the reference from object to the proxy object should be "toggled" from strong to weak (is_last_ref true) or weak to strong (is_last_ref false).

Since a (normal) reference must be held to the object before calling g_object_add_toggle_ref(), the initial state of the reverse link is always strong.

Multiple toggle references may be added to the same gobject, however if there are multiple toggle references to an object, none of them will ever be notified until all but one are removed. For this reason, you should only ever use a toggle reference if there is important state in the proxy object.

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Since: 2.8

g_object_remove_toggle_ref ()

void
g_object_remove_toggle_ref (GObject *object,
                            GToggleNotify notify,
                            gpointer data);

Removes a reference added with g_object_add_toggle_ref(). The reference count of the object is decreased by one.

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Since: 2.8

g_object_connect ()

gpointer
g_object_connect (gpointer object,
                  const gchar *signal_spec,
                  ...);

A convenience function to connect multiple signals at once.

The signal specs expected by this function have the form "modifier::signal_name", where modifier can be one of the following:

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menu->toplevel = g_object_connect (g_object_new (GTK_TYPE_WINDOW,
						   "type", GTK_WINDOW_POPUP,
						   "child", menu,
						   NULL),
				     "signal::event", gtk_menu_window_event, menu,
				     "signal::size_request", gtk_menu_window_size_request, menu,
				     "signal::destroy", gtk_widget_destroyed, &menu->toplevel,
				     NULL);

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g_object_disconnect ()

void
g_object_disconnect (gpointer object,
                     const gchar *signal_spec,
                     ...);

A convenience function to disconnect multiple signals at once.

The signal specs expected by this function have the form "any_signal", which means to disconnect any signal with matching callback and data, or "any_signal::signal_name", which only disconnects the signal named "signal_name".

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g_object_set ()

void
g_object_set (gpointer object,
              const gchar *first_property_name,
              ...);

Sets properties on an object.

The same caveats about passing integer literals as varargs apply as with g_object_new(). In particular, any integer literals set as the values for properties of type gint64 or guint64 must be 64 bits wide, using the G_GINT64_CONSTANT or G_GUINT64_CONSTANT macros.

Note that the "notify" signals are queued and only emitted (in reverse order) after all properties have been set. See g_object_freeze_notify().

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g_object_setv ()

void
g_object_setv (GObject *object,
               guint n_properties,
               const gchar *names[],
               const GValue values[]);

Sets n_properties properties for an object . Properties to be set will be taken from values . All properties must be valid. Warnings will be emitted and undefined behaviour may result if invalid properties are passed in.

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Since: 2.54

g_object_get ()

void
g_object_get (gpointer object,
              const gchar *first_property_name,
              ...);

Gets properties of an object.

In general, a copy is made of the property contents and the caller is responsible for freeing the memory in the appropriate manner for the type, for instance by calling g_free() or g_object_unref().

Here is an example of using g_object_get() to get the contents of three properties: an integer, a string and an object:

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gint intval;
guint64 uint64val;
gchar *strval;
GObject *objval;

g_object_get (my_object,
              "int-property", &intval,
              "uint64-property", &uint64val,
              "str-property", &strval,
              "obj-property", &objval,
              NULL);

// Do something with intval, uint64val, strval, objval

g_free (strval);
g_object_unref (objval);

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g_object_getv ()

void
g_object_getv (GObject *object,
               guint n_properties,
               const gchar *names[],
               GValue values[]);

Gets n_properties properties for an object . Obtained properties will be set to values . All properties must be valid. Warnings will be emitted and undefined behaviour may result if invalid properties are passed in.

Since: 2.54

g_object_notify ()

void
g_object_notify (GObject *object,
                 const gchar *property_name);

Emits a "notify" signal for the property property_name on object .

When possible, eg. when signaling a property change from within the class that registered the property, you should use g_object_notify_by_pspec() instead.

Note that emission of the notify signal may be blocked with g_object_freeze_notify(). In this case, the signal emissions are queued and will be emitted (in reverse order) when g_object_thaw_notify() is called.

g_object_notify_by_pspec ()

void
g_object_notify_by_pspec (GObject *object,
                          GParamSpec *pspec);

Emits a "notify" signal for the property specified by pspec on object .

This function omits the property name lookup, hence it is faster than g_object_notify().

One way to avoid using g_object_notify() from within the class that registered the properties, and using g_object_notify_by_pspec() instead, is to store the GParamSpec used with g_object_class_install_property() inside a static array, e.g.:

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typedef enum
{
  PROP_FOO = 1,
  PROP_LAST
} MyObjectProperty;

static GParamSpec *properties[PROP_LAST];

static void
my_object_class_init (MyObjectClass *klass)
{
  properties[PROP_FOO] = g_param_spec_int ("foo", "Foo", "The foo",
                                           0, 100,
                                           50,
                                           G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS);
  g_object_class_install_property (gobject_class,
                                   PROP_FOO,
                                   properties[PROP_FOO]);
}

and then notify a change on the "foo" property with:

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g_object_notify_by_pspec (self, properties[PROP_FOO]);

Since: 2.26

g_object_freeze_notify ()

void
g_object_freeze_notify (GObject *object);

Increases the freeze count on object . If the freeze count is non-zero, the emission of "notify" signals on object is stopped. The signals are queued until the freeze count is decreased to zero. Duplicate notifications are squashed so that at most one “notify” signal is emitted for each property modified while the object is frozen.

This is necessary for accessors that modify multiple properties to prevent premature notification while the object is still being modified.

g_object_thaw_notify ()

void
g_object_thaw_notify (GObject *object);

Reverts the effect of a previous call to g_object_freeze_notify(). The freeze count is decreased on object and when it reaches zero, queued "notify" signals are emitted.

Duplicate notifications for each property are squashed so that at most one “notify” signal is emitted for each property, in the reverse order in which they have been queued.

It is an error to call this function when the freeze count is zero.

g_object_get_data ()

gpointer
g_object_get_data (GObject *object,
                   const gchar *key);

Gets a named field from the objects table of associations (see g_object_set_data()).

g_object_set_data ()

void
g_object_set_data (GObject *object,
                   const gchar *key,
                   gpointer data);

Each object carries around a table of associations from strings to pointers. This function lets you set an association.

If the object already had an association with that name, the old association will be destroyed.

Internally, the key is converted to a GQuark using g_quark_from_string(). This means a copy of key is kept permanently (even after object has been finalized) — so it is recommended to only use a small, bounded set of values for key in your program, to avoid the GQuark storage growing unbounded.

g_object_set_data_full ()

void
g_object_set_data_full (GObject *object,
                        const gchar *key,
                        gpointer data,
                        GDestroyNotify destroy);

Like g_object_set_data() except it adds notification for when the association is destroyed, either by setting it to a different value or when the object is destroyed.

Note that the destroy callback is not called if data is NULL.

[skip]

g_object_steal_data ()

gpointer
g_object_steal_data (GObject *object,
                     const gchar *key);

Remove a specified datum from the object's data associations, without invoking the association's destroy handler.

g_object_dup_data ()

gpointer
g_object_dup_data (GObject *object,
                   const gchar *key,
                   GDuplicateFunc dup_func,
                   gpointer user_data);

This is a variant of g_object_get_data() which returns a 'duplicate' of the value. dup_func defines the meaning of 'duplicate' in this context, it could e.g. take a reference on a ref-counted object.

If the key is not set on the object then dup_func will be called with a NULL argument.

Note that dup_func is called while user data of object is locked.

This function can be useful to avoid races when multiple threads are using object data on the same key on the same object.

[skip]

Since: 2.34

g_object_replace_data ()

gboolean
g_object_replace_data (GObject *object,
                       const gchar *key,
                       gpointer oldval,
                       gpointer newval,
                       GDestroyNotify destroy,
                       GDestroyNotify *old_destroy);

Compares the user data for the key key on object with oldval , and if they are the same, replaces oldval with newval .

This is like a typical atomic compare-and-exchange operation, for user data on an object.

If the previous value was replaced then ownership of the old value (oldval ) is passed to the caller, including the registered destroy notify for it (passed out in old_destroy ). It’s up to the caller to free this as needed, which may or may not include using old_destroy as sometimes replacement should not destroy the object in the normal way.

See g_object_set_data() for guidance on using a small, bounded set of values for key .

[skip]

Since: 2.34

g_object_get_qdata ()

gpointer
g_object_get_qdata (GObject *object,
                    GQuark quark);

This function gets back user data pointers stored via g_object_set_qdata().

g_object_set_qdata ()

void
g_object_set_qdata (GObject *object,
                    GQuark quark,
                    gpointer data);

This sets an opaque, named pointer on an object. The name is specified through a GQuark (retrieved e.g. via g_quark_from_static_string()), and the pointer can be gotten back from the object with g_object_get_qdata() until the object is finalized. Setting a previously set user data pointer, overrides (frees) the old pointer set, using NULL as pointer essentially removes the data stored.

[skip]

g_object_set_qdata_full ()

void
g_object_set_qdata_full (GObject *object,
                         GQuark quark,
                         gpointer data,
                         GDestroyNotify destroy);

This function works like g_object_set_qdata(), but in addition, a void (*destroy) (gpointer) function may be specified which is called with data as argument when the object is finalized, or the data is being overwritten by a call to g_object_set_qdata() with the same quark .

[skip]

g_object_steal_qdata ()

gpointer
g_object_steal_qdata (GObject *object,
                      GQuark quark);

This function gets back user data pointers stored via g_object_set_qdata() and removes the data from object without invoking its destroy() function (if any was set). Usually, calling this function is only required to update user data pointers with a destroy notifier, for example:

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void
object_add_to_user_list (GObject     *object,
                         const gchar *new_string)
{
  // the quark, naming the object data
  GQuark quark_string_list = g_quark_from_static_string ("my-string-list");
  // retrieve the old string list
  GList *list = g_object_steal_qdata (object, quark_string_list);

  // prepend new string
  list = g_list_prepend (list, g_strdup (new_string));
  // this changed 'list', so we need to set it again
  g_object_set_qdata_full (object, quark_string_list, list, free_string_list);
}
static void
free_string_list (gpointer data)
{
  GList *node, *list = data;

  for (node = list; node; node = node->next)
    g_free (node->data);
  g_list_free (list);
}

Using g_object_get_qdata() in the above example, instead of g_object_steal_qdata() would have left the destroy function set, and thus the partial string list would have been freed upon g_object_set_qdata_full().

g_object_dup_qdata ()

gpointer
g_object_dup_qdata (GObject *object,
                    GQuark quark,
                    GDuplicateFunc dup_func,
                    gpointer user_data);

This is a variant of g_object_get_qdata() which returns a 'duplicate' of the value. dup_func defines the meaning of 'duplicate' in this context, it could e.g. take a reference on a ref-counted object.

If the quark is not set on the object then dup_func will be called with a NULL argument.

Note that dup_func is called while user data of object is locked.

This function can be useful to avoid races when multiple threads are using object data on the same key on the same object.

[skip]

Since: 2.34

g_object_replace_qdata ()

gboolean
g_object_replace_qdata (GObject *object,
                        GQuark quark,
                        gpointer oldval,
                        gpointer newval,
                        GDestroyNotify destroy,
                        GDestroyNotify *old_destroy);

Compares the user data for the key quark on object with oldval , and if they are the same, replaces oldval with newval .

This is like a typical atomic compare-and-exchange operation, for user data on an object.

If the previous value was replaced then ownership of the old value (oldval ) is passed to the caller, including the registered destroy notify for it (passed out in old_destroy ). It’s up to the caller to free this as needed, which may or may not include using old_destroy as sometimes replacement should not destroy the object in the normal way.

[skip]

Since: 2.34

g_object_set_property ()

void
g_object_set_property (GObject *object,
                       const gchar *property_name,
                       const GValue *value);

Sets a property on an object.

g_object_get_property ()

void
g_object_get_property (GObject *object,
                       const gchar *property_name,
                       GValue *value);

Gets a property of an object.

The value can be:

In general, a copy is made of the property contents and the caller is responsible for freeing the memory by calling g_value_unset().

Note that g_object_get_property() is really intended for language bindings, g_object_get() is much more convenient for C programming.

g_object_new_valist ()

GObject *
g_object_new_valist (GType object_type,
                     const gchar *first_property_name,
                     va_list var_args);

Creates a new instance of a GObject subtype and sets its properties.

Construction parameters (see G_PARAM_CONSTRUCT, G_PARAM_CONSTRUCT_ONLY) which are not explicitly specified are set to their default values.

[skip]

g_object_set_valist ()

void
g_object_set_valist (GObject *object,
                     const gchar *first_property_name,
                     va_list var_args);

Sets properties on an object.

[skip]

g_object_get_valist ()

void
g_object_get_valist (GObject *object,
                     const gchar *first_property_name,
                     va_list var_args);

Gets properties of an object.

In general, a copy is made of the property contents and the caller is responsible for freeing the memory in the appropriate manner for the type, for instance by calling g_free() or g_object_unref().

See g_object_get().

[skip]

g_object_watch_closure ()

void
g_object_watch_closure (GObject *object,
                        GClosure *closure);

This function essentially limits the life time of the closure to the life time of the object. That is, when the object is finalized, the closure is invalidated by calling g_closure_invalidate() on it, in order to prevent invocations of the closure with a finalized (nonexisting) object. Also, g_object_ref() and g_object_unref() are added as marshal guards to the closure , to ensure that an extra reference count is held on object during invocation of the closure . Usually, this function will be called on closures that use this object as closure data.

g_object_run_dispose ()

void
g_object_run_dispose (GObject *object);

Releases all references to other objects. This can be used to break reference cycles.

This function should only be called from object system implementations.

G_OBJECT_WARN_INVALID_PROPERTY_ID()

#define             G_OBJECT_WARN_INVALID_PROPERTY_ID(object, property_id, pspec)

This macro should be used to emit a standard warning about unexpected properties in set_property() and get_property() implementations.

g_weak_ref_init ()

void
g_weak_ref_init (GWeakRef *weak_ref,
                 gpointer object);

Initialise a non-statically-allocated GWeakRef.

This function also calls g_weak_ref_set() with object on the freshly-initialised weak reference.

This function should always be matched with a call to g_weak_ref_clear(). It is not necessary to use this function for a GWeakRef in static storage because it will already be properly initialised. Just use g_weak_ref_set() directly.

[skip]

Since: 2.32

g_weak_ref_clear ()

void
g_weak_ref_clear (GWeakRef *weak_ref);

Frees resources associated with a non-statically-allocated GWeakRef. After this call, the GWeakRef is left in an undefined state.

You should only call this on a GWeakRef that previously had g_weak_ref_init() called on it.

[skip]

Since: 2.32

g_weak_ref_get ()

gpointer
g_weak_ref_get (GWeakRef *weak_ref);

If weak_ref is not empty, atomically acquire a strong reference to the object it points to, and return that reference.

This function is needed because of the potential race between taking the pointer value and g_object_ref() on it, if the object was losing its last reference at the same time in a different thread.

The caller should release the resulting reference in the usual way, by using g_object_unref().

[skip]

Since: 2.32

g_weak_ref_set ()

void
g_weak_ref_set (GWeakRef *weak_ref,
                gpointer object);

Change the object to which weak_ref points, or set it to NULL.

You must own a strong reference on object while calling this function.

[skip]

Since: 2.32

g_assert_finalize_object ()

void
g_assert_finalize_object (GObject *object);

Assert that object is non-NULL, then release one reference to it with g_object_unref() and assert that it has been finalized (i.e. that there are no more references).

If assertions are disabled via G_DISABLE_ASSERT, this macro just calls g_object_unref() without any further checks.

This macro should only be used in regression tests.

[skip]

Since: 2.62

struct GObject

struct GObject;

The base object type.

All the fields in the GObject structure are private to the implementation and should never be accessed directly.

Since GLib 2.72, all GObjects are guaranteed to be aligned to at least the alignment of the largest basic GLib type (typically this is guint64 or gdouble). If you need larger alignment for an element in a GObject, you should allocate it on the heap (aligned), or arrange for your GObject to be appropriately padded. This guarantee applies to the GObject (or derived) struct, the GObjectClass (or derived) struct, and any private data allocated by G_ADD_PRIVATE().

struct GObjectClass

struct GObjectClass {
  GTypeClass   g_type_class;

  /* seldom overridden */
  GObject*   (*constructor)     (GType                  type,
                                 guint                  n_construct_properties,
                                 GObjectConstructParam *construct_properties);
  /* overridable methods */
  void       (*set_property)		(GObject        *object,
                                         guint           property_id,
                                         const GValue   *value,
                                         GParamSpec     *pspec);
  void       (*get_property)		(GObject        *object,
                                         guint           property_id,
                                         GValue         *value,
                                         GParamSpec     *pspec);
  void       (*dispose)			(GObject        *object);
  void       (*finalize)		(GObject        *object);
  /* seldom overridden */
  void       (*dispatch_properties_changed) (GObject      *object,
					     guint	   n_pspecs,
					     GParamSpec  **pspecs);
  /* signals */
  void	     (*notify)			(GObject *object,
					 GParamSpec *pspec);

  /* called when done constructing */
  void	     (*constructed)		(GObject *object);
};

The class structure for the GObject type.

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// Example of implementing a singleton using a constructor.
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;
}

struct GObjectConstructParam

struct GObjectConstructParam {
  GParamSpec *pspec;
  GValue     *value;
};

The GObjectConstructParam struct is an auxiliary structure used to hand GParamSpec/GValue pairs to the constructor of a GObjectClass.

struct GParameter

struct GParameter {
  const gchar *name;
  GValue       value;
};

The GParameter struct is an auxiliary structure used to hand parameter name/value pairs to g_object_newv().

GInitiallyUnowned

typedef struct _GObject                  GInitiallyUnowned;

A type for objects that have an initially floating reference.

All the fields in the GInitiallyUnowned structure are private to the implementation and should never be accessed directly.

GInitiallyUnownedClass

typedef struct _GObjectClass             GInitiallyUnownedClass;

The class structure for the GInitiallyUnowned type.

G_TYPE_INITIALLY_UNOWNED

#define G_TYPE_INITIALLY_UNOWNED	      (g_initially_unowned_get_type())

The type for GInitiallyUnowned.

GWeakRef

typedef struct {
} GWeakRef;

A structure containing a weak reference to a GObject.

A GWeakRef can either be empty (i.e. point to NULL), or point to an object for as long as at least one "strong" reference to that object exists. Before the object's GObjectClass.dispose method is called, every GWeakRef associated with becomes empty (i.e. points to NULL).

Like GValue, GWeakRef can be statically allocated, stack- or heap-allocated, or embedded in larger structures.

Unlike g_object_weak_ref() and g_object_add_weak_pointer(), this weak reference is thread-safe: converting a weak pointer to a reference is atomic with respect to invalidation of weak pointers to destroyed objects.

If the object's GObjectClass.dispose method results in additional references to the object being held (‘re-referencing’), any GWeakRefs taken before it was disposed will continue to point to NULL. Any GWeakRefs taken during disposal and after re-referencing, or after disposal has returned due to the re-referencing, will continue to point to the object until its refcount goes back to zero, at which point they too will be invalidated.

It is invalid to take a GWeakRef on an object during GObjectClass.dispose without first having or creating a strong reference to the object.

The “notify” signal

void
user_function (GObject    *gobject,
               GParamSpec *pspec,
               gpointer    user_data)

The notify signal is emitted on an object when one of its properties has its value set through g_object_set_property(), g_object_set(), et al.

Note that getting this signal doesn’t itself guarantee that the value of the property has actually changed. When it is emitted is determined by the derived GObject class. If the implementor did not create the property with G_PARAM_EXPLICIT_NOTIFY, then any call to g_object_set_property() results in ::notify being emitted, even if the new value is the same as the old. If they did pass G_PARAM_EXPLICIT_NOTIFY, then this signal is emitted only when they explicitly call g_object_notify() or g_object_notify_by_pspec(), and common practice is to do that only when the value has actually changed.

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:

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g_signal_connect (text_view->buffer, "notify::paste-target-list",
                  G_CALLBACK (gtk_text_view_target_list_notify),
                  text_view)

It is important to note that you must use canonical parameter names as detail strings for the notify signal.

Flags: No Hooks