glib/docs/reference/gobject/tut_gobject.xml

<?xml version='1.0' encoding="ISO-8859-1"?>
<chapter id="chapter-gobject">
  <title>The GObject base class</title>

  <para>
    The two previous chapters discussed the details of GLib's Dynamic Type System
    and its signal control system. The GObject library also contains an implementation
    for a base fundamental type named <type><link linkend="GObject">GObject</link></type>.
  </para>

  <para>
    <type><link linkend="GObject">GObject</link></type> is a fundamental classed instantiable type. It implements:
    <itemizedlist>
      <listitem><para>Memory management with reference counting</para></listitem>
      <listitem><para>Construction/Destruction of instances</para></listitem>
      <listitem><para>Generic per-object properties with set/get function pairs</para></listitem>
      <listitem><para>Easy use of signals</para></listitem>
    </itemizedlist>
    All the GNOME libraries which use the GLib type system (like GTK+ and GStreamer)
    inherit from <type><link linkend="GObject">GObject</link></type> which is why it is important to understand
    the details of how it works.
  </para>

  <sect1 id="gobject-instantiation">
    <title>Object instantiation</title>

    <para>
      The <function><link linkend="g-object-new">g_object_new</link></function> family of functions can be used to instantiate any
      GType which inherits from the GObject base type. All these functions make sure the class
      and instance structures have been correctly initialized by glib's type system and
      then invoke at one point or another the constructor class method which is used to:
      <itemizedlist>
        <listitem><para>
            Allocate and clear memory through <function><link linkend="g-type-create-instance">g_type_create_instance</link></function>,
          </para></listitem>
        <listitem><para>
            Initialize the object' instance with the construction properties.
          </para></listitem>
      </itemizedlist>
     Although one can expect all class and instance members (except the fields
     pointing to the parents) to be set to zero, some consider it good practice to explicitly set them.
    </para>

    <para>
      Objects which inherit from GObject are allowed to override this constructor class method:
      they should however chain to their parent constructor method before doing so:
<programlisting>
  GObject*   (*constructor)     (GType                  type,
                                 guint                  n_construct_properties,
                                 GObjectConstructParam *construct_properties);
</programlisting>
    </para>

    <para>
      The example below shows how <type>MamanBar</type> overrides the parent's constructor:
<programlisting>
#define MAMAN_TYPE_BAR                  (maman_bar_get_type ())
#define MAMAN_BAR(obj)                  (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_BAR, MamanBar))
#define MAMAN_BAR_CLASS(klass)          (G_TYPE_CHECK_CLASS_CAST ((klass), MAMAN_TYPE_BAR, MamanBarClass))
#define MAMAN_IS_BAR(obj)          (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_BAR))
#define MAMAN_IS_BAR_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), MAMAN_TYPE_BAR))
#define MAMAN_BAR_GET_CLASS(obj)  (G_TYPE_INSTANCE_GET_CLASS ((obj), MAMAN_TYPE_BAR, MamanBarClass))

typedef struct _MamanBar MamanBar;
typedef struct _MamanBarClass MamanBarClass;

struct _MamanBar {
  GObject parent;
  /* instance members */
};

struct _MamanBarClass {
  GObjectClass parent;

  /* class members */
};

/* used by MAMAN_TYPE_BAR */
GType maman_bar_get_type (void);

static GObject *
maman_bar_constructor (GType                  type,
                       guint                  n_construct_properties,
                       GObjectConstructParam *construct_properties)
{
  GObject *obj;

  {
    /* Invoke parent constructor. */
    MamanBarClass *klass;
    GObjectClass *parent_class;  
    klass = MAMAN_BAR_CLASS (g_type_class_peek (MAMAN_TYPE_BAR));
    parent_class = G_OBJECT_CLASS (g_type_class_peek_parent (klass));
    obj = parent_class->constructor (type,
                                     n_construct_properties,
                                     construct_properties);
  }
  
  /* do stuff. */

  return obj;
}

static void
maman_bar_instance_init (GTypeInstance   *instance,
                         gpointer         g_class)
{
  MamanBar *self = (MamanBar *)instance;
  /* do stuff */
}

static void
maman_bar_class_init (gpointer g_class,
                      gpointer g_class_data)
{
  GObjectClass *gobject_class = G_OBJECT_CLASS (g_class);
  MamanBarClass *klass = MAMAN_BAR_CLASS (g_class);

  gobject_class->constructor = maman_bar_constructor;
}

GType maman_bar_get_type (void)
{
  static GType type = 0;
  if (type == 0) {
    static const GTypeInfo info = {
      sizeof (MamanBarClass),
      NULL,   /* base_init */
      NULL,   /* base_finalize */
      maman_bar_class_init,   /* class_init */
      NULL,   /* class_finalize */
      NULL,   /* class_data */
      sizeof (MamanBar),
      0,      /* n_preallocs */
      maman_bar_instance_init    /* instance_init */
    };
    type = g_type_register_static (G_TYPE_OBJECT,
                                   "MamanBarType",
                                   &amp;info, 0);
  }
  return type;
}
</programlisting>
      If the user instantiates an object <type>MamanBar</type> with:
<programlisting>
MamanBar *bar = g_object_new (MAMAN_TYPE_BAR, NULL);
</programlisting>        
      If this is the first instantiation of such an object, the <function>maman_b_class_init</function>
      function will be invoked after any <function>maman_b_base_class_init</function> function. 
      This will make sure the class structure of this new object is correctly initialized. Here, 
      <function>maman_bar_class_init</function> is expected to override the object's class methods
      and setup the class' own methods. In the example above, the constructor method is the only 
      overridden method: it is set to <function>maman_bar_constructor</function>.
    </para>

    <para>
      Once <function><link linkend="g-object-new">g_object_new</link></function> has obtained a reference to an initialized
      class structure, it invokes its constructor method to create an instance of the new 
      object. Since it has just been overridden by <function>maman_bar_class_init</function> 
      to <function>maman_bar_constructor</function>, the latter is called and, because it
      was implemented correctly, it chains up to its parent's constructor. The problem here
      is how we can find the parent constructor. An approach (used in GTK+ source code) would be
      to save the original constructor in a static variable from <function>maman_bar_class_init</function>
      and then to re-use it from <function>maman_bar_constructor</function>. This is clearly possible
      and very simple but I was told it was not nice and the preferred way is to use the 
      <function><link linkend="g-type-class-peek">g_type_class_peek</link></function> and <function><link linkend="g-type-class-peek-parent">g_type_class_peek_parent</link></function> functions.
    </para>

    <para>
      Finally, at one point or another, <function>g_object_constructor</function> is invoked
      by the last constructor in the chain. This function allocates the object's instance' buffer 
      through <function><link linkend="g-type-create-instance">g_type_create_instance</link></function>
      which means that the instance_init function is invoked at this point if one
      was registered. After instance_init returns, the object is fully initialized and should be 
      ready to answer any user-request. When <function><link linkend="g-type-create-instance">g_type_create_instance</link></function>
      returns, <function>g_object_constructor</function> sets the construction properties
      (i.e. the properties which were given to <function><link linkend="g-object-new">g_object_new</link></function>) and returns
      to the user's constructor which is then allowed to do useful instance initialization...
    </para>

    <para>
      The process described above might seem a bit complicated (it <emphasis>is</emphasis> actually
      overly complicated in my opinion..) but it can be summarized easily by the table below which
      lists the functions invoked by <function><link linkend="g-object-new">g_object_new</link></function> and their order of
      invocation.
    </para>

    <para>
      The array below lists the functions invoked by <function><link linkend="g-object-new">g_object_new</link></function> and 
      their order of invocation:

      <table id="gobject-construction-table">
        <title><function><link linkend="g-object-new">g_object_new</link></function></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 morerows="3">First call to <function><link linkend="g-object-new">g_object_new</link></function> for target type</entry>
              <entry>target type's base_init function</entry>
              <entry>On the inheritance tree of classes from fundamental type to target type. 
                base_init is invoked once for each class structure.</entry>
              <entry>
                I have no real idea on how this can be used. If you have a good real-life
                example of how a class' base_init can be used, please, let me know.
              </entry>
            </row>
            <row>
              <!--entry>First call to <function><link linkend="g-object-new">g_object_new</link></function> for target type</entry-->
              <entry>target type's class_init function</entry>
              <entry>On target type's class structure</entry>
              <entry>
                Here, you should make sure to initialize or override class methods (that is,
                assign to each class' method its function pointer) and create the signals and
                the properties associated to your object.
              </entry>
            </row>
            <row>
              <!--entry>First call to <function><link linkend="g-object-new">g_object_new</link></function> for target type</entry-->
              <entry>interface' base_init function</entry>
              <entry>On interface' vtable</entry>
              <entry></entry>
            </row>
            <row>
              <!--entry>First call to <function><link linkend="g-object-new">g_object_new</link></function> for target type</entry-->
              <entry>interface' interface_init function</entry>
              <entry>On interface' vtable</entry>
              <entry></entry>
            </row>
            <row>
              <entry morerows="1">Each call to <function><link linkend="g-object-new">g_object_new</link></function> for target type</entry>
              <entry>target type's class constructor method: GObjectClass->constructor</entry>
              <entry>On object's instance</entry>
              <entry>
                If you need to complete the object initialization after all the construction properties
                are set, override the constructor method and make sure to chain up to the object's
                parent class before doing your own initialization.
                In doubt, do not override the constructor method.
              </entry>
            </row>
            <row>
              <!--entry>Each call to <function><link linkend="g-object-new">g_object_new</link></function> for target type</entry-->
              <entry>type's instance_init function</entry>
              <entry>On the inheritance tree of classes from fundamental type to target type. 
              the instance_init provided for each type is invoked once for each instance 
              structure.</entry>
              <entry>
                Provide an instance_init function to initialize your object before its construction
                properties are set. This is the preferred way to initialize a GObject instance.
                This function is equivalent to C++ constructors.
              </entry>
            </row>
          </tbody>
        </tgroup>
      </table>
    </para>

    <para>
      Readers should feel concerned about one little twist in the order in which functions 
      are invoked: while, technically, the class' constructor method is called
      <emphasis>before</emphasis> the GType's instance_init function (since 
      <function><link linkend="g-type-create-instance">g_type_create_instance</link></function> which calls instance_init is called by
      <function>g_object_constructor</function> which is the top-level class 
      constructor method and to which users are expected to chain to), the user's code
      which runs in a user-provided constructor will always run <emphasis>after</emphasis>
      GType's instance_init function since the user-provided constructor 
      <emphasis>must</emphasis> (you've been warned) chain up <emphasis>before</emphasis>
      doing anything useful.
    </para>
  </sect1>

  <sect1 id="gobject-memory">
    <title>Object memory management</title>

    <para>
      The memory-management API for GObjects is a bit complicated but the idea behind it
      is pretty simple: the goal is to provide a flexible model based on reference counting
      which can be integrated in applications which use or require different memory management
      models (such as garbage collection, aso...). The methods which are used to
      manipulate this reference count are described below.
<programlisting>
/*
  Refcounting
*/
gpointer    g_object_ref                      (gpointer        object);
void        g_object_unref                    (gpointer        object);

/*
  Weak References
*/
typedef void (*GWeakNotify)                (gpointer      data,
                                         GObject      *where_the_object_was);
void            g_object_weak_ref                      (GObject              *object,
                                               GWeakNotify     notify,
                                               gpointer               data);
void            g_object_weak_unref                      (GObject              *object,
                                               GWeakNotify     notify,
                                               gpointer               data);
void        g_object_add_weak_pointer         (GObject        *object, 
                                               gpointer       *weak_pointer_location);
void        g_object_remove_weak_pointer      (GObject        *object, 
                                               gpointer       *weak_pointer_location);
/*
  Cycle handling
*/
void        g_object_run_dispose              (GObject              *object);
</programlisting>
    </para>

    <sect2 id="gobject-memory-refcount">
      <title>Reference count</title>

      <para>
        The functions <function><link linkend="g-object-ref">g_object_ref</link></function>/<function><link linkend="g-object-unref">g_object_unref</link></function> respectively 
        increase and decrease the reference count.These functions are thread-safe as of GLib 2.8.
        The reference count is, unsurprisingly, initialized to one by 
        <function><link linkend="g-object-new">g_object_new</link></function> which means that the caller
        is currently the sole owner of the newly-created reference.
        When the reference count reaches zero, that is, 
        when <function><link linkend="g-object-unref">g_object_unref</link></function> is called by the last client holding
        a reference to the object, the <emphasis>dispose</emphasis> and the 
        <emphasis>finalize</emphasis> class methods are invoked.
      </para>
      <para>
        Finally, after <emphasis>finalize</emphasis> is invoked, 
        <function><link linkend="g-type-free-instance">g_type_free_instance</link></function> is called to free the object instance.
        Depending on the memory allocation policy decided when the type was registered (through
        one of the <function>g_type_register_*</function> functions), the object's instance 
        memory will be freed or returned to the object pool for this type.
        Once the object has been freed, if it was the last instance of the type, the type's class
        will be destroyed as described in <xref linkend="gtype-instantiable-classed"/> and 
          <xref linkend="gtype-non-instantiable-classed"/>.
      </para>

      <para>
        The table below summarizes the destruction process of a GObject:
        <table id="gobject-destruction-table">
          <title><function><link linkend="g-object-unref">g_object_unref</link></function></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 morerows="1">Last call to <function><link linkend="g-object-unref">g_object_unref</link></function> for an instance
                  of target type
                 </entry>
                <entry>target type's dispose class function</entry>
                <entry>GObject instance</entry>
                <entry>
                  When dispose ends, the object should not hold any reference to any other
                  member object. The object is also expected to be able to answer client
                  method invocations (with possibly an error code but no memory violation)
                  until finalize is executed. dispose can be executed more than once.
                dispose should chain up to its parent implementation just before returning
                to the caller.
                </entry>
              </row>
              <row>
                <!--entry>Last call to <function><link linkend="g-object-unref">g_object_unref</link></function> for an instance
                  of target type
                </entry-->
                <entry>target type's finalize class function</entry>
                <entry>GObject instance</entry>
                <entry>
                  Finalize is expected to complete the destruction process initiated by
                  dispose. It should complete the object's destruction. finalize will be
                  executed only once.
                finalize should chain up to its parent implementation just before returning
                to the caller.
                  The reason why the destruction process is split is two different phases is
                  explained in <xref linkend="gobject-memory-cycles"/>.
                </entry>
              </row>
              <row>
                <entry morerows="3">Last call to <function><link linkend="g-object-unref">g_object_unref</link></function> for the last
                  instance of target type
                 </entry>
                <entry>interface' interface_finalize function</entry>
                <entry>On interface' vtable</entry>
                <entry>Never used in practice. Unlikely you will need it.</entry>
              </row>
              <row>
                <!--entry>Last call to <function><link linkend="g-object-unref">g_object_unref</link></function>for the last
                  instance of target type
                 </entry-->
                <entry>interface' base_finalize function</entry>
                <entry>On interface' vtable</entry>
                <entry>Never used in practice. Unlikely you will need it.</entry>
              </row>
              <row>
                <!--entry>Last call to <function><link linkend="g-object-unref">g_object_unref</link></function> for the last
                  instance of target type
                 </entry-->
                <entry>target type's class_finalize function</entry>
                <entry>On target type's class structure</entry>
                <entry>Never used in practice. Unlikely you will need it.</entry>
              </row>
              <row>
                <!--entry>Last call to <function><link linkend="g-object-unref">g_object_unref</link></function> for the last
                  instance of target type
                 </entry-->
                <entry>type's base_finalize function</entry>
                <entry>On the inheritance tree of classes from fundamental type to target type.
                  base_init is invoked once for each class structure.</entry>
                <entry>Never used in practice. Unlikely you will need it.</entry>
              </row>
            </tbody>
          </tgroup>
        </table>                
      </para>

    </sect2>

    <sect2 id="gobject-memory-weakref">
      <title>Weak References</title>
    
      <para>
      Weak References are used to monitor object finalization: 
      <function><link linkend="g-object-weak-ref">g_object_weak_ref</link></function> adds a monitoring callback which does
      not hold a reference to the object but which is invoked when the object runs 
      its dispose method. As such, each weak ref can be invoked more than once upon
      object finalization (since dispose can run more than once during object 
      finalization).
      </para>

      <para>
        <function><link linkend="g-object-weak-unref">g_object_weak_unref</link></function> can be used to remove a monitoring
        callback from the object. 
      </para>
  
      <para>
        Weak References are also used to implement <function><link linkend="g-object-add-weak-pointer">g_object_add_weak_pointer</link></function>
        and <function><link linkend="g-object-remove-weak-pointer">g_object_remove_weak_pointer</link></function>. These functions add a weak reference
        to the object they are applied to which makes sure to nullify the pointer given by the user
        when object is finalized.
      </para>
  
    </sect2>
  
    <sect2 id="gobject-memory-cycles">
      <title>Reference counts and cycles</title>
      
      <para>
        Note: the following section was inspired by James Henstridge. I guess this means that 
        all praise and all curses will be directly forwarded to him.
      </para>
  
      <para>
        GObject's memory management model was designed to be easily integrated in existing code
        using garbage collection. This is why the destruction process is split in two phases:
        the first phase, executed in the dispose handler is supposed to release all references
        to other member objects. The second phase, executed by the finalize handler is supposed
        to complete the object's destruction process. Object methods should be able to run
        without program error (that is, without segfault :) in-between the two phases.
      </para>
  
      <para>
        This two-step destruction process is very useful to break reference counting cycles.
        While the detection of the cycles is up to the external code, once the cycles have been
        detected, the external code can invoke <function><link linkend="g-object-dispose">g_object_dispose</link></function> which 
        will indeed break any existing cycles since it will run the dispose handler associated
        to the object and thus release all references to other objects.
      </para>
  
      <para>
        Attentive readers might now have understood one of the rules about the dispose handler
        we stated a bit sooner: the dispose handler can be invoked multiple times. Let's say we
        have a reference count cycle: object A references B which itself references object A.
        Let's say we have detected the cycle and we want to destroy the two objects. One way to 
        do this would be to invoke <function><link linkend="g-object-dispose">g_object_dispose</link></function> on one of the 
        objects.
      </para>
  
      <para>
        If object A releases all its references to all objects, this means it releases its
        reference to object B. If object B was not owned by anyone else, this is its last
        reference count which means this last unref runs B's dispose handler which, in turn,
        releases B's reference on object A. If this is A's last reference count, this last 
        unref runs A's dispose handler which is running for the second time before
        A's finalize handler is invoked !
      </para>
  
      <para>
        The above example, which might seem a bit contrived can really happen if your
        GObject's are being handled by language bindings. I would thus suggest the rules stated above
        for object destruction are closely followed. Otherwise, <emphasis>Bad Bad Things</emphasis> 
        will happen.
      </para>
    </sect2>
  </sect1>

  <sect1 id="gobject-properties">
    <title>Object properties</title>
  
    <para>
      One of GObject's nice features is its generic get/set mechanism for object
      properties. When an object
      is instantiated, the object's class_init handler should be used to register
      the object's properties with <function><link linkend="g-object-class-install-property">g_object_class_install_property</link></function>
      (implemented in <filename>gobject.c</filename>).
    </para>
  
    <para>
      The best way to understand how object properties work is by looking at a real example
      on how it is used:
<programlisting>
/************************************************/
/* Implementation                               */
/************************************************/

enum {
  MAMAN_BAR_CONSTRUCT_NAME = 1,
  MAMAN_BAR_PAPA_NUMBER,
};

static void
maman_bar_instance_init (GTypeInstance   *instance,
                         gpointer         g_class)
{
  MamanBar *self = (MamanBar *)instance;
}

static void
maman_bar_set_property (GObject      *object,
                        guint         property_id,
                        const GValue *value,
                        GParamSpec   *pspec)
{
  MamanBar *self = (MamanBar *) object;

  switch (property_id) {
  case MAMAN_BAR_CONSTRUCT_NAME: {
    g_free (self->priv->name);
    self->priv->name = g_value_dup_string (value);
    g_print ("maman: %s\n",self->priv->name);
  }
    break;
  case MAMAN_BAR_PAPA_NUMBER: {
    self->priv->papa_number = g_value_get_uchar (value);
    g_print ("papa: %u\n",self->priv->papa_number);
  }
    break;
  default:
    /* We don't have any other property... */
    G_OBJECT_WARN_INVALID_PROPERTY_ID(object,property_id,pspec);
    break;
  }
}

static void
maman_bar_get_property (GObject      *object,
                        guint         property_id,
                        GValue       *value,
                        GParamSpec   *pspec)
{
  MamanBar *self = (MamanBar *) object;

  switch (property_id) {
  case MAMAN_BAR_CONSTRUCT_NAME: {
    g_value_set_string (value, self->priv->name);
  }
    break;
  case MAMAN_BAR_PAPA_NUMBER: {
    g_value_set_uchar (value, self->priv->papa_number);
  }
    break;
  default:
    /* We don't have any other property... */
    G_OBJECT_WARN_INVALID_PROPERTY_ID(object,property_id,pspec);
    break;
  }
}

static void
maman_bar_class_init (gpointer g_class,
                      gpointer g_class_data)
{
  GObjectClass *gobject_class = G_OBJECT_CLASS (g_class);
  MamanBarClass *klass = MAMAN_BAR_CLASS (g_class);
  GParamSpec *pspec;

  gobject_class->set_property = maman_bar_set_property;
  gobject_class->get_property = maman_bar_get_property;

  pspec = g_param_spec_string ("maman-name",
                               "Maman construct prop",
                               "Set maman's name",
                               "no-name-set" /* default value */,
                               G_PARAM_CONSTRUCT_ONLY | G_PARAM_READWRITE);
  g_object_class_install_property (gobject_class,
                                   MAMAN_BAR_CONSTRUCT_NAME,
                                   pspec);

  pspec = g_param_spec_uchar ("papa-number",
                              "Number of current Papa",
                              "Set/Get papa's number",
                              0  /* minimum value */,
                              10 /* maximum value */,
                              2  /* default value */,
                              G_PARAM_READWRITE);
  g_object_class_install_property (gobject_class,
                                   MAMAN_BAR_PAPA_NUMBER,
                                   pspec);
}

/************************************************/
/* Use                                          */
/************************************************/

GObject *bar;
GValue val = {0,};
bar = g_object_new (MAMAN_TYPE_SUBBAR, NULL);
g_value_init (&amp;val, G_TYPE_CHAR);
g_value_set_char (&amp;val, 11);
g_object_set_property (G_OBJECT (bar), "papa-number", &amp;val);
</programlisting>
      The client code just above looks simple but a lot of things happen under the hood:
    </para>
  
    <para>
      <function><link linkend="g-object-set-property">g_object_set_property</link></function> first ensures a property
      with this name was registered in bar's class_init handler. If so, it calls
      <function><link linkend="object-set-property">object_set_property</link></function> which first walks the class hierarchy,
      from bottom, most derived type, to top, fundamental type to find the class
      which registered that property. It then tries to convert the user-provided GValue
      into a GValue whose type if that of the associated property.
    </para>
  
    <para>
      If the user provides a signed char GValue, as is shown
      here, and if the object's property was registered as an unsigned int, 
      <function><link linkend="g-value-transform">g_value_transform</link></function> will try to transform the input signed char into
      an unsigned int. Of course, the success of the transformation depends on the availability
      of the required transform function. In practice, there will almost always be a transformation
      <footnote>
        <para>Its behaviour might not be what you expect but it is up to you to actually avoid
          relying on these transformations.
        </para>
      </footnote>
      which matches and conversion will be carried out if needed.
    </para>
  
    <para>
      After transformation, the <type><link linkend="GValue">GValue</link></type> is validated by 
      <function><link linkend="g-param-value-validate">g_param_value_validate</link></function> which makes sure the user's
      data stored in the <type><link linkend="GValue">GValue</link></type> matches the characteristics specified by
      the property's <type><link linkend="GParamSpec">GParamSpec</link></type>.  Here, the <type><link linkend="GParamSpec">GParamSpec</link></type> we 
      provided in class_init has a validation function which makes sure that the GValue
      contains a value which respects the minimum and maximum bounds of the 
      <type><link linkend="GParamSpec">GParamSpec</link></type>. In the example above, the client's GValue does not
      respect these constraints (it is set to 11, while the maximum is 10). As such, the
      <function><link linkend="g-object-set-property">g_object_set_property</link></function> function will return with an error.
    </para>
  
    <para>
      If the user's GValue had been set to a valid value, <function><link linkend="g-object-set-property">g_object_set_property</link></function>
      would have proceeded with calling the object's set_property class method. Here, since our
      implementation of Foo did override this method, the code path would jump to
      <function>foo_set_property</function> after having retrieved from the 
      <type><link linkend="GParamSpec">GParamSpec</link></type> the <emphasis>param_id</emphasis>
      <footnote>
        <para>
          It should be noted that the param_id used here need only to uniquely identify each 
          <type><link linkend="GParamSpec">GParamSpec</link></type> within the <type><link linkend="FooClass">FooClass</link></type> such that the switch
          used in the set and get methods actually works. Of course, this locally-unique 
          integer is purely an optimization: it would have been possible to use a set of 
          <emphasis>if (strcmp (a, b) == 0) {} else if (strcmp (a, b) == 0) {}</emphasis> statements.
        </para>
      </footnote>
      which had been stored by
      <function><link linkend="g-object-class-install-property">g_object_class_install_property</link></function>.
    </para>
  
    <para>
      Once the property has been set by the object's set_property class method, the code path
      returns to <function><link linkend="g-object-set-property">g_object_set_property</link></function> which calls 
      <function><link linkend="g-object-notify-queue-thaw">g_object_notify_queue_thaw</link></function>. This function makes sure that
      the "notify" signal is emitted on the object's instance with the changed property as
      parameter unless notifications were frozen by <function><link linkend="g-object-freeze-notify">g_object_freeze_notify</link></function>.
    </para>
  
    <para>
      <function><link linkend="g-object-thaw-notify">g_object_thaw_notify</link></function> can be used to re-enable notification of 
      property modifications through the "notify" signal. It is important to remember that
      even if properties are changed while property change notification is frozen, the "notify"
      signal will be emitted once for each of these changed properties as soon as the property
      change notification is thawed: no property change is lost for the "notify" signal. Signal
      can only be delayed by the notification freezing mechanism.
    </para>
    
    <para>
      It sounds like a tedious task to set up GValues every time when one wants to modify a property.
      In practice one will rarely do this. The functions <function><link linkend="g-object-set-property">g_object_set_property</link></function>
      and <function><link linkend="g-object-get-property">g_object_get_property</link></function>
      are meant to be used by language bindings. For application there is an easier way and
      that is described next.
    </para>

    <sect2 id="gobject-multi-properties">
      <title>Accessing multiple properties at once</title>
  
      <para>
        It is interesting to note that the <function><link linkend="g-object-set">g_object_set</link></function> and 
        <function><link linkend="g-object-set-valist">g_object_set_valist</link></function> (vararg version) functions can be used to set
        multiple properties at once. The client code shown above can then be re-written as:
<programlisting>
MamanBar *foo;
foo = /* */;
g_object_set (G_OBJECT (foo),
              "papa-number", 2, 
              "maman-name", "test", 
              NULL);
</programlisting>
        This saves us from managing the GValues that we were needing to handle when using
        <function><link linkend="g-object-set-property">g_object_set_property</link></function>.
        The code above will trigger one notify signal emission for each property modified.
      </para>
    
      <para>
        Of course, the _get versions are also available: <function><link linkend="g-object-get">g_object_get</link></function>
        and <function><link linkend="g-object-get-valist">g_object_get_valist</link></function> (vararg version) can be used to get numerous
        properties at once.
      </para>
      
      <para>
        These high level functions have one drawback - they don't provide a return result.
        One should pay attention to the argument types and ranges when using them.
        A know source of errors is to e.g. pass a gfloat instead of a gdouble and thus
        shifting all subsequent parameters by four bytes. Also forgetting the terminating
        NULL will lead to unexpected behaviour.
      </para>
    
      <para>
        Really attentive readers now understand how <function><link linkend="g-object-new">g_object_new</link></function>,
        <function><link linkend="g-object-newv">g_object_newv</link></function> and <function><link linkend="g-object-new-valist">g_object_new_valist</link></function> 
        work: they parse the user-provided variable number of parameters and invoke
        <function><link linkend="g-object-set">g_object_set</link></function> on the parameters only after the object has been successfully constructed.
        Of course, the "notify" signal will be emitted for each property set.
      </para>
    
    </sect2>

<!-- @todo tell here about how to pass use handle properties in derived classes -->

  </sect1>

</chapter>