docs/reference/gobject/html/gobject-memory.html

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Object memory management: GObject Reference Manual</title>
<meta name="generator" content="DocBook XSL Stylesheets V1.79.1">
<link rel="home" href="index.html" title="GObject Reference Manual">
<link rel="up" href="chapter-gobject.html" title="The GObject base class">
<link rel="prev" href="chapter-gobject.html" title="The GObject base class">
<link rel="next" href="gobject-properties.html" title="Object properties">
<meta name="generator" content="GTK-Doc V1.25.1 (XML mode)">
<link rel="stylesheet" href="style.css" type="text/css">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table class="navigation" id="top" width="100%" summary="Navigation header" cellpadding="2" cellspacing="5"><tr valign="middle">
<td width="100%" align="left" class="shortcuts"></td>
<td><a accesskey="h" href="index.html"><img src="home.png" width="16" height="16" border="0" alt="Home"></a></td>
<td><a accesskey="u" href="chapter-gobject.html"><img src="up.png" width="16" height="16" border="0" alt="Up"></a></td>
<td><a accesskey="p" href="chapter-gobject.html"><img src="left.png" width="16" height="16" border="0" alt="Prev"></a></td>
<td><a accesskey="n" href="gobject-properties.html"><img src="right.png" width="16" height="16" border="0" alt="Next"></a></td>
</tr></table>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="gobject-memory"></a>Object memory management</h2></div></div></div>
<p>
      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). The methods which are used to
      manipulate this reference count are described below.
    </p>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="gobject-memory-refcount"></a>Reference count</h3></div></div></div>
<p>
        The functions <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-ref" title="g_object_ref ()">g_object_ref</a></code>/<code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-unref" title="g_object_unref ()">g_object_unref</a></code> respectively 
        increase and decrease the reference count. These functions are
        thread-safe.
        <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-clear-object" title="g_clear_object ()">g_clear_object</a></code>
        is a convenience wrapper around <code class="function">g_object_unref</code>
        which also clears the pointer passed to it.
      </p>
<p>
        The reference count is initialized to one by 
        <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-new" title="g_object_new ()">g_object_new</a></code> which means that the caller
        is currently the sole owner of the newly-created reference.
        When the reference count reaches zero, that is, 
        when <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-unref" title="g_object_unref ()">g_object_unref</a></code> is called by the last client holding
        a reference to the object, the <span class="emphasis"><em>dispose</em></span> and the 
        <span class="emphasis"><em>finalize</em></span> class methods are invoked.
      </p>
<p>
        Finally, after <span class="emphasis"><em>finalize</em></span> is invoked, 
        <code class="function"><a class="link" href="gobject-Type-Information.html#g-type-free-instance" title="g_type_free_instance ()">g_type_free_instance</a></code> is called to free the object instance.
        Depending on the memory allocation policy decided when the type was registered (through
        one of the <code class="function">g_type_register_*</code> 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 <a class="xref" href="gtype-instantiable-classed.html" title="Instantiable classed types: objects">the section called “Instantiable classed types: objects”</a> and 
          <a class="xref" href="gtype-non-instantiable-classed.html" title="Non-instantiable classed types: interfaces">the section called “Non-instantiable classed types: interfaces”</a>.
      </p>
<p>
        The table below summarizes the destruction process of a GObject:
        </p>
<div class="table">
<a name="gobject-destruction-table"></a><p class="title"><b>Table 5. <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-unref" title="g_object_unref ()">g_object_unref</a></code></b></p>
<div class="table-contents"><table class="table" summary="g_object_unref" border="1">
<colgroup>
<col align="left">
<col align="left">
<col align="left">
</colgroup>
<thead><tr>
<th align="left">Invocation time</th>
<th align="left">Function invoked</th>
<th align="left">Function's parameters</th>
<th>Remark</th>
</tr></thead>
<tbody>
<tr>
<td rowspan="2" align="left">Last call to <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-unref" title="g_object_unref ()">g_object_unref</a></code> for an instance
                  of target type
                 </td>
<td align="left">target type's dispose class function</td>
<td align="left">GObject instance</td>
<td>
                  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.
                </td>
</tr>
<tr>
<td align="left">target type's finalize class function</td>
<td align="left">GObject instance</td>
<td>
                  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 <a class="xref" href="gobject-memory.html#gobject-memory-cycles" title="Reference counts and cycles">the section called “Reference counts and cycles”</a>.
                </td>
</tr>
<tr>
<td rowspan="4" align="left">Last call to <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-unref" title="g_object_unref ()">g_object_unref</a></code> for the last
                  instance of target type
                 </td>
<td align="left">interface's <code class="function">interface_finalize</code> function</td>
<td align="left">On interface's vtable</td>
<td>Never used in practice. Unlikely you will need it.</td>
</tr>
<tr>
<td align="left">interface's <code class="function">base_finalize</code> function</td>
<td align="left">On interface's vtable</td>
<td>Never used in practice. Unlikely you will need it.</td>
</tr>
<tr>
<td align="left">target type's <code class="function">class_finalize</code> function</td>
<td align="left">On target type's class structure</td>
<td>Never used in practice. Unlikely you will need it.</td>
</tr>
<tr>
<td align="left">type's <code class="function">base_finalize</code> function</td>
<td align="left">On the inheritance tree of classes from fundamental type to target type.
                  <code class="function">base_init</code> is invoked once for each class structure.</td>
<td>Never used in practice. Unlikely you will need it.</td>
</tr>
</tbody>
</table></div>
</div>
<p><br class="table-break">                
      </p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="gobject-memory-weakref"></a>Weak References</h3></div></div></div>
<p>
      Weak references are used to monitor object finalization:
      <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-weak-ref" title="g_object_weak_ref ()">g_object_weak_ref</a></code> 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).
      </p>
<p>
        <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-weak-unref" title="g_object_weak_unref ()">g_object_weak_unref</a></code> can be used to remove a monitoring
        callback from the object. 
      </p>
<p>
        Weak references are also used to implement <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-add-weak-pointer" title="g_object_add_weak_pointer ()">g_object_add_weak_pointer</a></code>
        and <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-remove-weak-pointer" title="g_object_remove_weak_pointer ()">g_object_remove_weak_pointer</a></code>. 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.
      </p>
<p>
        Similarly, <a class="link" href="gobject-The-Base-Object-Type.html#GWeakRef" title="GWeakRef"><span class="type">GWeakRef</span></a> can be
        used to implement weak references if thread safety is required.
      </p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="gobject-memory-cycles"></a>Reference counts and cycles</h3></div></div></div>
<p>
        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 in-between the two phases.
      </p>
<p>
        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 <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-run-dispose" title="g_object_run_dispose ()">g_object_run_dispose</a></code> 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.
      </p>
<p>
        This explains one of the rules about the dispose handler stated earlier:
        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 <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-run-dispose" title="g_object_run_dispose ()">g_object_run_dispose</a></code> on one of the 
        objects.
      </p>
<p>
        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 !
      </p>
<p>
        The above example, which might seem a bit contrived, can really happen if
        GObjects are being handled by language bindings — hence the rules for
        object destruction should be closely followed.
      </p>
</div>
</div>
<div class="footer">
<hr>Generated by GTK-Doc V1.25.1</div>
</body>
</html>