Changes between Initial Version and Version 1 of GRefPtr

Timestamp:

Jan 20, 2014 3:03:43 AM (10 years ago)

Author:

Adrian Perez de Castro

Comment:

Initial version

GRefPtr

@@ +1 @@
+= Reference counting with `GRefPtr<T>` = 
+
+In the GLib/GObject/GTK+ object model, instances are reference-counted. To ease handling those in !WebKitGTK+ code, the `GRefPtr<T>` template is provided. It keeps a pointer to a reference-counted and takes care of properly increasing and decreasing the reference counts for object being held. Helper functions are provided as well (e.g. `adoptGRef()`).
+
+Sources:
+
+ * [source:trunk/Source/WTF/wtf/gobject/GRefPtr.h Source/WTF/wtf/gobject/GRefPtr.h]
+ * [source:trunk/Source/WTF/wtf/gobject/GRefPtr.cpp Source/WTF/wtf/gobject/GRefPtr.h]
+
+
+== Usage ==
+
+=== Creation ===
+
+Creating a `GRefPtr<T>` initializes it to `nullptr`:
+{{{
+#!cpp
+GRefPtr<GtkWidget> widget;
+// Check always succeeds, nullptr is the default value.
+ASSERT(!widget);
+}}}
+
+It is also possible to pass an initial value to the constructor — it can be both a raw pointer or a `GRefPtr` of the same type:
+{{{
+#!cpp
+void doWithWidget(GtkWidget* widget) {
+    GRefPtr<GtkWidget> widgetLocal1(widget); // Increases the reference counter of “widget”
+    GRefPtr<GtkWidget> widgetLocal2(widgetLocal1); // ...and again.
+}
+}}}
+
+When a `GRefPtr<T>` goes is destroyed, it decreases the reference count of the object it holds.
+
+=== Adopting a pointer ===
+
+Sometimes it is desirable to store a pointer in a `GRefPtr` without increasing the reference counter. The main use case is assigning a newly created instance to a `GRefPtr`: the newly created instance already has a reference count of ''one'' and assigning it to a `GRefPtr` would increase the reference count to ''two'', which would be incorrect. To avoid this, the `adoptGRef()` helper function is provided. This function creates a `GRefPtr` without increasing the reference counter of the object it holds. This is incorrect:
+{{{
+#!cpp
+// Incorrect: The created object has *two* references, even
+// when referenced only from the “context” variable.
+GRefPtr<GMainContext> context = g_main_context_new();
+}}}
+
+And this is correct:
+{{{
+#!
+// Correct: The newly created object has only one reference
+GRefPtr<GMainContext> context = adoptGRef(g_main_context_new());
+}}}
+
+=== Assignment ===
+
+Assigning to a `GRefPtr` increases the reference count for the pointer being assigned. Both raw pointers and other `GRefPtr` instances can be assigned:
+{{{
+#!cpp
+void doWithWidget(GtkWidget* widget) {
+    GRefPtr<GtkWidget> widgetLocal1 = widget;
+    GRefPtr<GtkWidget> widgetLocal2 = widgetLocal1;
+}
+}}}
+
+
+=== Raw pointer ===
+
+To obtain the raw pointer use the `get()` method. This is typically used when calling functions from support libraries used by the port (GLib, GTK+, etc). For example:
+{{{
+#!cpp
+void show(GRefPtr<GtkWidget>& widget) {
+    gtk_widget_show_all(widget.get());
+}
+}}}
+
+
+== Floating references ==
+
+Instances of most types have a ''floating reference'' when created. Most `GObject`-derived types, but also some other types have floating references (`GVariant` is one of those). It is incorrect to use `adoptGRef()` on newly created instances of types which use floating references: the floating reference has to be converted on a full reference. Assigning to a `GRefPtr` will convert the floating reference to a full reference.
+
+Incorrect usage:
+{{{
+#!cpp
+GRefPtr<GVariant> variant = adoptGRef(g_variant_new("s", "Incorrect usage"));
+}}}
+
+Correct usage:
+{{{
+#!cpp
+GRefPtr<GVariant> variant = g_variant_new("s", "Floating reference is sank");
+}}}