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129 lines
4.7 KiB
Markdown
129 lines
4.7 KiB
Markdown
Title: Floating References
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# Floating References
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**Note**: Floating references are a C convenience API and should not be used
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in modern GObject code. Language bindings in particular find the concept
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highly problematic, as floating references are not identifiable through
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annotations, and neither are deviations from the floating reference
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behavior, like types that inherit from [class@GObject.InitiallyUnowned] and
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still return a full reference from [`id@g_object_new`].
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[class@GObject.InitiallyUnowned] is derived from [class@GObject.Object]. The
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only difference between the two is that the initial reference of a
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`GInitiallyUnowned` is flagged as a "floating" reference. This means that it
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is not specifically claimed to be "owned" by any code portion. The main
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motivation for providing floating references is C convenience. In
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particular, it allows code to be written as:
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```c
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container = create_container ();
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container_add_child (container, create_child());
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```
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If `container_add_child()` calls [`method@GObject.Object.ref_sink`] on the
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passed-in child, no reference of the newly created child is leaked. Without
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floating references, `container_add_child()` can only acquire a reference
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the new child, so to implement this code without reference leaks, it would
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have to be written as:
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```c
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Child *child;
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container = create_container ();
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child = create_child ();
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container_add_child (container, child);
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g_object_unref (child);
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```
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The floating reference can be converted into an ordinary reference by
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calling `g_object_ref_sink()`. For already sunken objects (objects that
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don't have a floating reference anymore), `g_object_ref_sink()` is
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equivalent to [`method@GObject.Object.ref`] and returns a new reference.
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Since floating references are useful almost exclusively for C convenience,
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language bindings that provide automated reference and memory ownership
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maintenance (such as smart pointers or garbage collection) should not expose
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floating references in their API. The best practice for handling types that
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have initially floating references is to immediately sink those references
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after `g_object_new()` returns, by checking if the `GType` inherits from
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`GInitiallyUnowned`. For instance:
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```c
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GObject *res = g_object_new_with_properties (gtype,
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n_props,
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prop_names,
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prop_values);
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// or: if (g_type_is_a (gtype, G_TYPE_INITIALLY_UNOWNED))
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if (G_IS_INITIALLY_UNOWNED (res))
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g_object_ref_sink (res);
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return res;
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```
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Some object implementations may need to save an object's floating state
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across certain code portions (an example is `GtkMenu` in GTK3), to achieve
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this, the following sequence can be used:
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```c
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// save floating state
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gboolean was_floating = g_object_is_floating (object);
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g_object_ref_sink (object);
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// protected code portion
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...
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// restore floating state
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if (was_floating)
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g_object_force_floating (object);
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else
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g_object_unref (object); // release previously acquired reference
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```
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## Replacing floating references with annotations
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You should avoid basing your object hierarchy on floating references, as
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they are hard to understand even in C, and introduce additional limitations
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when consuming a GObject-based API in other languages.
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One way to express the ownership transfer between ‘container’ and ‘child’
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instances is to use ownership transfer annotations in your documentation and
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introspection data. For instance, you can implement this pattern:
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```c
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container_add_child (container, create_child ());
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```
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without leaking by having `container_add_child()` defined as:
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```c
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/**
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* container_add_child:
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* @self: a container
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* @child: (transfer full): the child to add to the container
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*
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* ...
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*/
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void
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container_add_child (Container *container,
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Child *child)
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{
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container->children = g_list_append (container->children, child);
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}
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```
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The container does not explicitly acquire a reference on the child; instead,
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the ownership of the child is transferred to the container. The transfer
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annotation will be used by language bindings to ensure that there are no
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leaks; and documentation tools will explicitly note that the callee now owns
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the value passed by the caller.
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## Replacing floating references with weak references
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Another option for replacing floating references is to use weak references
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in place of strong ones. A ‘container’ can acquire a weak reference on the
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‘child’ instance by using [`method@GObject.Object.weak_ref`]. Once the
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child instance loses its last strong reference, the container holding a
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weak reference is notified, and it can either remove the child from its
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internal list, or turn a weak reference into a strong one.
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