A type, as manipulated by the GLib type system, is much more generic than what is usually understood as an Object type. It is best explained by looking at the structure and the functions used to register new types in the type system.
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typedef struct _GTypeInfo GTypeInfo; struct _GTypeInfo { /* interface types, classed types, instantiated types */ guint16 class_size; GBaseInitFunc base_init; GBaseFinalizeFunc base_finalize; /* classed types, instantiated types */ GClassInitFunc class_init; GClassFinalizeFunc class_finalize; gconstpointer class_data; /* instantiated types */ guint16 instance_size; guint16 n_preallocs; GInstanceInitFunc instance_init; /* value handling */ const GTypeValueTable *value_table; }; GType g_type_register_static (GType parent_type, const gchar *type_name, const GTypeInfo *info, GTypeFlags flags); GType g_type_register_fundamental (GType type_id, const gchar *type_name, const GTypeInfo *info, const GTypeFundamentalInfo *finfo, GTypeFlags flags); |
g_type_register_static
,
g_type_register_dynamic
and
g_type_register_fundamental
are the C functions, defined in
gtype.h
and implemented in gtype.c
which you should use to register a new GType in the program's type system.
It is not likely you will ever need to use
g_type_register_fundamental
but in case you want to, the last chapter explains how to create
new fundamental types.
Fundamental types are top-level types which do not derive from any other type while other non-fundamental types derive from other types. Upon initialization, the type system not only initializes its internal data structures but it also registers a number of core types: some of these are fundamental types. Others are types derived from these fundamental types.
Fundamental and non-fundamental types are defined by:
class size: the class_size field in GTypeInfo.
class initialization functions (C++ constructor): the base_init
and
class_init
fields in GTypeInfo.
class destruction functions (C++ destructor): the base_finalize and class_finalize fields in GTypeInfo.
instance size (C++ parameter to new): the instance_size field in GTypeInfo.
instantiation policy (C++ type of new operator): the n_preallocs field in GTypeInfo.
copy functions (C++ copy operators): the value_table field in GTypeInfo.
type characteristic flags: GTypeFlags.
Fundamental types are also defined by a set of GTypeFundamentalFlags
which are stored in a GTypeFundamentalInfo.
Non-fundamental types are furthermore defined by the type of their parent which is
passed as the parent_type parameter to g_type_register_static
and g_type_register_dynamic
.
The major common point between all GLib types (fundamental and non-fundamental, classed and non-classed, instantiatable and non-instantiatable) is that they can all be manipulated through a single API to copy/assign them.
The GValue structure is used as an abstract container for all of these
types. Its simplistic API (defined in gobject/gvalue.h
) can be
used to invoke the value_table functions registered
during type registration: for example g_value_copy
copies the
content of a GValue to another GValue. This is similar
to a C++ assignment which invokes the C++ copy operator to modify the default
bit-by-bit copy semantics of C++/C structures/classes.
The following code shows how you can copy around a 64 bit integer, as well as a GObject instance pointer:
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static void test_int (void) { GValue a_value = G_VALUE_INIT; GValue b_value = G_VALUE_INIT; guint64 a, b; a = 0xdeadbeef; g_value_init (&a_value, G_TYPE_UINT64); g_value_set_uint64 (&a_value, a); g_value_init (&b_value, G_TYPE_UINT64); g_value_copy (&a_value, &b_value); b = g_value_get_uint64 (&b_value); if (a == b) { g_print ("Yay !! 10 lines of code to copy around a uint64.\n"); } else { g_print ("Are you sure this is not a Z80 ?\n"); } } static void test_object (void) { GObject *obj; GValue obj_vala = G_VALUE_INIT; GValue obj_valb = G_VALUE_INIT; obj = g_object_new (VIEWER_TYPE_FILE, NULL); g_value_init (&obj_vala, VIEWER_TYPE_FILE); g_value_set_object (&obj_vala, obj); g_value_init (&obj_valb, G_TYPE_OBJECT); /* g_value_copy's semantics for G_TYPE_OBJECT types is to copy the reference. * This function thus calls g_object_ref. * It is interesting to note that the assignment works here because * VIEWER_TYPE_FILE is a G_TYPE_OBJECT. */ g_value_copy (&obj_vala, &obj_valb); g_object_unref (G_OBJECT (obj)); g_object_unref (G_OBJECT (obj)); } |
The important point about the above code is that the exact semantics of the copy calls is undefined since they depend on the implementation of the copy function. Certain copy functions might decide to allocate a new chunk of memory and then to copy the data from the source to the destination. Others might want to simply increment the reference count of the instance and copy the reference to the new GValue.
The value table used to specify these assignment functions is documented in GTypeValueTable.
Interestingly, it is also very unlikely you will ever need to specify a value_table during type registration because these value_tables are inherited from the parent types for non-fundamental types.