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<title>The GLib Dynamic Type System: GObject Reference Manual</title>
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<link rel="up" href="pt01.html" title="Part I. Concepts">
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<link rel="prev" href="ch01s02.html" title="Exporting a C API">
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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The GLib Dynamic Type System
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Copy functions
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Conventions
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Non-instantiable non-classed fundamental types
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Instantiable classed types: objects
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- Initialization and Destruction
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Non-instantiable classed types: interfaces
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Interface Initialization
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Interface Destruction
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A type, as manipulated by the GLib type system, is much more generic than what
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is usually understood as an Object type. It is best explained by looking at the
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structure and the functions used to register new types in the type system.
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typedef struct _GTypeInfo GTypeInfo;
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struct _GTypeInfo
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{
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guint16 class_size;
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GBaseInitFunc base_init;
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GBaseFinalizeFunc base_finalize;
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GClassInitFunc class_init;
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GClassFinalizeFunc class_finalize;
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gconstpointer class_data;
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guint16 instance_size;
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guint16 n_preallocs;
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GInstanceInitFunc instance_init;
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const GTypeValueTable *value_table;
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};
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GType g_type_register_static (GType parent_type,
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const gchar *type_name,
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const GTypeInfo *info,
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GTypeFlags flags);
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GType g_type_register_fundamental (GType type_id,
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const gchar *type_name,
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const GTypeInfo *info,
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const GTypeFundamentalInfo *finfo,
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GTypeFlags flags);
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g_type_register_static,
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g_type_register_dynamic and
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g_type_register_fundamental
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are the C functions, defined in
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gtype.h and implemented in gtype.c
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which you should use to register a new GType in the program's type system.
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It is not likely you will ever need to use
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g_type_register_fundamental
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but in case you want to, the last chapter explains how to create
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new fundamental types.
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Fundamental types are top-level types which do not derive from any other type
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while other non-fundamental types derive from other types.
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Upon initialization, the type system not only initializes its
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internal data structures but it also registers a number of core
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types: some of these are fundamental types. Others are types derived from these
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fundamental types.
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Fundamental and non-fundamental types are defined by:
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class size: the class_size field in GTypeInfo.
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class initialization functions (C++ constructor): the base_init and
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class_init fields in GTypeInfo.
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class destruction functions (C++ destructor): the base_finalize and
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class_finalize fields in GTypeInfo.
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instance size (C++ parameter to new): the instance_size field in
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GTypeInfo.
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instantiation policy (C++ type of new operator): the n_preallocs
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field in GTypeInfo.
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copy functions (C++ copy operators): the value_table field in
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GTypeInfo.
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type characteristic flags: GTypeFlags.
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Fundamental types are also defined by a set of GTypeFundamentalFlags
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which are stored in a GTypeFundamentalInfo.
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Non-fundamental types are furthermore defined by the type of their parent which is
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passed as the parent_type parameter to g_type_register_static
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and g_type_register_dynamic.
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Copy functions
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The major common point between all GLib types (fundamental and
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non-fundamental, classed and non-classed, instantiable and non-instantiable) is that
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they can all be manipulated through a single API to copy/assign them.
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The GValue structure is used as an abstract container for all of these
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types. Its simplistic API (defined in gobject/gvalue.h) can be
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used to invoke the value_table functions registered
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during type registration: for example g_value_copy copies the
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content of a GValue to another GValue. This is similar
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to a C++ assignment which invokes the C++ copy operator to modify the default
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bit-by-bit copy semantics of C++/C structures/classes.
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The following code shows how you can copy around a 64 bit integer, as well as a GObject
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instance pointer:
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static void test_int (void)
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{
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GValue a_value = G_VALUE_INIT;
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GValue b_value = G_VALUE_INIT;
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guint64 a, b;
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a = 0xdeadbeef;
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g_value_init (&a_value, G_TYPE_UINT64);
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g_value_set_uint64 (&a_value, a);
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g_value_init (&b_value, G_TYPE_UINT64);
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g_value_copy (&a_value, &b_value);
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b = g_value_get_uint64 (&b_value);
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if (a == b) {
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g_print ("Yay !! 10 lines of code to copy around a uint64.\n");
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} else {
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g_print ("Are you sure this is not a Z80 ?\n");
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}
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}
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static void test_object (void)
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{
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GObject *obj;
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GValue obj_vala = G_VALUE_INIT;
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GValue obj_valb = G_VALUE_INIT;
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obj = g_object_new (VIEWER_TYPE_FILE, NULL);
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g_value_init (&obj_vala, VIEWER_TYPE_FILE);
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g_value_set_object (&obj_vala, obj);
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g_value_init (&obj_valb, G_TYPE_OBJECT);
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g_value_copy (&obj_vala, &obj_valb);
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g_object_unref (G_OBJECT (obj));
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g_object_unref (G_OBJECT (obj));
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}
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The important point about the above code is that the exact semantics of the copy calls
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is undefined since they depend on the implementation of the copy function. Certain
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copy functions might decide to allocate a new chunk of memory and then to copy the
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data from the source to the destination. Others might want to simply increment
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the reference count of the instance and copy the reference to the new GValue.
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The value table used to specify these assignment functions is
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documented in
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GTypeValueTable.
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Interestingly, it is also very unlikely
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you will ever need to specify a value_table during type registration
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because these value_tables are inherited from the parent types for
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non-fundamental types.
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Generated by GTK-Doc V1.27
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