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| <div class="chapter" title="The GLib Dynamic Type System"> |
| <div class="titlepage"><div><div><h2 class="title"> |
| <a name="chapter-gtype"></a>The GLib Dynamic Type System</h2></div></div></div> |
| <div class="toc"><dl> |
| <dt><span class="sect1"><a href="chapter-gtype.html#gtype-copy">Copy functions</a></span></dt> |
| <dt><span class="sect1"><a href="gtype-conventions.html">Conventions</a></span></dt> |
| <dt><span class="sect1"><a href="gtype-non-instantiable.html">Non-instantiable non-classed fundamental types</a></span></dt> |
| <dt><span class="sect1"><a href="gtype-instantiable-classed.html">Instantiable classed types: objects</a></span></dt> |
| <dd><dl><dt><span class="sect2"><a href="gtype-instantiable-classed.html#gtype-instantiable-classed-init-done">Initialization and Destruction</a></span></dt></dl></dd> |
| <dt><span class="sect1"><a href="gtype-non-instantiable-classed.html">Non-instantiable classed types: interfaces</a></span></dt> |
| <dd><dl> |
| <dt><span class="sect2"><a href="gtype-non-instantiable-classed.html#gtype-non-instantiable-classed-init">Interface Initialization</a></span></dt> |
| <dt><span class="sect2"><a href="gtype-non-instantiable-classed.html#gtype-non-instantiable-classed-dest">Interface Destruction</a></span></dt> |
| </dl></dd> |
| </dl></div> |
| <p> |
| 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. |
| </p> |
| <pre class="programlisting"> |
| 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); |
| </pre> |
| <p> |
| </p> |
| <p> |
| <code class="function"><a class="link" href="gobject-Type-Information.html#g-type-register-static" title="g_type_register_static ()">g_type_register_static</a></code> and |
| <code class="function"><a class="link" href="gobject-Type-Information.html#g-type-register-fundamental" title="g_type_register_fundamental ()">g_type_register_fundamental</a></code> |
| are the C functions, defined in |
| <code class="filename">gtype.h</code> and implemented in <code class="filename">gtype.c</code> |
| which you should use to register a new <span class="type"><a class="link" href="gobject-Type-Information.html#GType" title="GType">GType</a></span> in the program's type system. |
| It is not likely you will ever need to use |
| <code class="function"><a class="link" href="gobject-Type-Information.html#g-type-register-fundamental" title="g_type_register_fundamental ()">g_type_register_fundamental</a></code> (you have to be Tim Janik |
| to do that) but in case you want to, the last chapter explains how to create |
| new fundamental types. |
| <sup>[<a name="id552292" href="#ftn.id552292" class="footnote">2</a>]</sup> |
| </p> |
| <p> |
| 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 by <code class="function"><a class="link" href="gobject-Type-Information.html#g-type-init" title="g_type_init ()">g_type_init</a></code>, 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. |
| </p> |
| <p> |
| Fundamental and non-fundamental types are defined by: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"><p> |
| class size: the class_size field in <span class="type"><a class="link" href="gobject-Type-Information.html#GTypeInfo" title="GTypeInfo">GTypeInfo</a></span>. |
| </p></li> |
| <li class="listitem"><p> |
| class initialization functions (C++ constructor): the base_init and |
| class_init fields in <span class="type"><a class="link" href="gobject-Type-Information.html#GTypeInfo" title="GTypeInfo">GTypeInfo</a></span>. |
| </p></li> |
| <li class="listitem"><p> |
| class destruction functions (C++ destructor): the base_finalize and |
| class_finalize fields in <span class="type"><a class="link" href="gobject-Type-Information.html#GTypeInfo" title="GTypeInfo">GTypeInfo</a></span>. |
| </p></li> |
| <li class="listitem"><p> |
| instance size (C++ parameter to new): the instance_size field in |
| <span class="type"><a class="link" href="gobject-Type-Information.html#GTypeInfo" title="GTypeInfo">GTypeInfo</a></span>. |
| </p></li> |
| <li class="listitem"><p> |
| instantiation policy (C++ type of new operator): the n_preallocs |
| field in <span class="type"><a class="link" href="gobject-Type-Information.html#GTypeInfo" title="GTypeInfo">GTypeInfo</a></span>. |
| </p></li> |
| <li class="listitem"><p> |
| copy functions (C++ copy operators): the value_table field in |
| <span class="type"><a class="link" href="gobject-Type-Information.html#GTypeInfo" title="GTypeInfo">GTypeInfo</a></span>. |
| </p></li> |
| <li class="listitem"><p> |
| type characteristic flags: <span class="type"><a class="link" href="gobject-Type-Information.html#GTypeFlags" title="enum GTypeFlags">GTypeFlags</a></span>. |
| </p></li> |
| </ul></div> |
| <p> |
| Fundamental types are also defined by a set of <span class="type"><a class="link" href="gobject-Type-Information.html#GTypeFundamentalFlags" title="enum GTypeFundamentalFlags">GTypeFundamentalFlags</a></span> |
| which are stored in a <span class="type"><a class="link" href="gobject-Type-Information.html#GTypeFundamentalInfo" title="GTypeFundamentalInfo">GTypeFundamentalInfo</a></span>. |
| Non-fundamental types are furthermore defined by the type of their parent which is |
| passed as the parent_type parameter to <code class="function"><a class="link" href="gobject-Type-Information.html#g-type-register-static" title="g_type_register_static ()">g_type_register_static</a></code> |
| and <code class="function"><a class="link" href="gobject-Type-Information.html#g-type-register-dynamic" title="g_type_register_dynamic ()">g_type_register_dynamic</a></code>. |
| </p> |
| <div class="sect1" title="Copy functions"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="gtype-copy"></a>Copy functions</h2></div></div></div> |
| <p> |
| The major common point between <span class="emphasis"><em>all</em></span> GLib types (fundamental and |
| non-fundamental, classed and non-classed, instantiable and non-instantiable) is that |
| they can all be manipulated through a single API to copy/assign them. |
| </p> |
| <p> |
| The <span class="type"><a class="link" href="gobject-Generic-values.html#GValue" title="GValue">GValue</a></span> structure is used as an abstract container for all of these |
| types. Its simplistic API (defined in <code class="filename">gobject/gvalue.h</code>) can be |
| used to invoke the value_table functions registered |
| during type registration: for example <code class="function"><a class="link" href="gobject-Generic-values.html#g-value-copy" title="g_value_copy ()">g_value_copy</a></code> copies the |
| content of a <span class="type"><a class="link" href="gobject-Generic-values.html#GValue" title="GValue">GValue</a></span> to another <span class="type"><a class="link" href="gobject-Generic-values.html#GValue" title="GValue">GValue</a></span>. 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. |
| </p> |
| <p> |
| The following code shows how you can copy around a 64 bit integer, as well as a <span class="type"><a class="link" href="gobject-The-Base-Object-Type.html#GObject">GObject</a></span> |
| instance pointer (sample code for this is located in the source tarball for this document in |
| <code class="filename">sample/gtype/test.c</code>): |
| </p> |
| <pre class="programlisting"> |
| static void test_int (void) |
| { |
| GValue a_value = {0, }; |
| GValue b_value = {0, }; |
| guint64 a, b; |
| |
| a = 0xdeadbeaf; |
| |
| 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 = {0, }; |
| GValue obj_valb = {0, }; |
| obj = g_object_new (MAMAN_TYPE_BAR, NULL); |
| |
| g_value_init (&obj_vala, MAMAN_TYPE_BAR); |
| 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 |
| MAMAN_TYPE_BAR is a G_TYPE_OBJECT. |
| */ |
| g_value_copy (&obj_vala, &obj_valb); |
| |
| g_object_unref (G_OBJECT (obj)); |
| g_object_unref (G_OBJECT (obj)); |
| } |
| </pre> |
| <p> |
| 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. |
| </p> |
| <p> |
| The value_table used to specify these assignment functions is defined in |
| <code class="filename">gtype.h</code> and is thoroughly described in the |
| API documentation provided with GObject (for once ;-) which is why we will |
| not detail its exact semantics. |
| </p> |
| <pre class="programlisting"> |
| typedef struct _GTypeValueTable GTypeValueTable; |
| struct _GTypeValueTable |
| { |
| void (*value_init) (GValue *value); |
| void (*value_free) (GValue *value); |
| void (*value_copy) (const GValue *src_value, |
| GValue *dest_value); |
| /* varargs functionality (optional) */ |
| gpointer (*value_peek_pointer) (const GValue *value); |
| gchar *collect_format; |
| gchar* (*collect_value) (GValue *value, |
| guint n_collect_values, |
| GTypeCValue *collect_values, |
| guint collect_flags); |
| gchar *lcopy_format; |
| gchar* (*lcopy_value) (const GValue *value, |
| guint n_collect_values, |
| GTypeCValue *collect_values, |
| guint collect_flags); |
| }; |
| </pre> |
| <p> |
| 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 which means that unless you want to write a |
| fundamental type (not a great idea!), you will not need to provide |
| a new value_table since you will inherit the value_table structure |
| from your parent type. |
| </p> |
| </div> |
| <div class="footnotes"> |
| <br><hr width="100" align="left"> |
| <div class="footnote"><p><sup>[<a name="ftn.id552292" href="#id552292" class="para">2</a>] </sup> |
| Please note that there exists another registration function: the |
| <code class="function"><a class="link" href="gobject-Type-Information.html#g-type-register-dynamic" title="g_type_register_dynamic ()">g_type_register_dynamic</a></code>. We will not discuss this |
| function here since its use is very similar to the <code class="function">_static</code> |
| version. |
| </p></div> |
| </div> |
| </div> |
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