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| <!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN" |
| "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" [ |
| ]> |
| <chapter id="chapter-gtype"> |
| <title>The GLib Dynamic Type System</title> |
| |
| <para> |
| 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. |
| <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); |
| </programlisting> |
| </para> |
| |
| <para> |
| <function><link linkend="g-type-register-static">g_type_register_static</link></function> and |
| <function><link linkend="g-type-register-fundamental">g_type_register_fundamental</link></function> |
| are the C functions, defined in |
| <filename>gtype.h</filename> and implemented in <filename>gtype.c</filename> |
| which you should use to register a new <type><link linkend="GType">GType</link></type> in the program's type system. |
| It is not likely you will ever need to use |
| <function><link linkend="g-type-register-fundamental">g_type_register_fundamental</link></function> (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. |
| <footnote> |
| <para> |
| Please note that there exists another registration function: the |
| <function><link linkend="g-type-register-dynamic">g_type_register_dynamic</link></function>. We will not discuss this |
| function here since its use is very similar to the <function>_static</function> |
| version. |
| </para> |
| </footnote> |
| </para> |
| |
| <para> |
| 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 <function><link linkend="g-type-init">g_type_init</link></function>, 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. |
| </para> |
| |
| <para> |
| Fundamental and non-fundamental types are defined by: |
| <itemizedlist> |
| <listitem><para> |
| class size: the class_size field in <type><link linkend="GTypeInfo">GTypeInfo</link></type>. |
| </para></listitem> |
| <listitem><para> |
| class initialization functions (C++ constructor): the base_init and |
| class_init fields in <type><link linkend="GTypeInfo">GTypeInfo</link></type>. |
| </para></listitem> |
| <listitem><para> |
| class destruction functions (C++ destructor): the base_finalize and |
| class_finalize fields in <type><link linkend="GTypeInfo">GTypeInfo</link></type>. |
| </para></listitem> |
| <listitem><para> |
| instance size (C++ parameter to new): the instance_size field in |
| <type><link linkend="GTypeInfo">GTypeInfo</link></type>. |
| </para></listitem> |
| <listitem><para> |
| instantiation policy (C++ type of new operator): the n_preallocs |
| field in <type><link linkend="GTypeInfo">GTypeInfo</link></type>. |
| </para></listitem> |
| <listitem><para> |
| copy functions (C++ copy operators): the value_table field in |
| <type><link linkend="GTypeInfo">GTypeInfo</link></type>. |
| </para></listitem> |
| <listitem><para> |
| type characteristic flags: <type><link linkend="GTypeFlags">GTypeFlags</link></type>. |
| </para></listitem> |
| </itemizedlist> |
| Fundamental types are also defined by a set of <type><link linkend="GTypeFundamentalFlags">GTypeFundamentalFlags</link></type> |
| which are stored in a <type><link linkend="GTypeFundamentalInfo">GTypeFundamentalInfo</link></type>. |
| Non-fundamental types are furthermore defined by the type of their parent which is |
| passed as the parent_type parameter to <function><link linkend="g-type-register-static">g_type_register_static</link></function> |
| and <function><link linkend="g-type-register-dynamic">g_type_register_dynamic</link></function>. |
| </para> |
| |
| <sect1 id="gtype-copy"> |
| <title>Copy functions</title> |
| |
| <para> |
| The major common point between <emphasis>all</emphasis> 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. |
| </para> |
| |
| <para> |
| The <type><link linkend="GValue">GValue</link></type> structure is used as an abstract container for all of these |
| types. Its simplistic API (defined in <filename>gobject/gvalue.h</filename>) can be |
| used to invoke the value_table functions registered |
| during type registration: for example <function><link linkend="g-value-copy">g_value_copy</link></function> copies the |
| content of a <type><link linkend="GValue">GValue</link></type> to another <type><link linkend="GValue">GValue</link></type>. 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. |
| </para> |
| |
| <para> |
| The following code shows how you can copy around a 64 bit integer, as well as a <type><link linkend="GObject">GObject</link></type> |
| instance pointer (sample code for this is located in the source tarball for this document in |
| <filename>sample/gtype/test.c</filename>): |
| <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)); |
| } |
| </programlisting> |
| 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. |
| </para> |
| |
| <para> |
| The value_table used to specify these assignment functions is defined in |
| <filename>gtype.h</filename> and is thoroughly described in the |
| API documentation provided with GObject (for once ;-) which is why we will |
| not detail its exact semantics. |
| <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); |
| }; |
| </programlisting> |
| 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. |
| </para> |
| </sect1> |
| |
| <sect1 id="gtype-conventions"> |
| <title>Conventions</title> |
| |
| |
| <para> |
| There are a number of conventions users are expected to follow when creating new types |
| which are to be exported in a header file: |
| <itemizedlist> |
| <listitem><para> |
| Use the <function>object_method</function> pattern for function names: to invoke |
| the method named foo on an instance of object type bar, call |
| <function>bar_foo</function>. |
| </para></listitem> |
| <listitem><para>Use prefixing to avoid namespace conflicts with other projects. |
| If your library (or application) is named <emphasis>Maman</emphasis>, |
| <footnote> |
| <para> |
| <emphasis>Maman</emphasis> is the French word for <emphasis>mum</emphasis> |
| or <emphasis>mother</emphasis> - nothing more and nothing less. |
| </para> |
| </footnote> |
| |
| prefix all your function names with <emphasis>maman_</emphasis>. |
| For example: <function>maman_object_method</function>. |
| </para></listitem> |
| <listitem><para>Create a macro named <function>PREFIX_TYPE_OBJECT</function> which always |
| returns the GType for the associated object type. For an object of type |
| <emphasis>Bar</emphasis> in a library prefixed by <emphasis>maman</emphasis>, |
| use: <function>MAMAN_TYPE_BAR</function>. |
| It is common although not a convention to implement this macro using either a global |
| static variable or a function named <function>prefix_object_get_type</function>. |
| We will follow the function pattern wherever possible in this document. |
| </para></listitem> |
| <listitem><para>Create a macro named <function>PREFIX_OBJECT (obj)</function> which |
| returns a pointer of type <type>PrefixObject</type>. This macro is used to enforce |
| static type safety by doing explicit casts wherever needed. It also enforces |
| dynamic type safety by doing runtime checks. It is possible to disable the dynamic |
| type checks in production builds (see <link linkend="glib-building">building glib</link>). |
| For example, we would create |
| <function>MAMAN_BAR (obj)</function> to keep the previous example. |
| </para></listitem> |
| <listitem><para>If the type is classed, create a macro named |
| <function>PREFIX_OBJECT_CLASS (klass)</function>. This macro |
| is strictly equivalent to the previous casting macro: it does static casting with |
| dynamic type checking of class structures. It is expected to return a pointer |
| to a class structure of type <type>PrefixObjectClass</type>. Again, an example is: |
| <function>MAMAN_BAR_CLASS</function>. |
| </para></listitem> |
| <listitem><para>Create a macro named <function>PREFIX_IS_BAR (obj)</function>: this macro is expected |
| to return a <type>gboolean</type> which indicates whether or not the input |
| object instance pointer of type BAR. |
| </para></listitem> |
| <listitem><para>If the type is classed, create a macro named |
| <function>PREFIX_IS_OBJECT_CLASS (klass)</function> which, as above, returns a boolean |
| if the input class pointer is a pointer to a class of type OBJECT. |
| </para></listitem> |
| <listitem><para>If the type is classed, create a macro named |
| <function>PREFIX_OBJECT_GET_CLASS (obj)</function> |
| which returns the class pointer associated to an instance of a given type. This macro |
| is used for static and dynamic type safety purposes (just like the previous casting |
| macros). |
| </para></listitem> |
| </itemizedlist> |
| The implementation of these macros is pretty straightforward: a number of simple-to-use |
| macros are provided in <filename>gtype.h</filename>. For the example we used above, we would |
| write the following trivial code to declare the macros: |
| <programlisting> |
| #define MAMAN_TYPE_BAR (maman_bar_get_type ()) |
| #define MAMAN_BAR(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_BAR, MamanBar)) |
| #define MAMAN_BAR_CLASS(klass) (G_TYPE_CHECK_CLASS_CAST ((klass), MAMAN_TYPE_BAR, MamanBarClass)) |
| #define MAMAN_IS_BAR(obj) (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_BAR)) |
| #define MAMAN_IS_BAR_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), MAMAN_TYPE_BAR)) |
| #define MAMAN_BAR_GET_CLASS(obj) (G_TYPE_INSTANCE_GET_CLASS ((obj), MAMAN_TYPE_BAR, MamanBarClass)) |
| </programlisting> |
| <note><simpara>Stick to the naming <varname>klass</varname> as <varname>class</varname> is a registered c++ keyword.</simpara></note> |
| </para> |
| |
| <para> |
| The following code shows how to implement the <function>maman_bar_get_type</function> |
| function: |
| <programlisting> |
| GType maman_bar_get_type (void) |
| { |
| static GType type = 0; |
| if (type == 0) { |
| static const GTypeInfo info = { |
| /* You fill this structure. */ |
| }; |
| type = g_type_register_static (G_TYPE_OBJECT, |
| "MamanBarType", |
| &info, 0); |
| } |
| return type; |
| } |
| </programlisting> |
| </para> |
| |
| <para> |
| When having no special requirements you also can use the <function>G_DEFINE_TYPE</function> |
| macro: |
| <programlisting> |
| G_DEFINE_TYPE (MamanBar, maman_bar, G_TYPE_OBJECT) |
| </programlisting> |
| </para> |
| |
| </sect1> |
| |
| <sect1 id="gtype-non-instantiable"> |
| <title>Non-instantiable non-classed fundamental types</title> |
| |
| <para> |
| A lot of types are not instantiable by the type system and do not have |
| a class. Most of these types are fundamental trivial types such as <emphasis>gchar</emphasis>, |
| registered in <function>g_value_types_init</function> (in <filename>gvaluetypes.c</filename>). |
| </para> |
| |
| <para> |
| To register such a type in the type system, you just need to fill the |
| <type><link linkend="GTypeInfo">GTypeInfo</link></type> structure with zeros since these types are also most of the time |
| fundamental: |
| <programlisting> |
| GTypeInfo info = { |
| 0, /* class_size */ |
| NULL, /* base_init */ |
| NULL, /* base_destroy */ |
| NULL, /* class_init */ |
| NULL, /* class_destroy */ |
| NULL, /* class_data */ |
| 0, /* instance_size */ |
| 0, /* n_preallocs */ |
| NULL, /* instance_init */ |
| NULL, /* value_table */ |
| }; |
| static const GTypeValueTable value_table = { |
| value_init_long0, /* value_init */ |
| NULL, /* value_free */ |
| value_copy_long0, /* value_copy */ |
| NULL, /* value_peek_pointer */ |
| "i", /* collect_format */ |
| value_collect_int, /* collect_value */ |
| "p", /* lcopy_format */ |
| value_lcopy_char, /* lcopy_value */ |
| }; |
| info.value_table = &value_table; |
| type = g_type_register_fundamental (G_TYPE_CHAR, "gchar", &info, &finfo, 0); |
| </programlisting> |
| </para> |
| |
| |
| <para> |
| Having non-instantiable types might seem a bit useless: what good is a type |
| if you cannot instantiate an instance of that type ? Most of these types |
| are used in conjunction with <type><link linkend="GValue">GValue</link></type>s: a GValue is initialized |
| with an integer or a string and it is passed around by using the registered |
| type's value_table. <type><link linkend="GValue">GValue</link></type>s (and by extension these trivial fundamental |
| types) are most useful when used in conjunction with object properties and signals. |
| </para> |
| |
| </sect1> |
| |
| <sect1 id="gtype-instantiable-classed"> |
| <title>Instantiable classed types: objects</title> |
| |
| <para> |
| Types which are registered with a class and are declared instantiable are |
| what most closely resembles an <emphasis>object</emphasis>. |
| Although <type><link linkend="GObject">GObject</link></type>s (detailed in <xref linkend="chapter-gobject"/>) |
| are the most well known type of instantiable |
| classed types, other kinds of similar objects used as the base of an inheritance |
| hierarchy have been externally developed and they are all built on the fundamental |
| features described below. |
| </para> |
| |
| <para> |
| For example, the code below shows how you could register |
| such a fundamental object type in the type system: |
| <programlisting> |
| typedef struct { |
| GObject parent; |
| /* instance members */ |
| int field_a; |
| } MamanBar; |
| |
| typedef struct { |
| GObjectClass parent; |
| /* class members */ |
| void (*do_action_public_virtual) (MamanBar *self, guint8 i); |
| |
| void (*do_action_public_pure_virtual) (MamanBar *self, guint8 i); |
| } MamanBarClass; |
| |
| #define MAMAN_TYPE_BAR (maman_bar_get_type ()) |
| |
| GType |
| maman_bar_get_type (void) |
| { |
| static GType type = 0; |
| if (type == 0) { |
| static const GTypeInfo info = { |
| sizeof (MamanBarClass), |
| NULL, /* base_init */ |
| NULL, /* base_finalize */ |
| (GClassInitFunc) foo_class_init, |
| NULL, /* class_finalize */ |
| NULL, /* class_data */ |
| sizeof (MamanBar), |
| 0, /* n_preallocs */ |
| (GInstanceInitFunc) NULL /* instance_init */ |
| }; |
| type = g_type_register_static (G_TYPE_OBJECT, |
| "BarType", |
| &info, 0); |
| } |
| return type; |
| } |
| </programlisting> |
| Upon the first call to <function>maman_bar_get_type</function>, the type named |
| <emphasis>BarType</emphasis> will be registered in the type system as inheriting |
| from the type <emphasis>G_TYPE_OBJECT</emphasis>. |
| </para> |
| |
| <para> |
| Every object must define two structures: its class structure and its |
| instance structure. All class structures must contain as first member |
| a <type><link linkend="GTypeClass">GTypeClass</link></type> structure. All instance structures must contain as first |
| member a <type><link linkend="GTypeInstance">GTypeInstance</link></type> structure. The declaration of these C types, |
| coming from <filename>gtype.h</filename> is shown below: |
| <programlisting> |
| struct _GTypeClass |
| { |
| GType g_type; |
| }; |
| struct _GTypeInstance |
| { |
| GTypeClass *g_class; |
| }; |
| </programlisting> |
| These constraints allow the type system to make sure that every object instance |
| (identified by a pointer to the object's instance structure) contains in its |
| first bytes a pointer to the object's class structure. |
| </para> |
| <para> |
| This relationship is best explained by an example: let's take object B which |
| inherits from object A: |
| <programlisting> |
| /* A definitions */ |
| typedef struct { |
| GTypeInstance parent; |
| int field_a; |
| int field_b; |
| } A; |
| typedef struct { |
| GTypeClass parent_class; |
| void (*method_a) (void); |
| void (*method_b) (void); |
| } AClass; |
| |
| /* B definitions. */ |
| typedef struct { |
| A parent; |
| int field_c; |
| int field_d; |
| } B; |
| typedef struct { |
| AClass parent_class; |
| void (*method_c) (void); |
| void (*method_d) (void); |
| } BClass; |
| </programlisting> |
| The C standard mandates that the first field of a C structure is stored starting |
| in the first byte of the buffer used to hold the structure's fields in memory. |
| This means that the first field of an instance of an object B is A's first field |
| which in turn is GTypeInstance's first field which in turn is g_class, a pointer |
| to B's class structure. |
| </para> |
| |
| <para> |
| Thanks to these simple conditions, it is possible to detect the type of every |
| object instance by doing: |
| <programlisting> |
| B *b; |
| b->parent.parent.g_class->g_type |
| </programlisting> |
| or, more quickly: |
| <programlisting> |
| B *b; |
| ((GTypeInstance*)b)->g_class->g_type |
| </programlisting> |
| </para> |
| |
| <sect2 id="gtype-instantiable-classed-init-done"> |
| <title>Initialization and Destruction</title> |
| |
| <para> |
| instantiation of these types can be done with |
| <function><link linkend="g-type-create-instance">g_type_create_instance</link></function>: |
| <programlisting> |
| GTypeInstance* g_type_create_instance (GType type); |
| void g_type_free_instance (GTypeInstance *instance); |
| </programlisting> |
| <function><link linkend="g-type-create-instance">g_type_create_instance</link></function> will look up the type information |
| structure associated to the type requested. Then, the instance size and instantiation |
| policy (if the n_preallocs field is set to a non-zero value, the type system allocates |
| the object's instance structures in chunks rather than mallocing for every instance) |
| declared by the user are used to get a buffer to hold the object's instance |
| structure. |
| </para> |
| |
| <para> |
| If this is the first instance of the object ever created, the type system must create |
| a class structure: it allocates a buffer to hold the object's class structure and |
| initializes it. It first copies the parent's class structure over this structure |
| (if there is no parent, it initializes it to zero). It then invokes the |
| base_class_initialization functions (<type><link linkend="GBaseInitFunc">GBaseInitFunc</link></type>) from topmost |
| fundamental object to bottom-most most derived object. The object's class_init |
| (<type><link linkend="GClassInitFunc">GClassInitFunc</link></type>) function is invoked afterwards to complete |
| initialization of the class structure. |
| Finally, the object's interfaces are initialized (we will discuss interface initialization |
| in more detail later). |
| </para> |
| |
| <para> |
| Once the type system has a pointer to an initialized class structure, it sets the object's |
| instance class pointer to the object's class structure and invokes the object's |
| instance_init (<type><link linkend="GInstanceInitFunc">GInstanceInitFunc</link></type>)functions, from top-most fundamental |
| type to bottom-most most derived type. |
| </para> |
| |
| <para> |
| Object instance destruction through <function><link linkend="g-type-free-instance">g_type_free_instance</link></function> is very simple: |
| the instance structure is returned to the instance pool if there is one and if this was the |
| last living instance of the object, the class is destroyed. |
| </para> |
| |
| |
| <para> |
| Class destruction (the concept of destruction is sometimes partly |
| referred to as finalization in GType) is the symmetric process of |
| the initialization: interfaces are destroyed first. |
| Then, the most derived |
| class_finalize (<type><link linkend="ClassFinalizeFunc">ClassFinalizeFunc</link></type>) function is invoked. The |
| base_class_finalize (<type><link linkend="GBaseFinalizeFunc">GBaseFinalizeFunc</link></type>) functions are |
| Finally invoked from bottom-most most-derived type to top-most fundamental type and |
| the class structure is freed. |
| </para> |
| |
| <para> |
| As many readers have now understood it, the base initialization/finalization process is |
| very similar to the C++ constructor/destructor paradigm. The practical details are different |
| though and it is important not to get confused by superficial similarities. |
| GTypes have no instance destruction mechanism. It is |
| the user's responsibility to implement correct destruction semantics on top |
| of the existing GType code. (this is what GObject does. See |
| <xref linkend="chapter-gobject"/>) |
| Furthermore, C++ code equivalent to the base_init |
| and class_init callbacks of GType is usually not needed because C++ cannot really create object |
| types at runtime. |
| </para> |
| |
| <para> |
| The instantiation/finalization process can be summarized as follows: |
| <table id="gtype-init-fini-table"> |
| <title>GType Instantiation/Finalization</title> |
| <tgroup cols="3"> |
| <colspec colwidth="*" colnum="1" align="left"/> |
| <colspec colwidth="*" colnum="2" align="left"/> |
| <colspec colwidth="8*" colnum="3" align="left"/> |
| |
| <thead> |
| <row> |
| <entry>Invocation time</entry> |
| <entry>Function Invoked</entry> |
| <entry>Function's parameters</entry> |
| </row> |
| </thead> |
| <tbody> |
| <row> |
| <entry morerows="2">First call to <function><link linkend="g-type-create-instance">g_type_create_instance</link></function> for target type</entry> |
| <entry>type's base_init function</entry> |
| <entry>On the inheritance tree of classes from fundamental type to target type. |
| base_init is invoked once for each class structure.</entry> |
| </row> |
| <row> |
| <!--entry>First call to <function><link linkend="g-type-create-instance">g_type_create_instance</link></function> for target type</entry--> |
| <entry>target type's class_init function</entry> |
| <entry>On target type's class structure</entry> |
| </row> |
| <row> |
| <!--entry>First call to <function><link linkend="g-type-create-instance">g_type_create_instance</link></function> for target type</entry--> |
| <entry>interface initialization, see |
| <xref linkend="gtype-non-instantiable-classed-init"/></entry> |
| <entry></entry> |
| </row> |
| <row> |
| <entry>Each call to <function><link linkend="g-type-create-instance">g_type_create_instance</link></function> for target type</entry> |
| <entry>target type's instance_init function</entry> |
| <entry>On object's instance</entry> |
| </row> |
| <row> |
| <entry morerows="2">Last call to <function><link linkend="g-type-free-instance">g_type_free_instance</link></function> for target type</entry> |
| <entry>interface destruction, see |
| <xref linkend="gtype-non-instantiable-classed-dest"/></entry> |
| <entry></entry> |
| </row> |
| <row> |
| <!--entry>Last call to <function><link linkend="g-type-free-instance">g_type_free_instance</link></function> for target type</entry--> |
| <entry>target type's class_finalize function</entry> |
| <entry>On target type's class structure</entry> |
| </row> |
| <row> |
| <!--entry>Last call to <function><link linkend="g-type-free-instance">g_type_free_instance</link></function> for target type</entry--> |
| <entry>type's base_finalize function</entry> |
| <entry>On the inheritance tree of classes from fundamental type to target type. |
| base_finalize is invoked once for each class structure.</entry> |
| </row> |
| </tbody> |
| </tgroup> |
| </table> |
| </para> |
| |
| </sect2> |
| |
| </sect1> |
| |
| <sect1 id="gtype-non-instantiable-classed"> |
| <title>Non-instantiable classed types: interfaces</title> |
| |
| <para> |
| GType's interfaces are very similar to Java's interfaces. They allow |
| to describe a common API that several classes will adhere to. |
| Imagine the play, pause and stop buttons on hi-fi equipment - those can |
| be seen as a playback interface. Once you know what they do, you can |
| control your CD player, MP3 player or anything that uses these symbols. |
| To declare an interface you have to register a non-instantiable |
| classed type which derives from |
| <type><link linkend="GTypeInterface">GTypeInterface</link></type>. The following piece of code declares such an interface. |
| <programlisting> |
| #define MAMAN_IBAZ_TYPE (maman_ibaz_get_type ()) |
| #define MAMAN_IBAZ(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_IBAZ_TYPE, MamanIbaz)) |
| #define MAMAN_IS_IBAZ(obj) (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_IBAZ_TYPE)) |
| #define MAMAN_IBAZ_GET_INTERFACE(inst) (G_TYPE_INSTANCE_GET_INTERFACE ((inst), MAMAN_IBAZ_TYPE, MamanIbazInterface)) |
| |
| typedef struct _MamanIbaz MamanIbaz; /* dummy object */ |
| typedef struct _MamanIbazInterface MamanIbazInterface; |
| |
| struct _MamanIbazInterface { |
| GTypeInterface parent; |
| |
| void (*do_action) (MamanIbaz *self); |
| }; |
| |
| GType maman_ibaz_get_type (void); |
| |
| void maman_ibaz_do_action (MamanIbaz *self); |
| </programlisting> |
| The interface function, <function>maman_ibaz_do_action</function> is implemented |
| in a pretty simple way: |
| <programlisting> |
| void maman_ibaz_do_action (MamanIbaz *self) |
| { |
| MAMAN_IBAZ_GET_INTERFACE (self)->do_action (self); |
| } |
| </programlisting> |
| <function>maman_ibaz_get_type</function> registers a type named <emphasis>MamanIBaz</emphasis> |
| which inherits from G_TYPE_INTERFACE. All interfaces must be children of G_TYPE_INTERFACE in the |
| inheritance tree. |
| </para> |
| |
| <para> |
| An interface is defined by only one structure which must contain as first member |
| a <type><link linkend="GTypeInterface">GTypeInterface</link></type> structure. The interface structure is expected to |
| contain the function pointers of the interface methods. It is good style to |
| define helper functions for each of the interface methods which simply call |
| the interface' method directly: <function>maman_ibaz_do_action</function> |
| is one of these. |
| </para> |
| |
| <para> |
| Once an interface type is registered, you must register implementations for these |
| interfaces. The function named <function>maman_baz_get_type</function> registers |
| a new GType named MamanBaz which inherits from <type><link linkend="GObject">GObject</link></type> and which |
| implements the interface <type>MamanIBaz</type>. |
| <programlisting> |
| static void maman_baz_do_action (MamanIbaz *self) |
| { |
| g_print ("Baz implementation of IBaz interface Action.\n"); |
| } |
| |
| |
| static void |
| baz_interface_init (gpointer g_iface, |
| gpointer iface_data) |
| { |
| MamanIbazInterface *iface = (MamanIbazInterface *)g_iface; |
| iface->do_action = maman_baz_do_action; |
| } |
| |
| GType |
| maman_baz_get_type (void) |
| { |
| static GType type = 0; |
| if (type == 0) { |
| static const GTypeInfo info = { |
| sizeof (MamanBazInterface), |
| NULL, /* base_init */ |
| NULL, /* base_finalize */ |
| NULL, /* class_init */ |
| NULL, /* class_finalize */ |
| NULL, /* class_data */ |
| sizeof (MamanBaz), |
| 0, /* n_preallocs */ |
| NULL /* instance_init */ |
| }; |
| static const GInterfaceInfo ibaz_info = { |
| (GInterfaceInitFunc) baz_interface_init, /* interface_init */ |
| NULL, /* interface_finalize */ |
| NULL /* interface_data */ |
| }; |
| type = g_type_register_static (G_TYPE_OBJECT, |
| "MamanBazType", |
| &info, 0); |
| g_type_add_interface_static (type, |
| MAMAN_IBAZ_TYPE, |
| &ibaz_info); |
| } |
| return type; |
| } |
| </programlisting> |
| </para> |
| |
| <para> |
| <function><link linkend="g-type-add-interface-static">g_type_add_interface_static</link></function> records in the type system that |
| a given type implements also <type>FooInterface</type> |
| (<function>foo_interface_get_type</function> returns the type of |
| <type>FooInterface</type>). |
| The <type><link linkend="GInterfaceInfo">GInterfaceInfo</link></type> structure holds |
| information about the implementation of the interface: |
| <programlisting> |
| struct _GInterfaceInfo |
| { |
| GInterfaceInitFunc interface_init; |
| GInterfaceFinalizeFunc interface_finalize; |
| gpointer interface_data; |
| }; |
| </programlisting> |
| </para> |
| <para> |
| When having no special requirements you also can use the <function>G_DEFINE_INTERFACE</function> macro: |
| <programlisting> |
| G_DEFINE_INTERFACE (MamanBaz, maman_baz, G_TYPE_OBJECT) |
| </programlisting> |
| </para> |
| |
| |
| <sect2 id="gtype-non-instantiable-classed-init"> |
| <title>Interface Initialization</title> |
| |
| <para> |
| When an instantiable classed type which registered an interface |
| implementation is created for the first time, its class structure |
| is initialized following the process |
| described in <xref linkend="gtype-instantiable-classed"/>. |
| After that, the interface implementations associated with |
| the type are initialized. |
| </para> |
| |
| <para> |
| First a memory buffer is allocated to hold the interface structure. The parent's |
| interface structure is then copied over to the new interface structure (the parent |
| interface is already initialized at that point). If there is no parent interface, |
| the interface structure is initialized with zeros. The g_type and the g_instance_type |
| fields are then initialized: g_type is set to the type of the most-derived interface |
| and g_instance_type is set to the type of the most derived type which implements |
| this interface. |
| </para> |
| |
| <para> |
| Finally, the interface' most-derived <function>base_init</function> function and then |
| the implementation's <function>interface_init</function> |
| function are invoked. It is important to understand that if there are multiple |
| implementations of an interface the <function>base_init</function> and |
| <function>interface_init</function> functions will be |
| invoked once for each implementation initialized. |
| </para> |
| |
| <para> |
| It is thus common for base_init functions to hold a local static boolean variable |
| which makes sure that the interface type is initialized only once even if there are |
| multiple implementations of the interface: |
| <programlisting> |
| static void |
| maman_ibaz_base_init (gpointer g_iface) |
| { |
| static gboolean initialized = FALSE; |
| |
| if (!initialized) { |
| /* create interface signals here. */ |
| initialized = TRUE; |
| } |
| } |
| </programlisting> |
| </para> |
| |
| <para> |
| If you have found the stuff about interface hairy, you are right: it is hairy but |
| there is not much I can do about it. What I can do is summarize what you need to know |
| about interfaces: |
| </para> |
| |
| <para> |
| The above process can be summarized as follows: |
| <table id="ginterface-init-table"> |
| <title>Interface Initialization</title> |
| <tgroup cols="3"> |
| <colspec colwidth="*" colnum="1" align="left"/> |
| <colspec colwidth="*" colnum="2" align="left"/> |
| <colspec colwidth="8*" colnum="3" align="left"/> |
| |
| <thead> |
| <row> |
| <entry>Invocation time</entry> |
| <entry>Function Invoked</entry> |
| <entry>Function's parameters</entry> |
| <entry>Remark</entry> |
| </row> |
| </thead> |
| <tbody> |
| <row> |
| <entry morerows="1">First call to <function><link linkend="g-type-create-instance">g_type_create_instance</link></function> for type |
| implementing interface |
| </entry> |
| <entry>interface' base_init function</entry> |
| <entry>On interface' vtable</entry> |
| <entry>Register interface' signals here (use a local static |
| boolean variable as described above to make sure not to register them |
| twice.).</entry> |
| </row> |
| <row> |
| <!--entry>First call to <function><link linkend="g-type-create-instance">g_type_create_instance</link></function> for type |
| implementing interface |
| </entry--> |
| <entry>interface' interface_init function</entry> |
| <entry>On interface' vtable</entry> |
| <entry> |
| Initialize interface' implementation. That is, initialize the interface |
| method pointers in the interface structure to the function's implementation. |
| </entry> |
| </row> |
| </tbody> |
| </tgroup> |
| </table> |
| It is highly unlikely (i.e. I do not know of <emphasis>anyone</emphasis> who actually |
| used it) you will ever need other more fancy things such as the ones described in the |
| following section (<xref linkend="gtype-non-instantiable-classed-dest"/>). |
| </para> |
| |
| </sect2> |
| |
| <sect2 id="gtype-non-instantiable-classed-dest"> |
| <title>Interface Destruction</title> |
| |
| <para> |
| When the last instance of an instantiable type which registered |
| an interface implementation is destroyed, the interface's |
| implementations associated to the type are destroyed. |
| </para> |
| |
| <para> |
| To destroy an interface implementation, GType first calls the |
| implementation's <function>interface_finalize</function> function |
| and then the interface's most-derived |
| <function>base_finalize</function> function. |
| </para> |
| |
| <para> |
| Again, it is important to understand, as in |
| <xref linkend="gtype-non-instantiable-classed-init"/>, |
| that both <function>interface_finalize</function> and <function>base_finalize</function> |
| are invoked exactly once for the destruction of each implementation of an interface. Thus, |
| if you were to use one of these functions, you would need to use a static integer variable |
| which would hold the number of instances of implementations of an interface such that |
| the interface's class is destroyed only once (when the integer variable reaches zero). |
| </para> |
| |
| <para> |
| The above process can be summarized as follows: |
| <table id="ginterface-fini-table"> |
| <title>Interface Finalization</title> |
| <tgroup cols="3"> |
| <colspec colwidth="*" colnum="1" align="left"/> |
| <colspec colwidth="*" colnum="2" align="left"/> |
| <colspec colwidth="8*" colnum="3" align="left"/> |
| |
| <thead> |
| <row> |
| <entry>Invocation time</entry> |
| <entry>Function Invoked</entry> |
| <entry>Function's parameters</entry> |
| </row> |
| </thead> |
| <tbody> |
| <row> |
| <entry morerows="1">Last call to <function><link linkend="g-type-free-instance">g_type_free_instance</link></function> for type |
| implementing interface |
| </entry> |
| <entry>interface' interface_finalize function</entry> |
| <entry>On interface' vtable</entry> |
| </row> |
| <row> |
| <!--entry>Last call to <function><link linkend="g-type-free-instance">g_type_free_instance</link></function>for type |
| implementing interface |
| </entry--> |
| <entry>interface' base_finalize function</entry> |
| <entry>On interface' vtable</entry> |
| </row> |
| </tbody> |
| </tgroup> |
| </table> |
| </para> |
| </sect2> |
| </sect1> |
| </chapter> |