Patch from Havoc Pennington to add functions for setting and getting a

[platform/upstream/glib.git] / docs / reference / gobject / tmpl / gtype.sgml

<!-- ##### SECTION Title ##### -->
GType

<!-- ##### SECTION Short_Description ##### -->
The GLib Runtime type identification and management system

<!-- ##### SECTION Long_Description ##### -->
<para>
The GType API is the foundation of the GObject system.  It provides the
facilities for registering and managing all fundamental data types,
user-defined object and interface types.  Before using any GType
or GObject functions, g_type_init() must be called to initialize the
type system.
</para>
<para>
For type creation and registration purposes, all types fall into one of
two categories: static or dynamic.  Static types are never loaded or
unloaded at run-time as dynamic types may be.  Static types are created
with g_type_register_static() that gets type specific information passed
in via a #GTypeInfo structure.
Dynamic types are created with g_type_register_dynamic() which takes a
#GTypePlugin structure instead. The remaining type information (the
#GTypeInfo structure) is retrieved during runtime through #GTypePlugin
and the g_type_plugin_*() API.
These registration functions are usually called only once from a 
function whose only purpose is to return the type identifier for a 
specific class.  Once the type (or class or interface) is registered,
it may be instantiated, inherited, or implemented depending on exactly
what sort of type it is.
There is also a third registration function for registering fundamental
types called g_type_register_fundamental() which requires both a #GTypeInfo
structure and a GTypeFundamentalInfo structure but it is seldom used
since most fundamental types are predefined rather than user-defined.
</para>

<!-- ##### SECTION See_Also ##### -->
<para>

</para>

<!-- ##### TYPEDEF GType ##### -->
<para>
A numerical value which represents the unique identifier of a registered
type.
</para>


<!-- ##### MACRO G_TYPE_FUNDAMENTAL ##### -->
<para>
Returns %TRUE if @type is a fundamental data type such as #G_TYPE_INT or
#G_TYPE_POINTER. Fundamental types are types that serve as fundaments for
the derived types, thus they are the roots of distinct inheritance hierarchies.
</para>

@type: A #GType value.


<!-- ##### MACRO G_TYPE_FUNDAMENTAL_MAX ##### -->
<para>
An integer constant that represents the number of identifiers reserved
for types that are assigned at compile-time.
</para>



<!-- ##### MACRO G_TYPE_MAKE_FUNDAMENTAL ##### -->
<para>
Returns the type ID for the fundamental type number @x.
Use g_type_fundamental_next() instead of this macro to create new fundamental 
types.
</para>

@x: the fundamental type number.


<!-- ##### MACRO G_TYPE_IS_ABSTRACT ##### -->
<para>
Returns %TRUE if @type is an abstract type.  An abstract type can not be
instantiated and is normally used as an abstract base class for
derived classes.
</para>

@type: A #GType value.


<!-- ##### MACRO G_TYPE_IS_DERIVED ##### -->
<para>
Returns %TRUE if @type is derived (or in object-oriented terminology:
inherited) from another type (this holds true for all non-fundamental
types).
</para>

@type: A #GType value.


<!-- ##### MACRO G_TYPE_IS_FUNDAMENTAL ##### -->
<para>
Returns %TRUE if @type is a fundamental type.
</para>

@type: A #GType value.


<!-- ##### MACRO G_TYPE_IS_VALUE_TYPE ##### -->
<para>

</para>

@type: A #GType value.


<!-- ##### MACRO G_TYPE_HAS_VALUE_TABLE ##### -->
<para>
Returns %TRUE if @type has a #GTypeValueTable.
</para>

@type: A #GType value.


<!-- ##### MACRO G_TYPE_IS_CLASSED ##### -->
<para>
Returns %TRUE if @type is a classed type.
</para>

@type: A #GType value.


<!-- ##### MACRO G_TYPE_IS_INSTANTIATABLE ##### -->
<para>
Returns %TRUE if @type can be instantiated.  Instantiation is the
process of creating an instance (object) of this type.
</para>

@type: A #GType value.


<!-- ##### MACRO G_TYPE_IS_DERIVABLE ##### -->
<para>
Returns %TRUE if @type is a derivable type.  A derivable type can
be used as the base class of a flat (single-level) class hierarchy.
</para>

@type: A #GType value.


<!-- ##### MACRO G_TYPE_IS_DEEP_DERIVABLE ##### -->
<para>
Returns %TRUE if @type is a deep derivable type.  A deep derivable type
can be used as the base class of a deep (multi-level) class hierarchy.
</para>

@type: A #GType value.


<!-- ##### MACRO G_TYPE_IS_INTERFACE ##### -->
<para>
Returns %TRUE if @type is an interface type.
Interface types are types that provide pure APIs, the implementation
of which is provided by another type (which is then said to conform
to the interface).  GLib interfaces are somewhat analogous to Java
interfaces and C++ classes containing only pure virtual functions, 
with the difference that GType interfaces are not derivable (but see
g_type_interface_add_prerequisite() for an alternative).
</para>

@type: A #GType value.


<!-- ##### STRUCT GTypeInterface ##### -->
<para>
An opaque structure used as the base of all interface types.
</para>


<!-- ##### STRUCT GTypeInstance ##### -->
<para>
An opaque structure used as the base of all type instances.
</para>


<!-- ##### STRUCT GTypeInfo ##### -->
<para>
This structure is used to provide the type system with the information
required to initialize and destruct (finalize) a type's class and
instances thereof.
The initialized structure is passed to the g_type_register_static() function
(or is copied into the provided #GTypeInfo structure in the
g_type_plugin_complete_type_info()). The type system will perform a deep
copy of this structure, so it's memory does not need to be persistent
across invocation of g_type_register_static().
</para>

@class_size:     Size of the class structure (required for interface, classed and instantiatable types).
@base_init:      Location of the base initialization function (optional).
@base_finalize:  Location of the base finalization function (optional).
@class_init:     Location of the class initialization function (optional, for classed and instantiatable types only).
@class_finalize: Location of the class finalization function (optional).
@class_data:     User-supplied data passed to the class init/finalize functions.
@instance_size:  Size of the instance (object) structure (required for instantiatable types only).
@n_preallocs:    Number of pre-allocated (cached) instances to reserve memory for (0 indicates no caching).
@instance_init:  Location of the instance initialization function (optional, for instantiatable types only).
@value_table:    A #GTypeValueTable function table for generic handling of GValues of this type (usually only
                 useful for fundamental types).

<!-- ##### STRUCT GTypeFundamentalInfo ##### -->
<para>
A structure that provides information to the type system which is
used specifically for managing fundamental types.  
</para>

@type_flags: 

<!-- ##### STRUCT GInterfaceInfo ##### -->
<para>
A structure that provides information to the type system which is
used specifically for managing interface types.
</para>

@interface_init:        Location of the function that initializes the interface.
@interface_finalize:    Location of the function that finalizes the interface.
@interface_data:        Location of user data passed to the @interface_init and
                        @interface_finalize functions (optional).

<!-- ##### STRUCT GTypeValueTable ##### -->
<para>
The #GTypeValueTable provides the functions required by the #GValue implementation,
to serve as a container for values of a type.
</para>

@value_init:            Default initialize @values contents by poking values
                        directly into the value-&gt;data array. The data array of
                        the #GValue passed into this function was zero-filled
                        with <function>memset()</function>, so no care has to
                        be taken to free any
                        old contents. E.g. for the implementation of a string
                        value that may never be %NULL, the implementation might
                        look like:
<programlisting>
{
  value-&gt;data[0].v_pointer = g_strdup ("");
}
</programlisting>
@value_free:            Free any old contents that might be left in the
                        data array of the passed in @value. No resources may
                        remain allocated through the #GValue contents after
                        this function returns. E.g. for our above string type:
<programlisting>
{
  /* only free strings without a specific flag for static storage */
  if (!(value-&gt;data[1].v_uint &amp; G_VALUE_NOCOPY_CONTENTS))
    g_free (value-&gt;data[0].v_pointer);
}
</programlisting>
@value_copy:            @dest_value is a #GValue with zero-filled data section
                        and @src_value is a properly setup #GValue of same or
                        derived type.
                        The purpose of this function is to copy the contents of
                        @src_value into @dest_value in a way, that even after
                        @src_value has been freed, the contents of @dest_value
                        remain valid. String type example:
<programlisting>
{
  dest_value-&gt;data[0].v_pointer = g_strdup (src_value-&gt;data[0].v_pointer);
}
</programlisting>
@value_peek_pointer:    If the value contents fit into a pointer, such as objects
                        or strings, return this pointer, so the caller can peek at
                        the current contents. To extend on our above string example:
<programlisting>
{
  return value-&gt;data[0].v_pointer;
}
</programlisting>
@collect_format:        A string format describing how to collect the contents of
                        this value, bit-by-bit. Each character in the format represents
                        an argument to be collected, the characters themselves indicate
                        the type of the argument. Currently supported arguments are:
<variablelist>
  <varlistentry><term></term><listitem><para>
        'i' - Integers. passed as collect_values[].v_int.
  </para></listitem></varlistentry>
  <varlistentry><term></term><listitem><para>
        'l' - Longs. passed as collect_values[].v_long.
  </para></listitem></varlistentry>
  <varlistentry><term></term><listitem><para>
        'd' - Doubles. passed as collect_values[].v_double.
  </para></listitem></varlistentry>
  <varlistentry><term></term><listitem><para>
        'p' - Pointers. passed as collect_values[].v_pointer.
  </para></listitem></varlistentry>
</variablelist>
                        It should be noted, that for variable argument list construction,
                        ANSI C promotes every type smaller than an integer to an int, and
                        floats to doubles. So for collection of short int or char, 'i'
                        needs to be used, and for collection of floats 'd'.
@collect_value:         The collect_value() function is responsible for converting the
                        values collected from a variable argument list into contents
                        suitable for storage in a GValue. This function should setup
                        @value similar to value_init(), e.g. for a string value that
                        does not allow %NULL pointers, it needs to either spew an error,
                        or do an implicit conversion by storing an empty string.
                        The @value passed in to this function has a zero-filled data
                        array, so just like for value_init() it is guaranteed to not
                        contain any old contents that might need freeing.
                        @n_collect_values is exactly the string length of @collect_format,
                        and @collect_values is an array of unions #GTypeCValue with
                        length @n_collect_values, containing the collected values
                        according to @collect_format.
                        @collect_flags is an argument provided as a hint by the caller,
                        which may contain the flag #G_VALUE_NOCOPY_CONTENTS indicating,
                        that the collected value contents may be considered "static"
                        for the duration of the @value lifetime.
                        Thus an extra copy of the contents stored in @collect_values is
                        not required for assignment to @value.
                        For our above string example, we continue with:
<programlisting>
{
  if (!collect_values[0].v_pointer)
    value->data[0].v_pointer = g_strdup ("");
  else if (collect_flags &amp; G_VALUE_NOCOPY_CONTENTS)
    {
      value-&gt;data[0].v_pointer = collect_values[0].v_pointer;
      /* keep a flag for the value_free() implementation to not free this string */
      value-&gt;data[1].v_uint = G_VALUE_NOCOPY_CONTENTS;
    }
  else
    value-&gt;data[0].v_pointer = g_strdup (collect_values[0].v_pointer);

  return NULL;
}
</programlisting>
                        It should be noted, that it is generally a bad idea to follow the
                        #G_VALUE_NOCOPY_CONTENTS hint for reference counted types. Due to
                        reentrancy requirements and reference count assertions performed
                        by the #GSignal code, reference counts should always be incremented
                        for reference counted contents stored in the value-&gt;data array.
                        To deviate from our string example for a moment, and taking a look
                        at an exemplary implementation for collect_value() of #GObject:
<programlisting>
{
  if (collect_values[0].v_pointer)
    {
      GObject *object = G_OBJECT (collect_values[0].v_pointer);

      /* never honour G_VALUE_NOCOPY_CONTENTS for ref-counted types */
      value-&gt;data[0].v_pointer = g_object_ref (object);
      return NULL;
    }
  else
    return g_strdup_printf ("Object passed as invalid NULL pointer");
}
</programlisting>
                        The reference count for valid objects is always incremented,
                        regardless of @collect_flags. For invalid objects, the example
                        returns a newly allocated string without altering @value.
                        Upon success, collect_value() needs to return %NULL, if however
                        a malicious condition occurred, collect_value() may spew an
                        error by returning a newly allocated non-%NULL string, giving
                        a suitable description of the error condition.
                        The calling code makes no assumptions about the @value
                        contents being valid upon error returns, @value
                        is simply thrown away without further freeing. As such, it is
                        a good idea to not allocate #GValue contents, prior to returning
                        an error, however, collect_values() is not obliged to return
                        a correctly setup @value for error returns, simply because
                        any non-%NULL return is considered a fatal condition so further
                        program behaviour is undefined.
@lcopy_format:          Format description of the arguments to collect for @lcopy_value,
                        analogous to @collect_format. Usually, @lcopy_format string consists
                        only of 'p's to provide lcopy_value() with pointers to storage locations.
@lcopy_value:           This function is responsible for storing the @value contents into
                        arguments passed through a variable argument list which got
                        collected into @collect_values according to @lcopy_format.
                        @n_collect_values equals the string length of @lcopy_format,
                        and @collect_flags may contain #G_VALUE_NOCOPY_CONTENTS.
                        In contrast to collect_value(), lcopy_value() is obliged to
                        always properly support #G_VALUE_NOCOPY_CONTENTS.
                        Similar to collect_value() the function may prematurely abort
                        by returning a newly allocated string describing an error condition.
                        To complete the string example:
<programlisting>
{
  gchar **string_p = collect_values[0].v_pointer;

  if (!string_p)
    return g_strdup_printf ("string location passed as NULL");

  if (collect_flags &amp; G_VALUE_NOCOPY_CONTENTS)
    *string_p = value-&gt;data[0].v_pointer;
  else
    *string_p = g_strdup (value-&gt;data[0].v_pointer);

}
</programlisting>
                        And an exemplary version of lcopy_value() for
                        reference-counted types:
<programlisting>
{
  GObject **object_p = collect_values[0].v_pointer;

  if (!object_p)
    return g_strdup_printf ("object location passed as NULL");
  if (!value-&gt;data[0].v_pointer)
    *object_p = NULL;
  else if (collect_flags &amp; G_VALUE_NOCOPY_CONTENTS) /* always honour */
    *object_p = value-&gt;data[0].v_pointer;
  else
    *object_p = g_object_ref (value-&gt;data[0].v_pointer);
  return NULL;
}
</programlisting>

<!-- ##### MACRO G_TYPE_FROM_INSTANCE ##### -->
<para>
Returns the type identifier from a given @instance structure.
</para>

@instance: Location of a valid #GTypeInstance structure.


<!-- ##### MACRO G_TYPE_FROM_CLASS ##### -->
<para>
Returns the type identifier from a given @class structure.
</para>

@g_class: Location of a valid #GTypeClass structure.


<!-- ##### MACRO G_TYPE_FROM_INTERFACE ##### -->
<para>
Returns the type identifier from a given @interface structure.
</para>

@g_iface: Location of a valid #GTypeInterface structure.


<!-- ##### MACRO G_TYPE_INSTANCE_GET_CLASS ##### -->
<para>
Returns the class structure of a given @instance, casted
to a specified anchestor type @g_type of the instance.
</para>

@instance:      Location of the #GTypeInstance structure.
@g_type:        The anchestor type of the class to be returned.
@c_type:        The corresponding C type of @g_Type.


<!-- ##### MACRO G_TYPE_INSTANCE_GET_INTERFACE ##### -->
<para>

</para>

@instance: 
@g_type: 
@c_type: 


<!-- ##### MACRO G_TYPE_CHECK_INSTANCE ##### -->
<para>

</para>

@instance: 


<!-- ##### MACRO G_TYPE_CHECK_INSTANCE_CAST ##### -->
<para>

</para>

@instance: 
@g_type: 
@c_type: 


<!-- ##### MACRO G_TYPE_CHECK_INSTANCE_TYPE ##### -->
<para>

</para>

@instance: 
@g_type: 


<!-- ##### MACRO G_TYPE_CHECK_CLASS_CAST ##### -->
<para>

</para>

@g_class: 
@g_type: 
@c_type: 


<!-- ##### MACRO G_TYPE_CHECK_CLASS_TYPE ##### -->
<para>

</para>

@g_class: 
@g_type: 


<!-- ##### MACRO G_TYPE_CHECK_VALUE ##### -->
<para>

</para>

@value: 


<!-- ##### MACRO G_TYPE_CHECK_VALUE_TYPE ##### -->
<para>

</para>

@value: 
@g_type: 


<!-- ##### MACRO G_TYPE_FLAG_RESERVED_ID_BIT ##### -->
<para>

</para>



<!-- ##### FUNCTION g_type_init ##### -->
<para>
Prior to any use of the type system, g_type_init() has to be called to initialize
the type system and assorted other code portions (such as the various fundamental
type implementations or the signal system).
</para>



<!-- ##### ENUM GTypeDebugFlags ##### -->
<para>
The <type>GTypeDebugFlags</type> enumeration values can be passed to
g_type_init_with_debug_flags() to trigger debugging messages during runtime.
Note that the messages can also be triggered by setting the
<envar>GOBJECT_DEBUG</envar> environment variable to a ':'-separated list of 
"objects" and "signals".

</para>

@G_TYPE_DEBUG_NONE: Print no messages.
@G_TYPE_DEBUG_OBJECTS: Print messages about object bookkeeping.
@G_TYPE_DEBUG_SIGNALS: Print messages about signal emissions.
@G_TYPE_DEBUG_MASK: 

<!-- ##### FUNCTION g_type_init_with_debug_flags ##### -->
<para>
Similar to g_type_init(), but additionally sets debug flags.
</para>

@debug_flags: Bitwise combination of #GTypeDebugFlags values for debugging purposes.


<!-- ##### FUNCTION g_type_name ##### -->
<para>
Return the unique name that is assigned to a type ID (this is the preferred method
to find out whether a specific type has been registered for the passed in ID yet).
</para>

@type:          Type to return name for.
@Returns:       Static type name or %NULL.


<!-- ##### FUNCTION g_type_qname ##### -->
<para>
Return the corresponding quark of the type IDs name.
</para>

@type:    Type to return quark of type name for.
@Returns: The type names quark or 0.


<!-- ##### FUNCTION g_type_from_name ##### -->
<para>
Lookup the type ID from a given type name, returns 0 if no type has been registered under this name
(this is the preferred method to find out by name whether a specific type has been registered yet).
</para>

@name:    Type name to lookup.
@Returns: Corresponding type ID or 0.


<!-- ##### FUNCTION g_type_parent ##### -->
<para>
Return the direct parent type of the passed in type.
If the passed in type has no parent, i.e. is a fundamental type, 0 is returned.
</para>

@type:    The derived type.
@Returns: The parent type.


<!-- ##### FUNCTION g_type_depth ##### -->
<para>
Returns the length of the ancestry of the passed in type. This includes the
type itself, so that e.g. a fundamental type has depth 1.
</para>

@type:    A #GType value.
@Returns: The depth of @type.


<!-- ##### FUNCTION g_type_next_base ##### -->
<para>
Given a @leaf_type and a @root_type which is contained in its anchestry, return
the type that @root_type is the immediate parent of.
In other words, this function determines the type that is derived directly from
@root_type which is also a base class of @leaf_type.  Given a root type and a
leaf type, this function can be used to determine the types and order in which
the leaf type is descended from the root type.
</para>

@leaf_type:     Descendant of @root_type and the type to be returned.
@root_type:     Immediate parent of the returned type.
@Returns:       Immediate child of @root_type and anchestor of @leaf_type.


<!-- ##### FUNCTION g_type_is_a ##### -->
<para>
If @is_a_type is a derivable type, check whether @type is a descendant of @is_a_type.
If @is_a_type is an interface, check whether @type conforms to it.
</para>

@type:      Type to check anchestry for.
@is_a_type: Possible anchestor of @type or interface @type could conform to.
@Returns:   %TRUE if @type is_a @is_a_type holds true.


<!-- ##### FUNCTION g_type_class_ref ##### -->
<para>
Increments the reference count of the class structure belonging to
@type. This function will demand-create the class if it doesn't
exist already.
</para>

@type:    Type ID of a classed type.
@Returns: The #GTypeClass structure for the given type ID.


<!-- ##### FUNCTION g_type_class_peek ##### -->
<para>
This function is essentially the same as g_type_class_ref(), except that
the classes reference count isn't incremented. Therefore, this function
may return %NULL if the class of the type passed in does not currently
exist (hasn't been referenced before).
</para>

@type:          Type ID of a classed type.
@Returns:       The #GTypeClass structure for the given type ID or %NULL
                if the class does not currently exist.


<!-- ##### FUNCTION g_type_class_unref ##### -->
<para>
Decrements the reference count of the class structure being passed in.
Once the last reference count of a class has been released, classes
may be finalized by the type system, so further dereferencing of a
class pointer after g_type_class_unref() are invalid.
</para>

@g_class:       The #GTypeClass structure to unreference.


<!-- ##### FUNCTION g_type_class_peek_parent ##### -->
<para>
This is a convenience function, often needed in class initializers.
It essentially takes the immediate parent type of the class passed in,
and returns the class structure thereof. Since derived classes hold
a reference count on their parent classes as long as they are instantiated,
the returned class will always exist. This function is essentially
equivalent to:

<programlisting>
g_type_class_peek (g_type_parent (G_TYPE_FROM_CLASS (g_class)));
</programlisting>

</para>

@g_class: The #GTypeClass structure to retrieve the parent class for.
@Returns: The parent class of @g_class.


<!-- ##### FUNCTION g_type_interface_peek ##### -->
<para>
Returns the #GTypeInterface structure of an interface to which the passed in 
class conforms.
</para>

@instance_class: A #GTypeClass structure.
@iface_type:     An interface ID which this class conforms to.
@Returns:        The #GTypeInterface structure of @iface_type, or %NULL if the
                 class is not instantiated.


<!-- ##### FUNCTION g_type_interface_peek_parent ##### -->
<para>

</para>

@g_iface: 
@Returns: 


<!-- ##### FUNCTION g_type_children ##### -->
<para>
Return a newly allocated and 0-terminated array of type IDs, listing the
child types of @type. The return value has to be g_free()ed after use.
</para>

@type:       The parent type.
@n_children: Optional #guint pointer to contain the number of child types.
@Returns:    Newly allocated and 0-terminated array of child types.


<!-- ##### FUNCTION g_type_interfaces ##### -->
<para>
Return a newly allocated and 0-terminated array of type IDs, listing the
interface types that @type conforms to. The return value has to be
g_free()ed after use.
</para>

@type:         The type to list interface types for.
@n_interfaces: Optional #guint pointer to contain the number of interface types.
@Returns:      Newly allocated and 0-terminated array of interface types.


<!-- ##### FUNCTION g_type_set_qdata ##### -->
<para>

</para>

@type: 
@quark: 
@data: 


<!-- ##### FUNCTION g_type_get_qdata ##### -->
<para>

</para>

@type: 
@quark: 
@Returns: 


<!-- ##### FUNCTION g_type_query ##### -->
<para>

</para>

@type: 
@query: 


<!-- ##### STRUCT GTypeQuery ##### -->
<para>

</para>

@type: 
@type_name: 
@class_size: 
@instance_size: 

<!-- ##### USER_FUNCTION GBaseInitFunc ##### -->
<para>
A callback function used by the type system to do base initialization
of the class structures of derived types. It is called as part of the
initialization process of all derived classes and should reallocate
or reset all dynamic class members copied over from the parent class.
Therefore class members, e.g. strings, that are not sufficiently
handled by a plain memory copy of the parent class into the derived class
have to be altered. See GClassInitFunc() for a discussion of the class
intialization process.
</para>

@g_class: The #GTypeClass structure to initialize.


<!-- ##### USER_FUNCTION GBaseFinalizeFunc ##### -->
<para>
A callback function used by the type system to finalize those portions
of a derived types class structure that were setup from the corresponding
GBaseInitFunc() function. Class finalization basically works the inverse
way in which class intialization is performed.
See GClassInitFunc() for a discussion of the class intialization process.
</para>

@g_class: The #GTypeClass structure to finalize.


<!-- ##### USER_FUNCTION GClassInitFunc ##### -->
<para>
A callback function used by the type system to initialize the class
of a specific type. This function should initialize all static class
members.
The initialization process of a class involves:
<variablelist>
  <varlistentry><term></term><listitem><para>
        1 - Copying common members from the parent class over to the
        derived class structure.
  </para></listitem></varlistentry>
  <varlistentry><term></term><listitem><para>
        2 -  Zero initialization of the remaining members not copied
        over from the parent class.
  </para></listitem></varlistentry>
  <varlistentry><term></term><listitem><para>
        3 - Invocation of the GBaseInitFunc() initializers of all parent
        types and the class' type.
  </para></listitem></varlistentry>
  <varlistentry><term></term><listitem><para>
        4 - Invocation of the class' GClassInitFunc() initializer.
  </para></listitem></varlistentry>
</variablelist>
Since derived classes are partially initialized through a memory copy
of the parent class, the general rule is that GBaseInitFunc() and
GBaseFinalizeFunc() should take care of necessary reinitialization
and release of those class members that were introduced by the type
that specified these GBaseInitFunc()/GBaseFinalizeFunc().
GClassInitFunc() should only care about initializing static
class members, while dynamic class members (such as allocated strings
or reference counted resources) are better handled by a GBaseInitFunc()
for this type, so proper initialization of the dynamic class members
is performed for class initialization of derived types as well.
An example may help to correspond the intend of the different class
initializers:

<programlisting>
typedef struct {
  GObjectClass parent_class;
  gint         static_integer;
  gchar       *dynamic_string;
} TypeAClass;
static void
type_a_base_class_init (TypeAClass *class)
{
  class->dynamic_string = g_strdup ("some string");
}
static void
type_a_base_class_finalize (TypeAClass *class)
{
  g_free (class->dynamic_string);
}
static void
type_a_class_init (TypeAClass *class)
{
  class->static_integer = 42;
}

typedef struct {
  TypeAClass   parent_class;
  gfloat       static_float;
  GString     *dynamic_gstring;
} TypeBClass;
static void
type_b_base_class_init (TypeBClass *class)
{
  class->dynamic_gstring = g_string_new ("some other string");
}
static void
type_b_base_class_finalize (TypeBClass *class)
{
  g_string_free (class->dynamic_gstring);
}
static void
type_b_class_init (TypeBClass *class)
{
  class->static_float = 3.14159265358979323846;
}
</programlisting>
Initialization of TypeBClass will first cause initialization of
TypeAClass (derived classes reference their parent classes, see
g_type_class_ref() on this).
Initialization of TypeAClass roughly involves zero-initializing its fields,
then calling its GBaseInitFunc() type_a_base_class_init() that allocates
its dynamic members (dynamic_string) and finally calling its GClassInitFunc()
type_a_class_init() to initialize its static members (static_integer).
The first step in the initialization process of TypeBClass is then
a plain memory copy of the contents of TypeAClass into TypeBClass and 
zero-initialization of the remaining fields in TypeBClass.
The dynamic members of TypeAClass within TypeBClass now need
reinitialization which is performed by calling type_a_base_class_init()
with an argument of TypeBClass.
After that, the GBaseInitFunc() of TypeBClass, type_b_base_class_init()
is called to allocate the dynamic members of TypeBClass (dynamic_gstring),
and finally the GClassInitFunc() of TypeBClass, type_b_class_init(),
is called to complete the initialization process with the static members
(static_float).
Corresponding finalization counter parts to the GBaseInitFunc() functions
have to be provided to release allocated resources at class finalization
time.
</para>

@g_class:       The #GTypeClass structure to initialize.
@class_data:    The @class_data member supplied via the #GTypeInfo structure.


<!-- ##### USER_FUNCTION GClassFinalizeFunc ##### -->
<para>
A callback function used by the type system to finalize a class.
This function is rarely needed, as dynamically allocated class resources
should be handled by GBaseInitFunc() and GBaseFinalizeFunc().
Also, specification of a GClassFinalizeFunc() in the #GTypeInfo
structure of a static type is invalid, because classes of static types
will never be finalized (they are artificially kept alive when their
reference count drops to zero).
</para>

@g_class:       The #GTypeClass structure to finalize.
@class_data:    The @class_data member supplied via the #GTypeInfo structure.


<!-- ##### USER_FUNCTION GInstanceInitFunc ##### -->
<para>
A callback function used by the type system to initialize a new
instance of a type. This function initializes all instance members and
allocates any resources required by it.
Initialization of a derived instance involves calling all its parent
types instance initializers, therefore the class member of the instance
is altered during its initialization to always point to the class that
belongs to the type the current initializer was introduced for.
</para>

@instance:      The instance to initialize.
@g_class:       The class of the type the instance is created for.


<!-- ##### USER_FUNCTION GInterfaceInitFunc ##### -->
<para>
A callback function used by the type system to initialize a new
interface.  This function should initialize all internal data and
allocate any resources required by the interface.
</para>

@g_iface:       The interface structure to initialize.
@iface_data:    The @class_data supplied via the #GTypeInfo structure.


<!-- ##### USER_FUNCTION GInterfaceFinalizeFunc ##### -->
<para>
A callback function used by the type system to finalize an interface.
This function should destroy any internal data and release any resources
allocated by the corresponding GInterfaceInitFunc() function.
</para>

@g_iface:       The interface structure to finalize.
@iface_data:    The @class_data supplied via the #GTypeInfo structure.


<!-- ##### USER_FUNCTION GTypeClassCacheFunc ##### -->
<para>

</para>

@cache_data: 
@g_class: 
@Returns: 


<!-- ##### ENUM GTypeFlags ##### -->
<para>
Bit masks used to check or determine characteristics of a type.
</para>

@G_TYPE_FLAG_ABSTRACT:  Indicates an abstract type. No instances can be
                        created for an abstract type.
@G_TYPE_FLAG_VALUE_ABSTRACT: 

<!-- ##### ENUM GTypeFundamentalFlags ##### -->
<para>
Bit masks used to check or determine specific characteristics of a
fundamental type.
</para>

@G_TYPE_FLAG_CLASSED:           Indicates a classed type.
@G_TYPE_FLAG_INSTANTIATABLE:    Indicates an instantiable type (implies classed).
@G_TYPE_FLAG_DERIVABLE:                 Indicates a flat derivable type.
@G_TYPE_FLAG_DEEP_DERIVABLE:    Indicates a deep derivable type (implies derivable).

<!-- ##### FUNCTION g_type_register_static ##### -->
<para>
Registers @type_name as the name of a new static type derived from
@parent_type.  The type system uses the information contained in the
#GTypeInfo structure pointed to by @info to manage the type and its
instances (if not abstract).  The value of @flags determines the nature
(e.g. abstract or not) of the type.
</para>

@parent_type:   Type which this type will be derived from.
@type_name:     0-terminated string used as the name of the new type.
@info:          The #GTypeInfo structure for this type.
@flags:                 Bitwise combination of #GTypeFlags values.
@Returns:       The new type identifier.


<!-- ##### FUNCTION g_type_register_dynamic ##### -->
<para>
Registers @type_name as the name of a new dynamic type derived from
@parent_type.  The type system uses the information contained in the
#GTypePlugin structure pointed to by @plugin to manage the type and its
instances (if not abstract).  The value of @flags determines the nature
(e.g. abstract or not) of the type.
</para>

@parent_type:   Type which this type will be derived from.
@type_name:     0-terminated string used as the name of the new type.
@plugin:        The #GTypePlugin structure to retrieve the #GTypeInfo from.
@flags:                 Bitwise combination of #GTypeFlags values.
@Returns:       The new type identifier or #G_TYPE_INVALID if registration failed.


<!-- ##### FUNCTION g_type_register_fundamental ##### -->
<para>
Registers @type_id as the predefined identifier and @type_name as the
name of a fundamental type.  The type system uses the information
contained in the #GTypeInfo structure pointed to by @info and the 
#GTypeFundamentalInfo structure pointed to by @finfo to manage the
type and its instances.  The value of @flags determines additional
characteristics of the fundamental type.
</para>

@type_id:       A predefined #GTypeFundamentals value.
@type_name:     0-terminated string used as the name of the new type.
@info:          The #GTypeInfo structure for this type.
@finfo:                 The #GTypeFundamentalInfo structure for this type.
@flags:                 Bitwise combination of #GTypeFlags values.
@Returns:       The predefined type identifier.


<!-- ##### FUNCTION g_type_add_interface_static ##### -->
<para>
Adds the static @interface_type to @instantiable_type.  The information
contained in the #GTypeInterfaceInfo structure pointed to by @info
is used to manage the relationship.
</para>

@instance_type:          #GType value of an instantiable type.
@interface_type: #GType value of an interface type.
@info:           The #GInterfaceInfo structure for this
                 (@instance_type, @interface_type) combination.


<!-- ##### FUNCTION g_type_add_interface_dynamic ##### -->
<para>
</para>

@instance_type: 
@interface_type: 
@plugin: 


<!-- ##### FUNCTION g_type_interface_add_prerequisite ##### -->
<para>

</para>

@interface_type: 
@prerequisite_type: 


<!-- ##### FUNCTION g_type_get_plugin ##### -->
<para>
Returns the #GTypePlugin structure for @type or
%NULL if @type does not have a #GTypePlugin structure.
</para>

@type:          The #GType to retrieve the plugin for.
@Returns:       The corresponding plugin if @type is a dynamic type,
                %NULL otherwise.


<!-- ##### FUNCTION g_type_interface_get_plugin ##### -->
<para>
</para>

@instance_type: 
@implementation_type: 
@Returns: 


<!-- ##### FUNCTION g_type_fundamental_next ##### -->
<para>
Returns the next free fundamental type id which can be used to
register a new fundamental type with g_type_register_fundamental().
The returned type ID represents the highest currently registered
fundamental type identifier.

</para>

@Returns: The nextmost fundamental type ID to be registered,
          or 0 if the type system ran out of fundamental type IDs.


<!-- ##### FUNCTION g_type_fundamental ##### -->
<para>
Internal function, used to extract the fundamental type ID portion.
use G_TYPE_FUNDAMENTAL() instead.
</para>

@type_id: valid type ID
@Returns: fundamental type ID


<!-- ##### FUNCTION g_type_create_instance ##### -->
<para>
Creates and initializes an instance of @type if @type is valid and can
be instantiated. The type system only performs basic allocation and
structure setups for instances, actual instance creation should happen
through functions supplied by the type's fundamental type implementation.
So use of g_type_create_instance() is reserved for implementators of
fundamental types only. E.g. instances of the #GObject hierarchy
should be created via g_object_new() and <emphasis>never</emphasis>
directly through g_type_create_instance() which doesn't handle
things like singleton objects or object construction.
Note: Do <emphasis>not</emphasis> use this function, unless you're
implementing a fundamental type. Also language bindings should <emphasis>not</emphasis>
use this function but g_object_new() instead.
</para>

@type:    An instantiabtable type to create an instance for.
@Returns: An allocated and initialized instance, subject to further
          treatment by the fundamental type implementation.


<!-- ##### FUNCTION g_type_free_instance ##### -->
<para>
</para>

@instance: 


<!-- ##### FUNCTION g_type_add_class_cache_func ##### -->
<para>

</para>

@cache_data: 
@cache_func: 


<!-- ##### FUNCTION g_type_remove_class_cache_func ##### -->
<para>

</para>

@cache_data: 
@cache_func: 


<!-- ##### FUNCTION g_type_class_unref_uncached ##### -->
<para>

</para>

@g_class: 


<!-- ##### FUNCTION g_type_value_table_peek ##### -->
<para>
Returns the location of the #GTypeValueTable associated with @type.
<emphasis>Note, this function should only be used from source code
that implements or has internal knowledge of the implementation of
@type.</emphasis>
</para>

@type:    A #GType value.
@Returns: Location of the #GTypeValueTable associated with @type or
          %NULL if there is no #GTypeValueTable associated with @type.


<!-- ##### MACRO G_TYPE_INVALID ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_NONE ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_INTERFACE ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_CHAR ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_UCHAR ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_BOOLEAN ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_INT ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_UINT ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_LONG ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_ULONG ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_INT64 ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_UINT64 ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_ENUM ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_FLAGS ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_FLOAT ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_DOUBLE ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_STRING ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_POINTER ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_BOXED ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_PARAM ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_OBJECT ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_RESERVED_GLIB_FIRST ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_RESERVED_GLIB_LAST ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_RESERVED_BSE_FIRST ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_RESERVED_BSE_LAST ##### -->
<para>

</para>



<!-- ##### MACRO G_TYPE_RESERVED_USER_FIRST ##### -->
<para>

</para>