## Type Information

Type Information — The GLib Runtime type identification and management system

## Synopsis

    #include <glib-object.h>

    typedef             GType;
    #define             G_TYPE_FUNDAMENTAL                  (type)
    #define             G_TYPE_FUNDAMENTAL_MAX
    #define             G_TYPE_MAKE_FUNDAMENTAL             (x)
    #define             G_TYPE_IS_ABSTRACT                  (type)
    #define             G_TYPE_IS_DERIVED                   (type)
    #define             G_TYPE_IS_FUNDAMENTAL               (type)
    #define             G_TYPE_IS_VALUE_TYPE                (type)
    #define             G_TYPE_HAS_VALUE_TABLE              (type)
    #define             G_TYPE_IS_CLASSED                   (type)
    #define             G_TYPE_IS_INSTANTIATABLE            (type)
    #define             G_TYPE_IS_DERIVABLE                 (type)
    #define             G_TYPE_IS_DEEP_DERIVABLE            (type)
    #define             G_TYPE_IS_INTERFACE                 (type)
                        GTypeInterface;
                        GTypeInstance;
                        GTypeClass;
                        GTypeInfo;
                        GTypeFundamentalInfo;
                        GInterfaceInfo;
                        GTypeValueTable;
    #define             G_TYPE_FROM_INSTANCE                (instance)
    #define             G_TYPE_FROM_CLASS                   (g_class)
    #define             G_TYPE_FROM_INTERFACE               (g_iface)
    #define             G_TYPE_INSTANCE_GET_CLASS           (instance, g_type, c_type)
    #define             G_TYPE_INSTANCE_GET_INTERFACE       (instance, g_type, c_type)
    #define             G_TYPE_INSTANCE_GET_PRIVATE         (instance, g_type, c_type)
    #define             G_TYPE_CHECK_INSTANCE               (instance)
    #define             G_TYPE_CHECK_INSTANCE_CAST          (instance, g_type, c_type)
    #define             G_TYPE_CHECK_INSTANCE_TYPE          (instance, g_type)
    #define             G_TYPE_CHECK_CLASS_CAST             (g_class, g_type, c_type)
    #define             G_TYPE_CHECK_CLASS_TYPE             (g_class, g_type)
    #define             G_TYPE_CHECK_VALUE                  (value)
    #define             G_TYPE_CHECK_VALUE_TYPE             (value, g_type)
    #define             G_TYPE_FLAG_RESERVED_ID_BIT
    void                g_type_init                         (void);
    enum                GTypeDebugFlags;
    void                g_type_init_with_debug_flags        (GTypeDebugFlags debug_flags);
    const gchar*        g_type_name                         (GType type);
    GQuark              g_type_qname                        (GType type);
    GType               g_type_from_name                    (const gchar *name);
    GType               g_type_parent                       (GType type);
    guint               g_type_depth                        (GType type);
    GType               g_type_next_base                    (GType leaf_type,
                                                             GType root_type);
    gboolean            g_type_is_a                         (GType type,
                                                             GType is_a_type);
    gpointer            g_type_class_ref                    (GType type);
    gpointer            g_type_class_peek                   (GType type);
    gpointer            g_type_class_peek_static            (GType type);
    void                g_type_class_unref                  (gpointer g_class);
    gpointer            g_type_class_peek_parent            (gpointer g_class);
    void                g_type_class_add_private            (gpointer g_class,
                                                             gsize private_size);
    gpointer            g_type_interface_peek               (gpointer instance_class,
                                                             GType iface_type);
    gpointer            g_type_interface_peek_parent        (gpointer g_iface);
    gpointer            g_type_default_interface_ref        (GType g_type);
    gpointer            g_type_default_interface_peek       (GType g_type);
    void                g_type_default_interface_unref      (gpointer g_iface);
    GType*              g_type_children                     (GType type,
                                                             guint *n_children);
    GType*              g_type_interfaces                   (GType type,
                                                             guint *n_interfaces);
    GType*              g_type_interface_prerequisites      (GType interface_type,
                                                             guint *n_prerequisites);
    void                g_type_set_qdata                    (GType type,
                                                             GQuark quark,
                                                             gpointer data);
    gpointer            g_type_get_qdata                    (GType type,
                                                             GQuark quark);
    void                g_type_query                        (GType type,
                                                             GTypeQuery *query);
                        GTypeQuery;
    void                (*GBaseInitFunc)                    (gpointer g_class);
    void                (*GBaseFinalizeFunc)                (gpointer g_class);
    void                (*GClassInitFunc)                   (gpointer g_class,
                                                             gpointer class_data);
    void                (*GClassFinalizeFunc)               (gpointer g_class,
                                                             gpointer class_data);
    void                (*GInstanceInitFunc)                (GTypeInstance *instance,
                                                             gpointer g_class);
    void                (*GInterfaceInitFunc)               (gpointer g_iface,
                                                             gpointer iface_data);
    void                (*GInterfaceFinalizeFunc)           (gpointer g_iface,
                                                             gpointer iface_data);
    gboolean            (*GTypeClassCacheFunc)              (gpointer cache_data,
                                                             GTypeClass *g_class);
    enum                GTypeFlags;
    enum                GTypeFundamentalFlags;
    GType               g_type_register_static              (GType parent_type,
                                                             const gchar *type_name,
                                                             const GTypeInfo *info,
                                                             GTypeFlags flags);
    GType               g_type_register_static_simple       (GType parent_type,
                                                             const gchar *type_name,
                                                             guint class_size,
                                                             GClassInitFunc class_init,
                                                             guint instance_size,
                                                             GInstanceInitFunc instance_init,
                                                             GTypeFlags flags);
    GType               g_type_register_dynamic             (GType parent_type,
                                                             const gchar *type_name,
                                                             GTypePlugin *plugin,
                                                             GTypeFlags flags);
    GType               g_type_register_fundamental         (GType type_id,
                                                             const gchar *type_name,
                                                             const GTypeInfo *info,
                                                             const GTypeFundamentalInfo *finfo,
                                                             GTypeFlags flags);
    void                g_type_add_interface_static         (GType instance_type,
                                                             GType interface_type,
                                                             const GInterfaceInfo *info);
    void                g_type_add_interface_dynamic        (GType instance_type,
                                                             GType interface_type,
                                                             GTypePlugin *plugin);
    void                g_type_interface_add_prerequisite   (GType interface_type,
                                                             GType prerequisite_type);
    GTypePlugin*        g_type_get_plugin                   (GType type);
    GTypePlugin*        g_type_interface_get_plugin         (GType instance_type,
                                                             GType interface_type);
    GType               g_type_fundamental_next             (void);
    GType               g_type_fundamental                  (GType type_id);
    GTypeInstance*      g_type_create_instance              (GType type);
    void                g_type_free_instance                (GTypeInstance *instance);
    void                g_type_add_class_cache_func         (gpointer cache_data,
                                                             GTypeClassCacheFunc cache_func);
    void                g_type_remove_class_cache_func      (gpointer cache_data,
                                                             GTypeClassCacheFunc cache_func);
    void                g_type_class_unref_uncached         (gpointer g_class);
    void                g_type_add_interface_check          (gpointer check_data,
                                                             GTypeInterfaceCheckFunc check_func);
    void                g_type_remove_interface_check       (gpointer check_data,
                                                             GTypeInterfaceCheckFunc check_func);
    void                (*GTypeInterfaceCheckFunc)          (gpointer check_data,
                                                             gpointer g_iface);
    GTypeValueTable*    g_type_value_table_peek             (GType type);
    #define             G_DEFINE_TYPE                       (TN, t_n, T_P)
    #define             G_DEFINE_TYPE_WITH_CODE             (TN, t_n, T_P, _C_)
    #define             G_DEFINE_ABSTRACT_TYPE              (TN, t_n, T_P)
    #define             G_DEFINE_ABSTRACT_TYPE_WITH_CODE    (TN, t_n, T_P, _C_)
    #define             G_IMPLEMENT_INTERFACE               (TYPE_IFACE, iface_init)
    #define             G_DEFINE_TYPE_EXTENDED              (TN, t_n, T_P, _f_, _C_)

    #define             G_TYPE_INVALID
    #define             G_TYPE_NONE
    #define             G_TYPE_INTERFACE
    #define             G_TYPE_CHAR
    #define             G_TYPE_UCHAR
    #define             G_TYPE_BOOLEAN
    #define             G_TYPE_INT
    #define             G_TYPE_UINT
    #define             G_TYPE_LONG
    #define             G_TYPE_ULONG
    #define             G_TYPE_INT64
    #define             G_TYPE_UINT64
    #define             G_TYPE_ENUM
    #define             G_TYPE_FLAGS
    #define             G_TYPE_FLOAT
    #define             G_TYPE_DOUBLE
    #define             G_TYPE_STRING
    #define             G_TYPE_POINTER
    #define             G_TYPE_BOXED
    #define             G_TYPE_PARAM
    #define             G_TYPE_OBJECT
    #define             G_TYPE_GTYPE

    #define             G_TYPE_RESERVED_GLIB_FIRST
    #define             G_TYPE_RESERVED_GLIB_LAST
    #define             G_TYPE_RESERVED_BSE_FIRST
    #define             G_TYPE_RESERVED_BSE_LAST
    #define             G_TYPE_RESERVED_USER_FIRST

## Description

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.

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.

A final word about type names. Such an identifier needs to be at least three characters long. There is no upper length limit. The first character needs to be a letter (a-z or A-Z) or an underscore '_'. Subsequent characters can be letters, numbers or any of '-_+'.

## Details

### GType

A numerical value which represents the unique identifier of a registered type.

### G_TYPE_FUNDAMENTAL()

    #define G_TYPE_FUNDAMENTAL(type) (g_type_fundamental (type))

The fundamental type which is the ancestor of `type`. Fundamental types are types that serve as ultimate bases for the derived types, thus they are the roots of distinct inheritance hierarchies.

`type` : | A GType value.

### G_TYPE_FUNDAMENTAL_MAX

    #define G_TYPE_FUNDAMENTAL_MAX		(255 << G_TYPE_FUNDAMENTAL_SHIFT)

An integer constant that represents the number of identifiers reserved for types that are assigned at compile-time.

### G_TYPE_MAKE_FUNDAMENTAL()

    #define G_TYPE_MAKE_FUNDAMENTAL(x) ((GType) ((x) << G_TYPE_FUNDAMENTAL_SHIFT))

Get the type ID for the fundamental type number `x`. Use `g_type_fundamental_next()` instead of this macro to create new fundamental types.

`x` : | the fundamental type number.
Returns : | the GType

### G_TYPE_IS_ABSTRACT()

    #define G_TYPE_IS_ABSTRACT(type)                (g_type_test_flags ((type), G_TYPE_FLAG_ABSTRACT))

Checks 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.

`type` : | A GType value.
Returns : | `TRUE` on success.

### G_TYPE_IS_DERIVED()

    #define G_TYPE_IS_DERIVED(type)                 ((type) > G_TYPE_FUNDAMENTAL_MAX)

Checks if `type` is derived (or in object-oriented terminology: inherited) from another type (this holds true for all non-fundamental types).

`type` : | A GType value.
Returns : | `TRUE` on success.

### G_TYPE_IS_FUNDAMENTAL()

    #define G_TYPE_IS_FUNDAMENTAL(type)             ((type) <= G_TYPE_FUNDAMENTAL_MAX)

Checks if `type` is a fundamental type.

`type` : | A GType value.
Returns : | `TRUE` on success.

### G_TYPE_IS_VALUE_TYPE()

    #define G_TYPE_IS_VALUE_TYPE(type)              (g_type_check_is_value_type (type))

Checks if `type` is a value type and can be used with `g_value_init()`.

`type` : | A GType value.
Returns : | `TRUE` on success.

### G_TYPE_HAS_VALUE_TABLE()

    #define G_TYPE_HAS_VALUE_TABLE(type)            (g_type_value_table_peek (type) != NULL)

Checks if `type` has a GTypeValueTable.

`type` : | A GType value.
Returns : | `TRUE` on success.

### G_TYPE_IS_CLASSED()

    #define G_TYPE_IS_CLASSED(type)                 (g_type_test_flags ((type), G_TYPE_FLAG_CLASSED))

Checks if `type` is a classed type.

`type` : | A GType value.
Returns : | `TRUE` on success.

### G_TYPE_IS_INSTANTIATABLE()

    #define G_TYPE_IS_INSTANTIATABLE(type)          (g_type_test_flags ((type), G_TYPE_FLAG_INSTANTIATABLE))

Checks if `type` can be instantiated. Instantiation is the process of creating an instance (object) of this type.

`type` : | A GType value.
Returns : | `TRUE` on success.

### G_TYPE_IS_DERIVABLE()

    #define G_TYPE_IS_DERIVABLE(type)               (g_type_test_flags ((type), G_TYPE_FLAG_DERIVABLE))

Checks if `type` is a derivable type. A derivable type can be used as the base class of a flat (single-level) class hierarchy.

`type` : | A GType value.
Returns : | `TRUE` on success.

### G_TYPE_IS_DEEP_DERIVABLE()

    #define G_TYPE_IS_DEEP_DERIVABLE(type)          (g_type_test_flags ((type), G_TYPE_FLAG_DEEP_DERIVABLE))

Checks 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.

`type` : | A GType value.
Returns : | `TRUE` on success.

### G_TYPE_IS_INTERFACE()

    #define G_TYPE_IS_INTERFACE(type)               (G_TYPE_FUNDAMENTAL (type) == G_TYPE_INTERFACE)

Checks if `type` is an interface type. An interface type provides a pure API, 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).

`type` : | A GType value.
Returns : | `TRUE` on success.

### GTypeInterface

    typedef struct {
    } GTypeInterface;

An opaque structure used as the base of all interface types.

### GTypeInstance

    typedef struct {
    } GTypeInstance;

An opaque structure used as the base of all type instances.

### GTypeClass

    typedef struct {
    } GTypeClass;

An opaque structure used as the base of all classes.

### GTypeInfo

    typedef struct {
      /* interface types, classed types, instantiated types */
      guint16                class_size;

      GBaseInitFunc          base_init;
      GBaseFinalizeFunc      base_finalize;

      /* interface types, 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;
    } GTypeInfo;

This structure is used to provide the type system with the information required to initialize and destruct (finalize) a type's class and its instances. 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 its memory does not need to be persistent across invocation of `g_type_register_static()`.

guint16 `class_size`; | Size of the class structure (required for interface, classed and instantiatable types).
GBaseInitFunc `base_init`; | Location of the base initialization function (optional).
GBaseFinalizeFunc `base_finalize`; | Location of the base finalization function (optional).
GClassInitFunc `class_init`; | Location of the class initialization function for classed and instantiatable types. Location of the default vtable inititalization function for interface types. (optional) This function is used both to fill in virtual functions in the class or default vtable, and to do type-specific setup such as registering signals and object properties.
GClassFinalizeFunc `class_finalize`; | Location of the class finalization function for classed and instantiatable types. Location fo the default vtable finalization function for interface types. (optional)
gconstpointer `class_data`; | User-supplied data passed to the class init/finalize functions.
guint16 `instance_size`; | Size of the instance (object) structure (required for instantiatable types only).
guint16 `n_preallocs`; | Prior to GLib 2.10, it specified the number of pre-allocated (cached) instances to reserve memory for (0 indicates no caching). Since GLib 2.10, it is ignored, since instances are allocated with the slice allocator now.
GInstanceInitFunc `instance_init`; | Location of the instance initialization function (optional, for instantiatable types only).
const GTypeValueTable *`value_table`; | A GTypeValueTable function table for generic handling of GValues of this type (usually only useful for fundamental types).

### GTypeFundamentalInfo

    typedef struct {
      GTypeFundamentalFlags  type_flags;
    } GTypeFundamentalInfo;

A structure that provides information to the type system which is used specifically for managing fundamental types.

GTypeFundamentalFlags `type_flags`; | GTypeFundamentalFlags describing the characteristics of the fundamental type

### GInterfaceInfo

    typedef struct {
      GInterfaceInitFunc     interface_init;
      GInterfaceFinalizeFunc interface_finalize;
      gpointer               interface_data;
    } GInterfaceInfo;

A structure that provides information to the type system which is used specifically for managing interface types.

GInterfaceInitFunc `interface_init`; | location of the interface initialization function
GInterfaceFinalizeFunc `interface_finalize`; | location of the interface finalization function
gpointer `interface_data`; | user-supplied data passed to the interface init/finalize functions

### GTypeValueTable

    typedef struct {
      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);
    } GTypeValueTable;

The GTypeValueTable provides the functions required by the GValue implementation, to serve as a container for values of a type.

`value_init` ()

Default initialize `values` contents by poking values directly into the value->data array. The data array of the GValue passed into this function was zero-filled with ``memset()``, 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:

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    value->data[0].v_pointer = g_strdup ("");

`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:

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    // only free strings without a specific flag for static storage
     if (!(value->data[1].v_uint & G_VALUE_NOCOPY_CONTENTS))
       g_free (value->data[0].v_pointer);

`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:

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    dest_value->data[0].v_pointer = g_strdup (src_value->data[0].v_pointer);

`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:

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    return value->data[0].v_pointer;

gchar *`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, and the characters themselves indicate the type of the argument. Currently supported arguments are:

| 'i' - Integers. passed as collect_values[].v_int.
| 'l' - Longs. passed as collect_values[].v_long.
| 'd' - Doubles. passed as collect_values[].v_double.
| 'p' - Pointers. passed as collect_values[].v_pointer.

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. It 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:

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    if (!collect_values[0].v_pointer)
       value->data[0].v_pointer = g_strdup ("");
     else if (collect_flags & G_VALUE_NOCOPY_CONTENTS)
     {
       value->data[0].v_pointer = collect_values[0].v_pointer;
       // keep a flag for the value_free() implementation to not free this string
       value->data[1].v_uint = G_VALUE_NOCOPY_CONTENTS;
     }
     else
       value->data[0].v_pointer = g_strdup (collect_values[0].v_pointer);
     return NULL;

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->data array. To deviate from our string example for a moment, and taking a look at an exemplary implementation for `collect_value()` of GObject:

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    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->data[0].v_pointer = g_object_ref (object);
       return NULL;
     }
     else
       return g_strdup_printf ("Object passed as invalid NULL pointer");
     }

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, an error 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.

gchar *`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:

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    gchar **string_p = collect_values[0].v_pointer;
     if (!string_p)
       return g_strdup_printf ("string location passed as NULL");
     if (collect_flags & G_VALUE_NOCOPY_CONTENTS)
       *string_p = value->data[0].v_pointer;
     else
       *string_p = g_strdup (value->data[0].v_pointer);

And an illustrative version of `lcopy_value()` for reference-counted types:

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

### G_TYPE_FROM_INSTANCE()

    #define G_TYPE_FROM_INSTANCE(instance)                          (G_TYPE_FROM_CLASS (((GTypeInstance*) (instance))->g_class))

Get the type identifier from a given `instance` structure.

This macro should only be used in type implementations.

`instance` : | Location of a valid GTypeInstance structure.
Returns : | the GType

### G_TYPE_FROM_CLASS()

    #define G_TYPE_FROM_CLASS(g_class)                              (((GTypeClass*) (g_class))->g_type)

Get the type identifier from a given `class` structure.

This macro should only be used in type implementations.

`g_class` : | Location of a valid GTypeClass structure.
Returns : | the GType

### G_TYPE_FROM_INTERFACE()

    #define G_TYPE_FROM_INTERFACE(g_iface)                          (((GTypeInterface*) (g_iface))->g_type)

Get the type identifier from a given `interface` structure.

This macro should only be used in type implementations.

`g_iface` : | Location of a valid GTypeInterface structure.
Returns : | the GType

### G_TYPE_INSTANCE_GET_CLASS()

    #define G_TYPE_INSTANCE_GET_CLASS(instance, g_type, c_type)     (_G_TYPE_IGC ((instance), (g_type), c_type))

Get the class structure of a given `instance`, casted to a specified ancestor type `g_type` of the instance.

Note that while calling a `GInstanceInitFunc()`, the class pointer gets modified, so it might not always return the expected pointer.

This macro should only be used in type implementations.

`instance` : | Location of the GTypeInstance structure.
`g_type` : | The GType of the class to be returned.
`c_type` : | The C type of the class structure.
Returns : | a pointer to the class structure

### G_TYPE_INSTANCE_GET_INTERFACE()

    #define G_TYPE_INSTANCE_GET_INTERFACE(instance, g_type, c_type) (_G_TYPE_IGI ((instance), (g_type), c_type))

Get the interface structure for interface `g_type` of a given `instance`.

This macro should only be used in type implementations.

`instance` : | Location of the GTypeInstance structure.
`g_type` : | The GType of the interface to be returned.
`c_type` : | The C type of the interface structure.
Returns : | a pointer to the interface structure

### G_TYPE_INSTANCE_GET_PRIVATE()

    #define G_TYPE_INSTANCE_GET_PRIVATE(instance, g_type, c_type)   ((c_type*) g_type_instance_get_private ((GTypeInstance*) (instance), (g_type)))

Gets the private structure for a particular type. The private structure must have been registered in the class_init function with `g_type_class_add_private()`.

This macro should only be used in type implementations.

`instance` : | the instance of a type deriving from `private_type`.
`g_type` : | the type identifying which private data to retrieve.
`c_type` : | The C type for the private structure.
Returns : | a pointer to the private data structure.

Since 2.4

### G_TYPE_CHECK_INSTANCE()

    #define G_TYPE_CHECK_INSTANCE(instance)				(_G_TYPE_CHI ((GTypeInstance*) (instance)))

Checks if `instance` is a valid GTypeInstance structure, otherwise issues a warning and returns `FALSE`.

This macro should only be used in type implementations.

`instance` : | Location of a GTypeInstance structure.
Returns : | `TRUE` on success.

### G_TYPE_CHECK_INSTANCE_CAST()

    #define G_TYPE_CHECK_INSTANCE_CAST(instance, g_type, c_type)    (_G_TYPE_CIC ((instance), (g_type), c_type))

Checks that `instance` is an instance of the type identified by `g_type` and issues a warning if this is not the case. Returns `instance` casted to a pointer to `c_type`.

This macro should only be used in type implementations.

`instance` : | Location of a GTypeInstance structure.
`g_type` : | The type to be returned.
`c_type` : | The corresponding C type of `g_type`.

### G_TYPE_CHECK_INSTANCE_TYPE()

    #define G_TYPE_CHECK_INSTANCE_TYPE(instance, g_type)            (_G_TYPE_CIT ((instance), (g_type)))

Checks if `instance` is an instance of the type identified by `g_type`.

This macro should only be used in type implementations.

`instance` : | Location of a GTypeInstance structure.
`g_type` : | The type to be checked
Returns : | `TRUE` on success.

### G_TYPE_CHECK_CLASS_CAST()

    #define G_TYPE_CHECK_CLASS_CAST(g_class, g_type, c_type)        (_G_TYPE_CCC ((g_class), (g_type), c_type))

Checks that `g_class` is a class structure of the type identified by `g_type` and issues a warning if this is not the case. Returns `g_class` casted to a pointer to `c_type`.

This macro should only be used in type implementations.

`g_class` : | Location of a GTypeClass structure.
`g_type` : | The type to be returned.
`c_type` : | The corresponding C type of class structure of `g_type`.

### G_TYPE_CHECK_CLASS_TYPE()

    #define G_TYPE_CHECK_CLASS_TYPE(g_class, g_type)                (_G_TYPE_CCT ((g_class), (g_type)))

Checks if `g_class` is a class structure of the type identified by `g_type`.

This macro should only be used in type implementations.

`g_class` : | Location of a GTypeClass structure.
`g_type` : | The type to be checked.
Returns : | `TRUE` on success.

### G_TYPE_CHECK_VALUE()

    #define G_TYPE_CHECK_VALUE(value)				(_G_TYPE_CHV ((value)))

Checks if `value` has been initialized to hold values of a value type.

This macro should only be used in type implementations.

`value` : | a GValue
Returns : | `TRUE` on success.

### G_TYPE_CHECK_VALUE_TYPE()

    #define G_TYPE_CHECK_VALUE_TYPE(value, g_type)			(_G_TYPE_CVH ((value), (g_type)))

Checks if `value` has been initialized to hold values of type `g_type`.

This macro should only be used in type implementations.

`value` : | a GValue
`g_type` : | The type to be checked.
Returns : | `TRUE` on success.

### G_TYPE_FLAG_RESERVED_ID_BIT

    #define G_TYPE_FLAG_RESERVED_ID_BIT ((GType) (1 << 0))

A bit in the type number that's supposed to be left untouched.

### g_type_init ()

    void                g_type_init                         (void);

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).

### enum GTypeDebugFlags

    typedef enum /*< skip >*/
    {
      G_TYPE_DEBUG_NONE = 0,
      G_TYPE_DEBUG_OBJECTS = 1 << 0,
      G_TYPE_DEBUG_SIGNALS = 1 << 1,
      G_TYPE_DEBUG_MASK = 0x03
    } GTypeDebugFlags;

The GTypeDebugFlags 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 `GOBJECT_DEBUG` environment variable to a ':'-separated list of "objects" and "signals".

`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` | Mask covering all debug flags.

### g_type_init_with_debug_flags ()

    void                g_type_init_with_debug_flags        (GTypeDebugFlags debug_flags);

Similar to `g_type_init()`, but additionally sets debug flags.

`debug_flags` : | Bitwise combination of GTypeDebugFlags values for debugging purposes.

### g_type_name ()

    const gchar*        g_type_name                         (GType type);

Get the unique name that is assigned to a type ID. Note that this function (like all other GType API) cannot cope with invalid type IDs. `G_TYPE_INVALID` may be passed to this function, as may be any other validly registered type ID, but randomized type IDs should not be passed in and will most likely lead to a crash.

`type` : | Type to return name for.
Returns : | Static type name or `NULL`.

### g_type_qname ()

    GQuark              g_type_qname                        (GType type);

Get the corresponding quark of the type IDs name.

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

### g_type_from_name ()

    GType               g_type_from_name                    (const gchar *name);

Lookup the type ID from a given type name, returning 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).

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

### g_type_parent ()

    GType               g_type_parent                       (GType type);

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.

`type` : | The derived type.
Returns : | The parent type.

### g_type_depth ()

    guint               g_type_depth                        (GType type);

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.

`type` : | A GType value.
Returns : | The depth of `type`.

### g_type_next_base ()

    GType               g_type_next_base                    (GType leaf_type,
                                                             GType root_type);

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.

`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`.

### g_type_is_a ()

    gboolean            g_type_is_a                         (GType type,
                                                             GType is_a_type);

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.

`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.

### g_type_class_ref ()

    gpointer            g_type_class_ref                    (GType type);

Increments the reference count of the class structure belonging to `type`. This function will demand-create the class if it doesn't exist already.

`type` : | Type ID of a classed type.
Returns : | The GTypeClass structure for the given type ID.

### g_type_class_peek ()

    gpointer            g_type_class_peek                   (GType type);

This function is essentially the same as `g_type_class_ref()`, except that the classes reference count isn't incremented. As a consequence, this function may return `NULL` if the class of the type passed in does not currently exist (hasn't been referenced before).

`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.

### g_type_class_peek_static ()

    gpointer            g_type_class_peek_static            (GType type);

A more efficient version of `g_type_class_peek()` which works only for static types.

`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 or is dynamically loaded.

Since 2.4

### g_type_class_unref ()

    void                g_type_class_unref                  (gpointer g_class);

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.

`g_class` : | The GTypeClass structure to unreference.

### g_type_class_peek_parent ()

    gpointer            g_type_class_peek_parent            (gpointer g_class);

This is a convenience function often needed in class initializers. It returns the class structure of the immediate parent type of the class passed in. 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:

    g_type_class_peek (g_type_parent (G_TYPE_FROM_CLASS (g_class)));

`g_class` : | The GTypeClass structure to retrieve the parent class for.
Returns : | The parent class of `g_class`.

### g_type_class_add_private ()

    void                g_type_class_add_private            (gpointer g_class,
                                                             gsize private_size);

Registers a private structure for an instantiatable type; when an object is allocated, the private structures for the type and all of its parent types are allocated sequentially in the same memory block as the public structures. This function should be called in the type's `class_init()` function. The private structure can be retrieved using the `G_TYPE_INSTANCE_GET_PRIVATE()` macro. The following example shows attaching a private structure MyObjectPrivate to an object MyObject defined in the standard GObject fashion.

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    typedef struct _MyObjectPrivate MyObjectPrivate;

    struct _MyObjectPrivate {
      int some_field;
    };

    #define MY_OBJECT_GET_PRIVATE(o)  \
       (G_TYPE_INSTANCE_GET_PRIVATE ((o), MY_TYPE_OBJECT, MyObjectPrivate))

    static void
    my_object_class_init (MyObjectClass *klass)
    {
      g_type_class_add_private (klass, sizeof (MyObjectPrivate));
    }

    static int
    my_object_get_some_field (MyObject *my_object)
    {
      MyObjectPrivate *priv = MY_OBJECT_GET_PRIVATE (my_object);

      return priv->some_field;
    }

`g_class` : | class structure for an instantiatable type
`private_size` : | size of private structure.

Since 2.4

### g_type_interface_peek ()

    gpointer            g_type_interface_peek               (gpointer instance_class,
                                                             GType iface_type);

Returns the GTypeInterface structure of an interface to which the passed in class conforms.

`instance_class` : | A GTypeClass structure.
`iface_type` : | An interface ID which this class conforms to.
Returns : | The GTypeInterface structure of iface_type if implemented by `instance_class`, `NULL` otherwise

### g_type_interface_peek_parent ()

    gpointer            g_type_interface_peek_parent        (gpointer g_iface);

Returns the corresponding GTypeInterface structure of the parent type of the instance type to which `g_iface` belongs. This is useful when deriving the implementation of an interface from the parent type and then possibly overriding some methods.

`g_iface` : | A GTypeInterface structure.
Returns : | The corresponding GTypeInterface structure of the parent type of the instance type to which `g_iface` belongs, or `NULL` if the parent type doesn't conform to the interface.

### g_type_default_interface_ref ()

    gpointer            g_type_default_interface_ref        (GType g_type);

Increments the reference count for the interface type `g_type`, and returns the default interface vtable for the type.

If the type is not currently in use, then the default vtable for the type will be created and initalized by calling the base interface init and default vtable init functions for the type (the @`base_init` and `class_init` members of GTypeInfo). Calling `g_type_default_interface_ref()` is useful when you want to make sure that signals and properties for an interface have been installed.

`g_type` : | an interface type
Returns : | the default vtable for the interface; call `g_type_default_interface_unref()` when you are done using the interface.

Since 2.4

### g_type_default_interface_peek ()

    gpointer            g_type_default_interface_peek       (GType g_type);

If the interface type `g_type` is currently in use, returns its default interface vtable.

`g_type` : | an interface type
Returns : | the default vtable for the interface, or `NULL` if the type is not currently in use.

Since 2.4

### g_type_default_interface_unref ()

    void                g_type_default_interface_unref      (gpointer g_iface);

Decrements the reference count for the type corresponding to the interface default vtable `g_iface`. If the type is dynamic, then when no one is using the interface and all references have been released, the finalize function for the interface's default vtable (the `class_finalize` member of GTypeInfo) will be called.

`g_iface` : | the default vtable structure for a interface, as returned by `g_type_default_interface_ref()`

Since 2.4

### g_type_children ()

    GType*              g_type_children                     (GType type,
                                                             guint *n_children);

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.

`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.

### g_type_interfaces ()

    GType*              g_type_interfaces                   (GType type,
                                                             guint *n_interfaces);

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.

`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.

### g_type_interface_prerequisites ()

    GType*              g_type_interface_prerequisites      (GType interface_type,
                                                             guint *n_prerequisites);

Returns the prerequisites of an interfaces type.

`interface_type` : | an interface type
`n_prerequisites` : | location to return the number of prerequisites, or `NULL`
Returns : | a newly-allocated zero-terminated array of GType containing the prerequisites of `interface_type`

Since 2.2

### g_type_set_qdata ()

    void                g_type_set_qdata                    (GType type,
                                                             GQuark quark,
                                                             gpointer data);

Attaches arbitrary data to a type.

`type` : | a GType
`quark` : | a GQuark id to identify the data
`data` : | the data

### g_type_get_qdata ()

    gpointer            g_type_get_qdata                    (GType type,
                                                             GQuark quark);

Obtains data which has previously been attached to `type` with `g_type_set_qdata()`.

`type` : | a GType
`quark` : | a GQuark id to identify the data
Returns : | the data, or `NULL` if no data was found

### g_type_query ()

    void                g_type_query                        (GType type,
                                                             GTypeQuery *query);

Queries the type system for information about a specific type. This function will fill in a user-provided structure to hold type-specific information. If an invalid GType is passed in, the `type` member of the GTypeQuery is 0. All members filled into the GTypeQuery structure should be considered constant and have to be left untouched.

`type` : | the GType value of a static, classed type.
`query` : | A user provided structure that is filled in with constant values upon success.

### GTypeQuery

    typedef struct {
      GType		type;
      const gchar  *type_name;
      guint		class_size;
      guint		instance_size;
    } GTypeQuery;

A structure holding information for a specific type. It is filled in by the `g_type_query()` function.

GType `type`; | the GType value of the type.
const gchar *`type_name`; | the name of the type.
guint `class_size`; | the size of the class structure.
guint `instance_size`; | the size of the instance structure.

### GBaseInitFunc ()

    void                (*GBaseInitFunc)                    (gpointer g_class);

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. For example, class members (such as 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.

`g_class` : | The GTypeClass structure to initialize.

### GBaseFinalizeFunc ()

    void                (*GBaseFinalizeFunc)                (gpointer g_class);

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.

`g_class` : | The GTypeClass structure to finalize.

### GClassInitFunc ()

    void                (*GClassInitFunc)                   (gpointer g_class,
                                                             gpointer class_data);

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:

- 1 - Copying common members from the parent class over to the derived class structure.

- 2 - Zero initialization of the remaining members not copied over from the parent class.

- 3 - Invocation of the `GBaseInitFunc()` initializers of all parent types and the class' type.

- 4 - Invocation of the class' `GClassInitFunc()` initializer.

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:

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    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;
    }

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()` to allocate 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.

`g_class` : | The GTypeClass structure to initialize.
`class_data` : | The `class_data` member supplied via the GTypeInfo structure.

### GClassFinalizeFunc ()

    void                (*GClassFinalizeFunc)               (gpointer g_class,
                                                             gpointer class_data);

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).

`g_class` : | The GTypeClass structure to finalize.
`class_data` : | The `class_data` member supplied via the GTypeInfo structure.

### GInstanceInitFunc ()

    void                (*GInstanceInitFunc)                (GTypeInstance *instance,
                                                             gpointer g_class);

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, so 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.

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

### GInterfaceInitFunc ()

    void                (*GInterfaceInitFunc)               (gpointer g_iface,
                                                             gpointer iface_data);

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.

`g_iface` : | The interface structure to initialize.
`iface_data` : | The `interface_data` supplied via the GInterfaceInfo structure.

### GInterfaceFinalizeFunc ()

    void                (*GInterfaceFinalizeFunc)           (gpointer g_iface,
                                                             gpointer iface_data);

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.

`g_iface` : | The interface structure to finalize.
`iface_data` : | The `interface_data` supplied via the GInterfaceInfo structure.

### GTypeClassCacheFunc ()

    gboolean            (*GTypeClassCacheFunc)              (gpointer cache_data,
                                                             GTypeClass *g_class);

A callback function which is called when the reference count of a class drops to zero. It may use `g_type_class_ref()` to prevent the class from being freed. You should not call `g_type_class_unref()` from a GTypeClassCacheFunc function to prevent infinite recursion, use `g_type_class_unref_uncached()` instead.

The functions have to check the class id passed in to figure whether they actually want to cache the class of this type, since all classes are routed through the same GTypeClassCacheFunc chain.

`cache_data` : | data that was given to the `g_type_add_class_cache_func()` call
`g_class` : | The GTypeClass structure which is unreferenced
Returns : | `TRUE` to stop further GTypeClassCacheFuncs from being called, `FALSE` to continue.

### enum GTypeFlags

    typedef enum    /*< skip >*/
    {
      G_TYPE_FLAG_ABSTRACT		= (1 << 4),
      G_TYPE_FLAG_VALUE_ABSTRACT = (1 << 5)
    } GTypeFlags;

Bit masks used to check or determine characteristics of a type.

`G_TYPE_FLAG_ABSTRACT` | Indicates an abstract type. No instances can be created for an abstract type.
`G_TYPE_FLAG_VALUE_ABSTRACT` | Indicates an abstract value type, i.e. a type that introduces a value table, but can't be used for `g_value_init()`.

### enum GTypeFundamentalFlags

    typedef enum    /*< skip >*/
    {
      G_TYPE_FLAG_CLASSED           = (1 << 0),
      G_TYPE_FLAG_INSTANTIATABLE    = (1 << 1),
      G_TYPE_FLAG_DERIVABLE         = (1 << 2),
      G_TYPE_FLAG_DEEP_DERIVABLE    = (1 << 3)
    } GTypeFundamentalFlags;

Bit masks used to check or determine specific characteristics of a fundamental type.

`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).

### g_type_register_static ()

    GType               g_type_register_static              (GType parent_type,
                                                             const gchar *type_name,
                                                             const GTypeInfo *info,
                                                             GTypeFlags flags);

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.

`parent_type` : | Type from which this type will be derived.
`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.

### g_type_register_static_simple ()

    GType               g_type_register_static_simple       (GType parent_type,
                                                             const gchar *type_name,
                                                             guint class_size,
                                                             GClassInitFunc class_init,
                                                             guint instance_size,
                                                             GInstanceInitFunc instance_init,
                                                             GTypeFlags flags);

Registers `type_name` as the name of a new static type derived from `parent_type`. The value of `flags` determines the nature (e.g. abstract or not) of the type. It works by filling a GTypeInfo struct and calling `g_type_register_static()`.

`parent_type` : | Type from which this type will be derived.
`type_name` : | 0-terminated string used as the name of the new type.
`class_size` : | Size of the class structure (see GTypeInfo)
`class_init` : | Location of the class initialization function (see GTypeInfo)
`instance_size` : | Size of the instance structure (see GTypeInfo)
`instance_init` : | Location of the instance initialization function (see GTypeInfo)
`flags` : | Bitwise combination of GTypeFlags values.
Returns : | The new type identifier.

Since 2.12

### g_type_register_dynamic ()

    GType               g_type_register_dynamic             (GType parent_type,
                                                             const gchar *type_name,
                                                             GTypePlugin *plugin,
                                                             GTypeFlags flags);

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.

`parent_type` : | Type from which this type will be derived.
`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.

### g_type_register_fundamental ()

    GType               g_type_register_fundamental         (GType type_id,
                                                             const gchar *type_name,
                                                             const GTypeInfo *info,
                                                             const GTypeFundamentalInfo *finfo,
                                                             GTypeFlags flags);

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.

`type_id` : | A predefined type identifier.
`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.

### g_type_add_interface_static ()

    void                g_type_add_interface_static         (GType instance_type,
                                                             GType interface_type,
                                                             const GInterfaceInfo *info);

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.

`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.

### g_type_add_interface_dynamic ()

    void                g_type_add_interface_dynamic        (GType instance_type,
                                                             GType interface_type,
                                                             GTypePlugin *plugin);

Adds the dynamic `interface_type` to `instantiable_type`. The information contained in the GTypePlugin structure pointed to by `plugin` is used to manage the relationship.

`instance_type` : | the GType value of an instantiable type.
`interface_type` : | the GType value of an interface type.
`plugin` : | the GTypePlugin structure to retrieve the GInterfaceInfo from.

### g_type_interface_add_prerequisite ()

    void                g_type_interface_add_prerequisite   (GType interface_type,
                                                             GType prerequisite_type);

Adds `prerequisite_type` to the list of prerequisites of `interface_type`. This means that any type implementing `interface_type` must also implement `prerequisite_type`. Prerequisites can be thought of as an alternative to interface derivation (which GType doesn't support). An interface can have at most one instantiatable prerequisite type.

`interface_type` : | GType value of an interface type.
`prerequisite_type` : | GType value of an interface or instantiatable type.

### g_type_get_plugin ()

    GTypePlugin*        g_type_get_plugin                   (GType type);

Returns the GTypePlugin structure for `type` or `NULL` if `type` does not have a GTypePlugin structure.

`type` : | The GType to retrieve the plugin for.
Returns : | The corresponding plugin if `type` is a dynamic type, `NULL` otherwise.

### g_type_interface_get_plugin ()

    GTypePlugin*        g_type_interface_get_plugin         (GType instance_type,
                                                             GType interface_type);

Returns the GTypePlugin structure for the dynamic interface `interface_type` which has been added to `instance_type`, or `NULL` if `interface_type` has not been added to `instance_type` or does not have a GTypePlugin structure. See `g_type_add_interface_dynamic()`.

`instance_type` : | the GType value of an instantiatable type.
`interface_type` : | the GType value of an interface type.
Returns : | the GTypePlugin for the dynamic interface `interface_type` of `instance_type`.

### g_type_fundamental_next ()

    GType               g_type_fundamental_next             (void);

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.

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

### g_type_fundamental ()

    GType               g_type_fundamental                  (GType type_id);

Internal function, used to extract the fundamental type ID portion. use `G_TYPE_FUNDAMENTAL()` instead.

`type_id` : | valid type ID
Returns : | fundamental type ID

### g_type_create_instance ()

    GTypeInstance*      g_type_create_instance              (GType type);

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 never directly through `g_type_create_instance()` which doesn't handle things like singleton objects or object construction. Note: Do not use this function, unless you're implementing a fundamental type. Also language bindings should not use this function but `g_object_new()` instead.

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

### g_type_free_instance ()

    void                g_type_free_instance                (GTypeInstance *instance);

Frees an instance of a type, returning it to the instance pool for the type, if there is one.

Like `g_type_create_instance()`, this function is reserved for implementors of fundamental types.

`instance` : | an instance of a type.

### g_type_add_class_cache_func ()

    void                g_type_add_class_cache_func         (gpointer cache_data,
                                                             GTypeClassCacheFunc cache_func);

Adds a GTypeClassCacheFunc to be called before the reference count of a class goes from one to zero. This can be used to prevent premature class destruction. All installed GTypeClassCacheFunc functions will be chained until one of them returns `TRUE`. The functions have to check the class id passed in to figure whether they actually want to cache the class of this type, since all classes are routed through the same GTypeClassCacheFunc chain.

`cache_data` : | data to be passed to `cache_func`
`cache_func` : | a GTypeClassCacheFunc

### g_type_remove_class_cache_func ()

    void                g_type_remove_class_cache_func      (gpointer cache_data,
                                                             GTypeClassCacheFunc cache_func);

Removes a previously installed GTypeClassCacheFunc. The cache maintained by `cache_func` has to be empty when calling `g_type_remove_class_cache_func()` to avoid leaks.

`cache_data` : | data that was given when adding `cache_func`
`cache_func` : | a GTypeClassCacheFunc

### g_type_class_unref_uncached ()

    void                g_type_class_unref_uncached         (gpointer g_class);

A variant of `g_type_class_unref()` for use in GTypeClassCacheFunc implementations. It unreferences a class without consulting the chain of GTypeClassCacheFuncs, avoiding the recursion which would occur otherwise.

`g_class` : | The GTypeClass structure to unreference.

### g_type_add_interface_check ()

    void                g_type_add_interface_check          (gpointer check_data,
                                                             GTypeInterfaceCheckFunc check_func);

Adds a function to be called after an interface vtable is initialized for any class (i.e. after the `interface_init` member of GInterfaceInfo has been called).

This function is useful when you want to check an invariant that depends on the interfaces of a class. For instance, the implementation of GObject uses this facility to check that an object implements all of the properties that are defined on its interfaces.

`check_data` : | data to pass to `check_func`
`check_func` : | function to be called after each interface is initialized.

Since 2.4

### g_type_remove_interface_check ()

    void                g_type_remove_interface_check       (gpointer check_data,
                                                             GTypeInterfaceCheckFunc check_func);

Removes an interface check function added with `g_type_add_interface_check()`.

`check_data` : | callback data passed to `g_type_add_interface_check()`
`check_func` : | callback function passed to `g_type_add_interface_check()`

Since 2.4

### GTypeInterfaceCheckFunc ()

    void                (*GTypeInterfaceCheckFunc)          (gpointer check_data,
                                                             gpointer g_iface);

A callback called after an interface vtable is initialized. See `g_type_add_interface_check()`.

`check_data` : | data passed to `g_type_add_interface_check()`.
`g_iface` : | the interface that has been initialized

Since 2.4

### g_type_value_table_peek ()

    GTypeValueTable*    g_type_value_table_peek             (GType type);

Returns the location of the GTypeValueTable associated with `type`. Note that this function should only be used from source code that implements or has internal knowledge of the implementation of `type`.

`type` : | A GType value.
Returns : | Location of the GTypeValueTable associated with `type` or `NULL` if there is no GTypeValueTable associated with `type`.

### G_DEFINE_TYPE()

    #define G_DEFINE_TYPE(TN, t_n, T_P)			    G_DEFINE_TYPE_EXTENDED (TN, t_n, T_P, 0, {})

A convenience macro for type implementations, which declares a class initialization function, an instance initialization function (see GTypeInfo for information about these) and a static variable named `t_n`_parent_class pointing to the parent class. Furthermore, it defines a *_get_type() function. See `G_DEFINE_TYPE_EXTENDED()` for an example.

`TN` : | The name of the new type, in Camel case.
`t_n` : | The name of the new type, in lowercase, with words separated by '_'.
`T_P` : | The GType of the parent type.

Since 2.4

### G_DEFINE_TYPE_WITH_CODE()

    #define G_DEFINE_TYPE_WITH_CODE(TN, t_n, T_P, _C_)	    _G_DEFINE_TYPE_EXTENDED_BEGIN (TN, t_n, T_P, 0) {_C_;} _G_DEFINE_TYPE_EXTENDED_END()

A convenience macro for type implementations. Similar to `G_DEFINE_TYPE()`, but allows to insert custom code into the *_get_type() function, e.g. interface implementations via `G_IMPLEMENT_INTERFACE()`. See `G_DEFINE_TYPE_EXTENDED()` for an example.

`TN` : | The name of the new type, in Camel case.
`t_n` : | The name of the new type in lowercase, with words separated by '_'.
`T_P` : | The GType of the parent type.
`_C_` : | Custom code that gets inserted in the *_get_type() function.

Since 2.4

### G_DEFINE_ABSTRACT_TYPE()

    #define G_DEFINE_ABSTRACT_TYPE(TN, t_n, T_P)		    G_DEFINE_TYPE_EXTENDED (TN, t_n, T_P, G_TYPE_FLAG_ABSTRACT, {})

A convenience macro for type implementations. Similar to `G_DEFINE_TYPE()`, but defines an abstract type. See `G_DEFINE_TYPE_EXTENDED()` for an example.

`TN` : | The name of the new type, in Camel case.
`t_n` : | The name of the new type, in lowercase, with words separated by '_'.
`T_P` : | The GType of the parent type.

Since 2.4

### G_DEFINE_ABSTRACT_TYPE_WITH_CODE()

    #define G_DEFINE_ABSTRACT_TYPE_WITH_CODE(TN, t_n, T_P, _C_) _G_DEFINE_TYPE_EXTENDED_BEGIN (TN, t_n, T_P, G_TYPE_FLAG_ABSTRACT) {_C_;} _G_DEFINE_TYPE_EXTENDED_END()

A convenience macro for type implementations. Similar to `G_DEFINE_TYPE_WITH_CODE()`, but defines an abstract type and allows to insert custom code into the *_get_type() function, e.g. interface implementations via `G_IMPLEMENT_INTERFACE()`. See `G_DEFINE_TYPE_EXTENDED()` for an example.

`TN` : | The name of the new type, in Camel case.
`t_n` : | The name of the new type, in lowercase, with words separated by '_'.
`T_P` : | The GType of the parent type.
`_C_` : | Custom code that gets inserted in the @`type_name_get_type()` function.

Since 2.4

### G_IMPLEMENT_INTERFACE()

    #define             G_IMPLEMENT_INTERFACE(TYPE_IFACE, iface_init)

A convenience macro to ease interface addition in the `_C_` section of `G_DEFINE_TYPE_WITH_CODE()` or `G_DEFINE_ABSTRACT_TYPE_WITH_CODE()`. See `G_DEFINE_TYPE_EXTENDED()` for an example.

Note that this macro can only be used together with the G_DEFINE_TYPE_* macros, since it depends on variable names from those macros.

`TYPE_IFACE` : | The GType of the interface to add
`iface_init` : | The interface init function

Since 2.4

### G_DEFINE_TYPE_EXTENDED()

    #define G_DEFINE_TYPE_EXTENDED(TN, t_n, T_P, _f_, _C_)	    _G_DEFINE_TYPE_EXTENDED_BEGIN (TN, t_n, T_P, _f_) {_C_;} _G_DEFINE_TYPE_EXTENDED_END()

The most general convenience macro for type implementations, on which `G_DEFINE_TYPE()`, etc are based.

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    G_DEFINE_TYPE_EXTENDED (GtkGadget,
                            gtk_gadget,
                            GTK_TYPE_WIDGET,
                            0,
                            G_IMPLEMENT_INTERFACE (TYPE_GIZMO,
                                                   gtk_gadget_gizmo_init));

expands to

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    static void     gtk_gadget_init       (GtkGadget      *self);
    static void     gtk_gadget_class_init (GtkGadgetClass *klass);
    static gpointer gtk_gadget_parent_class = NULL;
    static void     gtk_gadget_class_intern_init (gpointer klass)
    {
      gtk_gadget_parent_class = g_type_class_peek_parent (klass);
      gtk_gadget_class_init ((GtkGadgetClass*) klass);
    }

    GType
    gtk_gadget_get_type (void)
    {
      static volatile gsize g_define_type_id__volatile = 0;
      if (g_once_init_enter (&g_define_type_id__volatile))
        {
          GType g_define_type_id =
            g_type_register_static_simple (GTK_TYPE_WIDGET,
                                           g_intern_static_string ("GtkGadget"),
                                           sizeof (GtkGadgetClass),
                                           (GClassInitFunc) gtk_gadget_class_intern_init,
                                           sizeof (GtkGadget),
                                           (GInstanceInitFunc) gtk_gadget_init,
                                           (GTypeFlags) flags);
          {
            static const GInterfaceInfo g_implement_interface_info = {
              (GInterfaceInitFunc) gtk_gadget_gizmo_init
            };
            g_type_add_interface_static (g_define_type_id, TYPE_GIZMO, &g_implement_interface_info);
          }
          g_once_init_leave (&g_define_type_id__volatile, g_define_type_id);
        }
      return g_define_type_id__volatile;
    }

The only pieces which have to be manually provided are the definitions of the instance and class structure and the definitions of the instance and class init functions.

`TN` : | The name of the new type, in Camel case.
`t_n` : | The name of the new type, in lowercase, with words separated by '_'.
`T_P` : | The GType of the parent type.
`_f_` : | GTypeFlags to pass to `g_type_register_static()`
`_C_` : | Custom code that gets inserted in the *_get_type() function.

Since 2.4

### G_TYPE_INVALID

    #define G_TYPE_INVALID			G_TYPE_MAKE_FUNDAMENTAL (0)

An invalid GType used as error return value in some functions which return a GType.

### G_TYPE_NONE

    #define G_TYPE_NONE			G_TYPE_MAKE_FUNDAMENTAL (1)

A fundamental type which is used as a replacement for the C `void` return type.

### G_TYPE_INTERFACE

    #define G_TYPE_INTERFACE		G_TYPE_MAKE_FUNDAMENTAL (2)

The fundamental type from which all interfaces are derived.

### G_TYPE_CHAR

    #define G_TYPE_CHAR			G_TYPE_MAKE_FUNDAMENTAL (3)

The fundamental type corresponding to gchar. The type designated by G_TYPE_CHAR is unconditionally an 8-bit signed integer. This may or may not be the same type a the C type "gchar".

### G_TYPE_UCHAR

    #define G_TYPE_UCHAR			G_TYPE_MAKE_FUNDAMENTAL (4)

The fundamental type corresponding to guchar.

### G_TYPE_BOOLEAN

    #define G_TYPE_BOOLEAN			G_TYPE_MAKE_FUNDAMENTAL (5)

The fundamental type corresponding to gboolean.

### G_TYPE_INT

    #define G_TYPE_INT			G_TYPE_MAKE_FUNDAMENTAL (6)

The fundamental type corresponding to gint.

### G_TYPE_UINT

    #define G_TYPE_UINT			G_TYPE_MAKE_FUNDAMENTAL (7)

The fundamental type corresponding to guint.

### G_TYPE_LONG

    #define G_TYPE_LONG			G_TYPE_MAKE_FUNDAMENTAL (8)

The fundamental type corresponding to glong.

### G_TYPE_ULONG

    #define G_TYPE_ULONG			G_TYPE_MAKE_FUNDAMENTAL (9)

The fundamental type corresponding to gulong.

### G_TYPE_INT64

    #define G_TYPE_INT64			G_TYPE_MAKE_FUNDAMENTAL (10)

The fundamental type corresponding to gint64.

### G_TYPE_UINT64

    #define G_TYPE_UINT64			G_TYPE_MAKE_FUNDAMENTAL (11)

The fundamental type corresponding to guint64.

### G_TYPE_ENUM

    #define G_TYPE_ENUM			G_TYPE_MAKE_FUNDAMENTAL (12)

The fundamental type from which all enumeration types are derived.

### G_TYPE_FLAGS

    #define G_TYPE_FLAGS			G_TYPE_MAKE_FUNDAMENTAL (13)

The fundamental type from which all flags types are derived.

### G_TYPE_FLOAT

    #define G_TYPE_FLOAT			G_TYPE_MAKE_FUNDAMENTAL (14)

The fundamental type corresponding to gfloat.

### G_TYPE_DOUBLE

    #define G_TYPE_DOUBLE			G_TYPE_MAKE_FUNDAMENTAL (15)

The fundamental type corresponding to gdouble.

### G_TYPE_STRING

    #define G_TYPE_STRING			G_TYPE_MAKE_FUNDAMENTAL (16)

The fundamental type corresponding to nul-terminated C strings.

### G_TYPE_POINTER

    #define G_TYPE_POINTER			G_TYPE_MAKE_FUNDAMENTAL (17)

The fundamental type corresponding to gpointer.

### G_TYPE_BOXED

    #define G_TYPE_BOXED			G_TYPE_MAKE_FUNDAMENTAL (18)

The fundamental type from which all boxed types are derived.

### G_TYPE_PARAM

    #define G_TYPE_PARAM			G_TYPE_MAKE_FUNDAMENTAL (19)

The fundamental type from which all GParamSpec types are derived.

### G_TYPE_OBJECT

    #define G_TYPE_OBJECT			G_TYPE_MAKE_FUNDAMENTAL (20)

The fundamental type for GObject.

### G_TYPE_GTYPE

    #define G_TYPE_GTYPE			 (g_gtype_get_type())

The type for GType.

### G_TYPE_RESERVED_GLIB_FIRST

    #define G_TYPE_RESERVED_GLIB_FIRST (21)

First fundamental type number to create a new fundamental type id with `G_TYPE_MAKE_FUNDAMENTAL()` reserved for GLib.

### G_TYPE_RESERVED_GLIB_LAST

    #define G_TYPE_RESERVED_GLIB_LAST (31)

Last fundamental type number reserved for GLib.

### G_TYPE_RESERVED_BSE_FIRST

    #define G_TYPE_RESERVED_BSE_FIRST (32)

First fundamental type number to create a new fundamental type id with `G_TYPE_MAKE_FUNDAMENTAL()` reserved for BSE.

### G_TYPE_RESERVED_BSE_LAST

    #define G_TYPE_RESERVED_BSE_LAST (48)

Last fundamental type number reserved for BSE.

### G_TYPE_RESERVED_USER_FIRST

    #define G_TYPE_RESERVED_USER_FIRST (49)

First available fundamental type number to create new fundamental type id with `G_TYPE_MAKE_FUNDAMENTAL()`.