## Generic values

Generic values — A polymorphic type that can hold values of any other type

## Synopsis

    #include <glib-object.h>

    #define             G_VALUE_HOLDS                       (value,
                                                             type)
    #define             G_VALUE_TYPE                        (value)
    #define             G_VALUE_TYPE_NAME                   (value)
    #define             G_TYPE_IS_VALUE                     (type)
    #define             G_TYPE_IS_VALUE_ABSTRACT            (type)
    #define             G_IS_VALUE                          (value)
                        GValue;
    #define             G_TYPE_VALUE
    #define             G_TYPE_VALUE_ARRAY
    GValue*             g_value_init                        (GValue *value,
                                                             GType g_type);
    void                g_value_copy                        (const GValue *src_value,
                                                             GValue *dest_value);
    GValue*             g_value_reset                       (GValue *value);
    void                g_value_unset                       (GValue *value);
    void                g_value_set_instance                (GValue *value,
                                                             gpointer instance);
    gboolean            g_value_fits_pointer                (const GValue *value);
    gpointer            g_value_peek_pointer                (const GValue *value);
    gboolean            g_value_type_compatible             (GType src_type,
                                                             GType dest_type);
    gboolean            g_value_type_transformable          (GType src_type,
                                                             GType dest_type);
    gboolean            g_value_transform                   (const GValue *src_value,
                                                             GValue *dest_value);
    void                (*GValueTransform)                  (const GValue *src_value,
                                                             GValue *dest_value);
    void                g_value_register_transform_func     (GType src_type,
                                                             GType dest_type,
                                                             GValueTransform transform_func);
    gchar*              g_strdup_value_contents             (const GValue *value);

## Description

The GValue structure is basically a variable container that consists of a type identifier and a specific value of that type. The type identifier within a GValue structure always determines the type of the associated value. To create a undefined GValue structure, simply create a zero-filled GValue structure. To initialize the GValue, use the `g_value_init()` function. A GValue cannot be used until it is initialized. The basic type operations (such as freeing and copying) are determined by the GTypeValueTable associated with the type ID stored in the GValue. Other GValue operations (such as converting values between types) are provided by this interface.

The code in the example program below demonstrates GValue's features.

    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19
    20
    21
    22
    23
    24
    25
    26
    27
    28
    29
    30
    31
    32
    33
    34
    35
    36
    37
    38
    39
    40
    41
    42
    43
    44
    45
    46
    47
    48
    49
    50
    51
    52
    53
    54

    #include <glib-object.h>

    static void
    int2string (const GValue *src_value,
                GValue       *dest_value)
    {
      if (g_value_get_int (src_value) == 42)
        g_value_set_static_string (dest_value, "An important number");
      else
        g_value_set_static_string (dest_value, "What's that?");
    }

    int
    main (int   argc,
          char *argv[])
    {
      /* GValues must start zero-filled */
      GValue a = {0};
      GValue b = {0};
      const gchar *message;

      g_type_init ();

      /* The GValue starts empty */
      g_assert (!G_VALUE_HOLDS_STRING (&a));

      /* Put a string in it */
      g_value_init (&a, G_TYPE_STRING);
      g_assert (G_VALUE_HOLDS_STRING (&a));
      g_value_set_static_string (&a, "Hello, world!");
      g_printf ("%s\n", g_value_get_string (&a));

      /* Reset it to its pristine state */
      g_value_unset (&a);

      /* It can then be reused for another type */
      g_value_init (&a, G_TYPE_INT);
      g_value_set_int (&a, 42);

      /* Attempt to transform it into a GValue of type STRING */
      g_value_init (&b, G_TYPE_STRING);

      /* An INT is transformable to a STRING */
      g_assert (g_value_type_transformable (G_TYPE_INT, G_TYPE_STRING));

      g_value_transform (&a, &b);
      g_printf ("%s\n", g_value_get_string (&b));

      /* Attempt to transform it again using a custom transform function */
      g_value_register_transform_func (G_TYPE_INT, G_TYPE_STRING, int2string);
      g_value_transform (&a, &b);
      g_printf ("%s\n", g_value_get_string (&b));
      return 0;
    }

## Details

### G_VALUE_HOLDS()

    #define G_VALUE_HOLDS(value,type) (G_TYPE_CHECK_VALUE_TYPE ((value), (type)))

Checks if `value` holds (or contains) a value of `type`. This macro will also check for `value` != `NULL` and issue a warning if the check fails.

`value` : | A GValue structure.
`type` : | A GType value.
Returns : | `TRUE` if `value` holds the `type`.

### G_VALUE_TYPE()

    #define G_VALUE_TYPE(value)		(((GValue*) (value))->g_type)

Get the type identifier of `value`.

`value` : | A GValue structure.
Returns : | the GType.

### G_VALUE_TYPE_NAME()

    #define G_VALUE_TYPE_NAME(value) (g_type_name (G_VALUE_TYPE (value)))

Gets the the type name of `value`.

`value` : | A GValue structure.
Returns : | the type name.

### G_TYPE_IS_VALUE()

    #define G_TYPE_IS_VALUE(type)		(g_type_check_is_value_type (type))

Checks whether the passed in type ID can be used for `g_value_init()`. That is, this macro checks whether this type provides an implementation of the GTypeValueTable functions required for a type to create a GValue of.

`type` : | A GType value.
Returns : | Whether `type` is suitable as a GValue type.

### G_TYPE_IS_VALUE_ABSTRACT()

    #define G_TYPE_IS_VALUE_ABSTRACT(type)          (g_type_test_flags ((type), G_TYPE_FLAG_VALUE_ABSTRACT))

Checks if `type` is an abstract value type. An abstract value type introduces a value table, but can't be used for `g_value_init()` and is normally used as an abstract base type for derived value types.

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

### G_IS_VALUE()

    #define G_IS_VALUE(value)		(G_TYPE_CHECK_VALUE (value))

Checks if `value` is a valid and initialized GValue structure.

`value` : | A GValue structure.
Returns : | `TRUE` on success.

### GValue

    typedef struct {
    } GValue;

An opaque structure used to hold different types of values. The data within the structure has protected scope: it is accessible only to functions within a GTypeValueTable structure, or implementations of the g_value_*() API. That is, code portions which implement new fundamental types. GValue users can not make any assumptions about how data is stored within the 2 element `data` union, and the `g_type` member should only be accessed through the `G_VALUE_TYPE()` macro.

### G_TYPE_VALUE

    #define G_TYPE_VALUE		(g_value_get_type ())

The type ID of the "GValue" type which is a boxed type, used to pass around pointers to GValues.

### G_TYPE_VALUE_ARRAY

    #define G_TYPE_VALUE_ARRAY (g_value_array_get_type ())

The type ID of the "GValueArray" type which is a boxed type, used to pass around pointers to GValueArrays.

### g_value_init ()

    GValue*             g_value_init                        (GValue *value,
                                                             GType g_type);

Initializes `value` with the default value of `type`.

`value` : | A zero-filled (uninitialized) GValue structure.
`g_type` : | Type the GValue should hold values of.
Returns : | the GValue structure that has been passed in

### g_value_copy ()

    void                g_value_copy                        (const GValue *src_value,
                                                             GValue *dest_value);

Copies the value of `src_value` into `dest_value`.

`src_value` : | An initialized GValue structure.
`dest_value` : | An initialized GValue structure of the same type as `src_value`.

### g_value_reset ()

    GValue*             g_value_reset                       (GValue *value);

Clears the current value in `value` and resets it to the default value (as if the value had just been initialized).

`value` : | An initialized GValue structure.
Returns : | the GValue structure that has been passed in

### g_value_unset ()

    void                g_value_unset                       (GValue *value);

Clears the current value in `value` and "unsets" the type, this releases all resources associated with this GValue. An unset value is the same as an uninitialized (zero-filled) GValue structure.

`value` : | An initialized GValue structure.

### g_value_set_instance ()

    void                g_value_set_instance                (GValue *value,
                                                             gpointer instance);

Sets `value` from an instantiatable type via the value_table's `collect_value()` function.

`value` : | An initialized GValue structure.
`instance` : | the instance

### g_value_fits_pointer ()

    gboolean            g_value_fits_pointer                (const GValue *value);

Determines if `value` will fit inside the size of a pointer value. This is an internal function introduced mainly for C marshallers.

`value` : | An initialized GValue structure.
Returns : | `TRUE` if `value` will fit inside a pointer value.

### g_value_peek_pointer ()

    gpointer            g_value_peek_pointer                (const GValue *value);

Return the value contents as pointer. This function asserts that `g_value_fits_pointer()` returned `TRUE` for the passed in value. This is an internal function introduced mainly for C marshallers.

`value` : | An initialized GValue structure.
Returns : | `TRUE` if `value` will fit inside a pointer value.

### g_value_type_compatible ()

    gboolean            g_value_type_compatible             (GType src_type,
                                                             GType dest_type);

Returns whether a GValue of type `src_type` can be copied into a GValue of type `dest_type`.

`src_type` : | source type to be copied.
`dest_type` : | destination type for copying.
Returns : | `TRUE` if `g_value_copy()` is possible with `src_type` and `dest_type`.

### g_value_type_transformable ()

    gboolean            g_value_type_transformable          (GType src_type,
                                                             GType dest_type);

Check whether `g_value_transform()` is able to transform values of type `src_type` into values of type `dest_type`.

`src_type` : | Source type.
`dest_type` : | Target type.
Returns : | `TRUE` if the transformation is possible, `FALSE` otherwise.

### g_value_transform ()

    gboolean            g_value_transform                   (const GValue *src_value,
                                                             GValue *dest_value);

Tries to cast the contents of `src_value` into a type appropriate to store in `dest_value`, e.g. to transform a `G_TYPE_INT` value into a `G_TYPE_FLOAT` value. Performing transformations between value types might incur precision lossage. Especially transformations into strings might reveal seemingly arbitrary results and shouldn't be relied upon for production code (such as rcfile value or object property serialization).

`src_value` : | Source value.
`dest_value` : | Target value.
Returns : | Whether a transformation rule was found and could be applied. Upon failing transformations, `dest_value` is left untouched.

### GValueTransform ()

    void                (*GValueTransform)                  (const GValue *src_value,
                                                             GValue *dest_value);

The type of value transformation functions which can be registered with `g_value_register_transform_func()`.

`src_value` : | Source value.
`dest_value` : | Target value.

### g_value_register_transform_func ()

    void                g_value_register_transform_func     (GType src_type,
                                                             GType dest_type,
                                                             GValueTransform transform_func);

Registers a value transformation function for use in `g_value_transform()`. A previously registered transformation function for `src_type` and `dest_type` will be replaced.

`src_type` : | Source type.
`dest_type` : | Target type.
`transform_func` : | a function which transforms values of type `src_type` into value of type `dest_type`

### g_strdup_value_contents ()

    gchar*              g_strdup_value_contents             (const GValue *value);

Return a newly allocated string, which describes the contents of a GValue. The main purpose of this function is to describe GValue contents for debugging output, the way in which the contents are described may change between different GLib versions.

`value` : | GValue which contents are to be described.
Returns : | Newly allocated string.

## See Also

The fundamental types which all support GValue operations and thus can be used as a type initializer for `g_value_init()` are defined by a separate interface. See the Standard Values API for details.