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Generic values

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

Types and Values

#define G_VALUE_INIT
  GValue
#define G_TYPE_VALUE
#define G_TYPE_VALUE_ARRAY

Includes

#include <glib-object.h>

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.

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#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 be initialized
  GValue a = G_VALUE_INIT;
  GValue b = G_VALUE_INIT;
  const gchar *message;

  // 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;
}

Functions

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.

Parameters

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 .

Parameters

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 type name of value .

Parameters

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.

Parameters

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.

Parameters

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.

Parameters

value

A GValue structure.

 

Returns

TRUE on success.


g_value_init ()

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

Initializes value with the default value of type .

Parameters

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.

[transfer none]


g_value_copy ()

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

Copies the value of src_value into dest_value .

Parameters

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).

Parameters

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 (if any) 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.

Parameters

value

An initialized GValue structure.

 

g_value_init_from_instance ()

void
g_value_init_from_instance (GValue *value,
                            gpointer instance);

Initializes and sets value from an instantiatable type via the value_table's collect_value() function.

Note: The value will be initialised with the exact type of instance . If you wish to set the value 's type to a different GType (such as a parent class GType), you need to manually call g_value_init() and g_value_set_instance().

Parameters

value

An uninitialized GValue structure.

 

instance

the instance.

[type GObject.TypeInstance]

Since: 2.42


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.

Parameters

value

An initialized GValue structure.

 

instance

the instance.

[nullable]

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.

Parameters

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);

Returns 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.

Parameters

value

An initialized GValue structure

 

Returns

the value contents as pointer.

[transfer none]


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 .

Parameters

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 . Note that for the types to be transformable, they must be compatible or a transformation function must be registered.

Parameters

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).

Parameters

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().

Parameters

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.

[skip]

Parameters

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.

Parameters

value

GValue which contents are to be described.

 

Returns

Newly allocated string.

Types and Values

G_VALUE_INIT

#define G_VALUE_INIT  { 0, { { 0 } } }

A GValue must be initialized before it can be used. This macro can be used as initializer instead of an explicit { 0 } when declaring a variable, but it cannot be assigned to a variable.

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GValue value = G_VALUE_INIT;

Since: 2.30


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 cannot 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 ())

G_TYPE_VALUE_ARRAY has been deprecated since version 2.32 and should not be used in newly-written code.

Use GArray instead of GValueArray

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

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

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