glib/docs/reference/gobject/tmpl/generic_values.sgml

<!-- ##### SECTION Title ##### -->
Generic Values

<!-- ##### SECTION Short_Description ##### -->
A polymorphic type that can hold values of any other type

<!-- ##### SECTION Long_Description ##### -->
<para>
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.
</para>

<!-- ##### SECTION See_Also ##### -->
<para>
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 <link 
linkend="gobject-Standard-Parameter-and-Value-Types">Standard Values 
API</link> for details.
</para>

<!-- ##### SECTION Stability_Level ##### -->


<!-- ##### MACRO G_VALUE_HOLDS ##### -->
<para>
Returns %TRUE 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.
</para>

@value: A #GValue structure.
@type: A #GType value.


<!-- ##### MACRO G_VALUE_TYPE ##### -->
<para>
Returns the type identifier of @value.
</para>

@value: A #GValue structure.


<!-- ##### MACRO G_VALUE_TYPE_NAME ##### -->
<para>
Returns the type name of @value.
</para>

@value: A #GValue structure.


<!-- ##### MACRO G_TYPE_IS_VALUE ##### -->
<para>
Return 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.
</para>

@type: 	  A #GType value.
@Returns: Whether @type is suitable as a #GValue type.


<!-- ##### MACRO G_TYPE_IS_VALUE_ABSTRACT ##### -->
<para>
Returns %TRUE 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.
</para>

@type: A #GType value.


<!-- ##### MACRO G_IS_VALUE ##### -->
<para>
Returns %TRUE if @value is a valid and initialized #GValue structure.
</para>

@value: A #GValue structure.


<!-- ##### STRUCT GValue ##### -->
<para>
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.
</para>


<!-- ##### MACRO G_TYPE_VALUE ##### -->
<para>
Returns the type ID of the "GValue" type which is a boxed type,
used to pass around pointers to GValues.
</para>



<!-- ##### MACRO G_TYPE_VALUE_ARRAY ##### -->
<para>
Returns the type ID of the "GValueArray" type which is a boxed type,
used to pass around pointers to GValueArrays.
</para>



<!-- ##### FUNCTION g_value_init ##### -->
<para>
Initializes @value with the default value of @type.
</para>

@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


<!-- ##### FUNCTION g_value_copy ##### -->
<para>
Copies the value of @src_value into @dest_value.
</para>

@src_value: An initialized #GValue structure.
@dest_value: An initialized #GValue structure of the same type as @src_value.


<!-- ##### FUNCTION g_value_reset ##### -->
<para>
Clears the current value in @value and resets it to the default value
(as if the value had just been initialized).
</para>

@value: An initialized #GValue structure.
@Returns: the #GValue structure that has been passed in


<!-- ##### FUNCTION g_value_unset ##### -->
<para>
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.
</para>

@value: 	An initialized #GValue structure.


<!-- ##### FUNCTION g_value_set_instance ##### -->
<para>
Sets @value from an instantiatable type via the 
value_table's collect_value() function.
</para>

@value: An initialized #GValue structure.
@instance: 


<!-- ##### FUNCTION g_value_fits_pointer ##### -->
<para>
Determines if @value will fit inside the size of a pointer value.
This is an internal function introduced mainly for C marshallers.
</para>

@value: An initialized #GValue structure.
@Returns: %TRUE if @value will fit inside a pointer value.


<!-- ##### FUNCTION g_value_peek_pointer ##### -->
<para>
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.
</para>

@value: An initialized #GValue structure.
@Returns: %TRUE if @value will fit inside a pointer value.


<!-- ##### FUNCTION g_value_type_compatible ##### -->
<para>
Returns whether a #GValue of type @src_type can be copied into
a #GValue of type @dest_type.
</para>

@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.


<!-- ##### FUNCTION g_value_type_transformable ##### -->
<para>
Check whether g_value_transform() is able to transform values
of type @src_type into values of type @dest_type.
</para>

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


<!-- ##### FUNCTION g_value_transform ##### -->
<para>
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).
</para>

@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.


<!-- ##### USER_FUNCTION GValueTransform ##### -->
<para>
The type of value transformation functions which can be registered with
g_value_register_transform_func().
</para>

@src_value:  Source value.
@dest_value: Target value.


<!-- ##### FUNCTION g_value_register_transform_func ##### -->
<para>
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.
</para>

@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


<!-- ##### FUNCTION g_strdup_value_contents ##### -->
<para>
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.
</para>

@value:   #GValue which contents are to be described.
@Returns: Newly allocated string.