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| #define | G_VALUE_HOLDS() |
| #define | G_VALUE_TYPE() |
| #define | G_VALUE_TYPE_NAME() |
| #define | G_TYPE_IS_VALUE() |
| #define | G_TYPE_IS_VALUE_ABSTRACT() |
| #define | G_IS_VALUE() |
| GValue * | g_value_init () |
| void | g_value_copy () |
| GValue * | g_value_reset () |
| void | g_value_unset () |
| void | g_value_init_from_instance () |
| void | g_value_set_instance () |
| gboolean | g_value_fits_pointer () |
| gpointer | g_value_peek_pointer () |
| gboolean | g_value_type_compatible () |
| gboolean | g_value_type_transformable () |
| gboolean | g_value_transform () |
| void | (*GValueTransform) () |
| void | g_value_register_transform_func () |
| gchar * | g_strdup_value_contents () |
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 an 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. Before
destruction you must always use g_value_unset() to make sure allocated
memory is freed.
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; } |
See also [gobject-Standard-Parameter-and-Value-Types] for more information on validation of GValue.
For letting a GValue own (and memory manage) arbitrary types or pointers,
they need to become a boxed type. The example below shows how
the pointer mystruct of type MyStruct is used as a boxed type.
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typedef struct { ... } MyStruct; G_DEFINE_BOXED_TYPE (MyStruct, my_struct, my_struct_copy, my_struct_free) // These two lines normally go in a public header. By GObject convention, // the naming scheme is NAMESPACE_TYPE_NAME: #define MY_TYPE_STRUCT (my_struct_get_type ()) GType my_struct_get_type (void); void foo () { GValue *value = g_new0 (GValue, 1); g_value_init (value, MY_TYPE_STRUCT); g_value_set_boxed (value, mystruct); // [... your code ....] g_value_unset (value); g_value_free (value); } |
#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.
#define G_VALUE_TYPE(value) (((GValue*) (value))->g_type)
Get the type identifier of value
.
#define G_VALUE_TYPE_NAME(value) (g_type_name (G_VALUE_TYPE (value)))
Gets the type name of 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.
#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.
#define G_IS_VALUE(value) (G_TYPE_CHECK_VALUE (value))
Checks if value
is a valid and initialized GValue structure.
GValue * g_value_init (GValue *value,GType g_type);
Initializes value
with the default value of type
.
void g_value_copy (const GValue *src_value,GValue *dest_value);
Copies the value of src_value
into dest_value
.
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).
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.
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().
value |
An uninitialized GValue structure. |
|
instance |
the instance. |
[type GObject.TypeInstance] |
Since: 2.42
void g_value_set_instance (GValue *value,gpointer instance);
Sets value
from an instantiatable type via the
value_table's collect_value() function.
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.
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.
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
.
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.
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).
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().
dest_value
will be initialized to the correct destination type.
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]
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.
#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
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.
#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.
#define G_TYPE_VALUE_ARRAY (g_value_array_get_type ()) GLIB_DEPRECATED_MACRO_IN_2_32_FOR(G_TYPE_ARRAY)
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.
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