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GObject Reference Manual | ![]() |
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Top | Description |
#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);
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.
#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; }
#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
.
|
A GValue structure. |
#define G_VALUE_TYPE_NAME(value) (g_type_name (G_VALUE_TYPE (value)))
Gets the the type name of value
.
|
A GValue structure. |
#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.
|
A GType value. |
#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.
|
A GType value. |
#define G_IS_VALUE(value) (G_TYPE_CHECK_VALUE (value))
Checks if value
is a valid and initialized GValue structure.
|
A GValue structure. |
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.
#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 ())
The type ID of the "GValueArray" type which is a boxed type, used to pass around pointers to GValueArrays.
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
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.
|
An initialized GValue structure. |
void g_value_set_instance (GValue *value, gpointer instance);
Sets value
from an instantiatable type via the
value_table's collect_value()
function.
|
An initialized GValue structure. |
|
the instance |
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);
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.
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
.
|
source type to be copied. |
|
destination type for copying. |
Returns : |
TRUE if g_value_copy() is possible with src_type and 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
.
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).
|
Source value. |
|
Target value. |
Returns : |
Whether a transformation rule was found and could be applied.
Upon failing transformations, dest_value is left untouched.
|
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()
.
|
Source value. |
|
Target value. |
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.
|
Source type. |
|
Target type. |
|
a function which transforms values of type src_type
into value of type dest_type
|
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.
|
GValue which contents are to be described. |
Returns : |
Newly allocated string. |
g_value_init()
are defined by a separate interface. See the Standard
Values API for details.