Function
GObjectTypeValueCollectFunc
since: 2.78
Declaration
gchar*
(* GTypeValueCollectFunc) (
GValue* value,
guint n_collect_values,
GTypeCValue* collect_values,
guint collect_flags
)
Description [src]
This function is responsible for converting the values collected from
a variadic argument list into contents suitable for storage in a GValue.
This function should setup value similar to GTypeValueInitFunc; e.g.
for a string value that does not allow NULL pointers, it needs to either
emit an error, or do an implicit conversion by storing an empty string.
The value passed in to this function has a zero-filled data array, so
just like for GTypeValueInitFunc it is guaranteed to not contain any old
contents that might need freeing.
The n_collect_values argument is the string length of the collect_format
field of GTypeValueTable, and collect_values is an array of GTypeCValue
with length of n_collect_values, containing the collected values according
to collect_format.
The collect_flags argument provided as a hint by the caller. It may
contain the flag G_VALUE_NOCOPY_CONTENTS indicating that the collected
value contents may be considered ‘static’ for the duration of the value
lifetime. Thus an extra copy of the contents stored in collect_values is
not required for assignment to value.
For our above string example, we continue with:
if (!collect_values[0].v_pointer)
value->data[0].v_pointer = g_strdup ("");
else if (collect_flags & G_VALUE_NOCOPY_CONTENTS)
{
value->data[0].v_pointer = collect_values[0].v_pointer;
// keep a flag for the `value_free()` implementation to not free this string
value->data[1].v_uint = G_VALUE_NOCOPY_CONTENTS;
}
else
value->data[0].v_pointer = g_strdup (collect_values[0].v_pointer);
return NULL;
It should be noted, that it is generally a bad idea to follow the
G_VALUE_NOCOPY_CONTENTS hint for reference counted types. Due to
reentrancy requirements and reference count assertions performed
by the signal emission code, reference counts should always be
incremented for reference counted contents stored in the value->data
array. To deviate from our string example for a moment, and taking
a look at an exemplary implementation for GTypeValueTable.collect_value()
of GObject:
GObject *object = G_OBJECT (collect_values[0].v_pointer);
g_return_val_if_fail (object != NULL,
g_strdup_printf ("Object %p passed as invalid NULL pointer", object));
// never honour G_VALUE_NOCOPY_CONTENTS for ref-counted types
value->data[0].v_pointer = g_object_ref (object);
return NULL;
The reference count for valid objects is always incremented, regardless
of collect_flags. For invalid objects, the example returns a newly
allocated string without altering value.
Upon success, collect_value() needs to return NULL. If, however,
an error condition occurred, collect_value() should return a newly
allocated string containing an error diagnostic.
The calling code makes no assumptions about the value contents being
valid upon error returns, value is simply thrown away without further
freeing. As such, it is a good idea to not allocate GValue contents
prior to returning an error; however, collect_values() is not obliged
to return a correctly setup value for error returns, simply because
any non-NULL return is considered a fatal programming error, and
further program behaviour is undefined.
Available since: 2.78
Parameters
value-
Type:
GValueThe value to initialize.
The data is owned by the caller of the function. n_collect_values-
Type:
guintThe number of collected values.
collect_values-
Type: An array of
GTypeCValueThe collected values.
The length of the array is specified in the n_collect_valuesargument.The data is owned by the caller of the function. collect_flags-
Type:
guintOptional flags.