Struct
GLibVariantType
since: 2.24
Description [src]
struct GVariantType {
/* No available fields */
}
A type in the GVariant
type system.
This section introduces the GVariant
type system. It is based, in
large part, on the D-Bus type system, with two major changes and
some minor lifting of restrictions. The
D-Bus specification,
therefore, provides a significant amount of
information that is useful when working with GVariant
.
The first major change with respect to the D-Bus type system is the
introduction of maybe (or ‘nullable’) types. Any type in GVariant
can be converted to a maybe type, in which case, nothing
(or null
)
becomes a valid value. Maybe types have been added by introducing the
character m
to type strings.
The second major change is that the GVariant
type system supports
the concept of ‘indefinite types’ — types that are less specific than
the normal types found in D-Bus. For example, it is possible to speak
of ‘an array of any type’ in GVariant
, where the D-Bus type system
would require you to speak of ‘an array of integers’ or ‘an array of
strings’. Indefinite types have been added by introducing the
characters *
, ?
and r
to type strings.
Finally, all arbitrary restrictions relating to the complexity of types are lifted along with the restriction that dictionary entries may only appear nested inside of arrays.
Just as in D-Bus, GVariant
types are described with strings (‘type
strings’). Subject to the differences mentioned above, these strings
are of the same form as those found in D-Bus. Note, however: D-Bus
always works in terms of messages and therefore individual type
strings appear nowhere in its interface. Instead, ‘signatures’
are a concatenation of the strings of the type of each argument in a
message. GVariant
deals with single values directly so
GVariant
type strings always describe the type of exactly one
value. This means that a D-Bus signature string is generally not a valid
GVariant
type string — except in the case that it is the signature
of a message containing exactly one argument.
An indefinite type is similar in spirit to what may be called an
abstract type in other type systems. No value can exist that has an
indefinite type as its type, but values can exist that have types
that are subtypes of indefinite types. That is to say,
g_variant_get_type()
will never return an indefinite type, but
calling g_variant_is_of_type()
with an indefinite type may return
true. For example, you cannot have a value that represents ‘an
array of no particular type’, but you can have an ‘array of integers’
which certainly matches the type of ‘an array of no particular type’,
since ‘array of integers’ is a subtype of ‘array of no particular type’.
This is similar to how instances of abstract classes may not
directly exist in other type systems, but instances of their
non-abstract subtypes may. For example, in GTK, no object that has
the type of GtkWidget
can
exist (since GtkWidget
is an abstract class), but a GtkWindow
can certainly be instantiated, and you would say that a GtkWindow
is a
GtkWidget
(since GtkWindow
is a subclass of GtkWidget
).
Two types may not be compared by value; use g_variant_type_equal()
or g_variant_type_is_subtype_of()
May be copied using
g_variant_type_copy()
and freed using g_variant_type_free()
.
GVariant Type Strings
A GVariant
type string can be any of the following:
- any basic type string (listed below)
v
,r
or*
- one of the characters
a
orm
, followed by another type string - the character
(
, followed by a concatenation of zero or more other type strings, followed by the character)
- the character
{
, followed by a basic type string (see below), followed by another type string, followed by the character}
A basic type string describes a basic type (as per
g_variant_type_is_basic()
) and is always a single character in
length. The valid basic type strings are b
, y
, n
, q
, i
, u
, x
,
t
, h
, d
, s
, o
, g
and ?
.
The above definition is recursive to arbitrary depth. aaaaai
and
(ui(nq((y)))s)
are both valid type strings, as is
a(aa(ui)(qna{ya(yd)}))
. In order to not hit memory limits,
GVariant
imposes a limit on recursion depth of 65 nested
containers. This is the limit in the D-Bus specification (64) plus one to
allow a GDBusMessage
to be nested in
a top-level tuple.
The meaning of each of the characters is as follows:
b
: the type string ofG_VARIANT_TYPE_BOOLEAN
; a boolean value.y
: the type string ofG_VARIANT_TYPE_BYTE
; a byte.n
: the type string ofG_VARIANT_TYPE_INT16
; a signed 16 bit integer.q
: the type string ofG_VARIANT_TYPE_UINT16
; an unsigned 16 bit integer.i
: the type string ofG_VARIANT_TYPE_INT32
; a signed 32 bit integer.u
: the type string ofG_VARIANT_TYPE_UINT32
; an unsigned 32 bit integer.x
: the type string ofG_VARIANT_TYPE_INT64
; a signed 64 bit integer.t
: the type string ofG_VARIANT_TYPE_UINT64
; an unsigned 64 bit integer.h
: the type string ofG_VARIANT_TYPE_HANDLE
; a signed 32 bit value that, by convention, is used as an index into an array of file descriptors that are sent alongside a D-Bus message.d
: the type string ofG_VARIANT_TYPE_DOUBLE
; a double precision floating point value.s
: the type string ofG_VARIANT_TYPE_STRING
; a string.o
: the type string ofG_VARIANT_TYPE_OBJECT_PATH
; a string in the form of a D-Bus object path.g
: the type string ofG_VARIANT_TYPE_SIGNATURE
; a string in the form of a D-Bus type signature.?
: the type string ofG_VARIANT_TYPE_BASIC
; an indefinite type that is a supertype of any of the basic types.v
: the type string ofG_VARIANT_TYPE_VARIANT
; a container type that contain any other type of value.a
: used as a prefix on another type string to mean an array of that type; the type stringai
, for example, is the type of an array of signed 32-bit integers.m
: used as a prefix on another type string to mean a ‘maybe’, or ‘nullable’, version of that type; the type stringms
, for example, is the type of a value that maybe contains a string, or maybe contains nothing.()
: used to enclose zero or more other concatenated type strings to create a tuple type; the type string(is)
, for example, is the type of a pair of an integer and a string.r
: the type string ofG_VARIANT_TYPE_TUPLE
; an indefinite type that is a supertype of any tuple type, regardless of the number of items.{}
: used to enclose a basic type string concatenated with another type string to create a dictionary entry type, which usually appears inside of an array to form a dictionary; the type stringa{sd}
, for example, is the type of a dictionary that maps strings to double precision floating point values.
The first type (the basic type) is the key type and the second type is
the value type. The reason that the first type is restricted to being a
basic type is so that it can easily be hashed.
- *
: the type string of G_VARIANT_TYPE_ANY
; the indefinite type that is
a supertype of all types. Note that, as with all type strings, this
character represents exactly one type. It cannot be used inside of tuples
to mean ‘any number of items’.
Any type string of a container that contains an indefinite type is,
itself, an indefinite type. For example, the type string a*
(corresponding to G_VARIANT_TYPE_ARRAY
) is an indefinite type
that is a supertype of every array type. (*s)
is a supertype
of all tuples that contain exactly two items where the second
item is a string.
a{?*}
is an indefinite type that is a supertype of all arrays
containing dictionary entries where the key is any basic type and
the value is any type at all. This is, by definition, a dictionary,
so this type string corresponds to G_VARIANT_TYPE_DICTIONARY
. Note
that, due to the restriction that the key of a dictionary entry must
be a basic type, {**}
is not a valid type string.
Available since: 2.24
Constructors
g_variant_type_new
Creates a new GVariantType
corresponding to the type string given
by type_string
. It is appropriate to call g_variant_type_free()
on
the return value.
since: 2.24
g_variant_type_new_array
Constructs the type corresponding to an array of elements of the
type type
.
g_variant_type_new_dict_entry
Constructs the type corresponding to a dictionary entry with a key
of type key
and a value of type value
.
g_variant_type_new_maybe
Constructs the type corresponding to a maybe instance containing
type type
or Nothing.
Functions
g_variant_type_string_is_valid
Checks if type_string
is a valid GVariant type string. This call is
equivalent to calling g_variant_type_string_scan()
and confirming
that the following character is a nul terminator.
g_variant_type_string_scan
Scan for a single complete and valid GVariant type string in string
.
The memory pointed to by limit
(or bytes beyond it) is never accessed.
since: 2.24
Instance methods
g_variant_type_copy
Makes a copy of a GVariantType
. It is appropriate to call
g_variant_type_free()
on the return value. type
may not be NULL
.
g_variant_type_dup_string
Returns a newly-allocated copy of the type string corresponding to
type
. The returned string is nul-terminated. It is appropriate to
call g_free()
on the return value.
g_variant_type_free
Frees a GVariantType
that was allocated with
g_variant_type_copy(), g_variant_type_new()
or one of the container
type constructor functions.
g_variant_type_get_string_length
Returns the length of the type string corresponding to the given
type
. This function must be used to determine the valid extent of
the memory region returned by g_variant_type_peek_string().
g_variant_type_is_array
Determines if the given type
is an array type. This is true if the
type string for type
starts with an ‘a’.
g_variant_type_is_dict_entry
Determines if the given type
is a dictionary entry type. This is
true if the type string for type
starts with a ‘{‘.
g_variant_type_is_maybe
Determines if the given type
is a maybe type. This is true if the
type string for type
starts with an ‘m’.
g_variant_type_is_tuple
Determines if the given type
is a tuple type. This is true if the
type string for type
starts with a ‘(‘ or if type
is
G_VARIANT_TYPE_TUPLE
.
g_variant_type_n_items
Determines the number of items contained in a tuple or dictionary entry type.
g_variant_type_peek_string
Returns the type string corresponding to the given type
. The
result is not nul-terminated; in order to determine its length you
must call g_variant_type_get_string_length().