GLibVariantType

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 GtkBin can exist (since GtkBin is an abstract class), but a GtkWindow can certainly be instantiated, and you would say that the GtkWindow is a GtkBin (since GtkWindow is a subclass of GtkBin).

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’ or ‘m’, 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 of G_VARIANT_TYPE_BOOLEAN; a boolean value. - y: the type string of G_VARIANT_TYPE_BYTE; a byte. - n: the type string of G_VARIANT_TYPE_INT16; a signed 16 bit integer. - q: the type string of G_VARIANT_TYPE_UINT16; an unsigned 16 bit integer. - i: the type string of G_VARIANT_TYPE_INT32; a signed 32 bit integer. - u: the type string of G_VARIANT_TYPE_UINT32; an unsigned 32 bit integer. - x: the type string of G_VARIANT_TYPE_INT64; a signed 64 bit integer. - t: the type string of G_VARIANT_TYPE_UINT64; an unsigned 64 bit integer. - h: the type string of G_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 of G_VARIANT_TYPE_DOUBLE; a double precision floating point value. - s: the type string of G_VARIANT_TYPE_STRING; a string. - o: the type string of G_VARIANT_TYPE_OBJECT_PATH; a string in the form of a D-Bus object path. - g: the type string of G_VARIANT_TYPE_SIGNATURE; a string in the form of a D-Bus type signature. - ?: the type string of G_VARIANT_TYPE_BASIC; an indefinite type that is a supertype of any of the basic types. - v: the type string of G_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 string “ai”, 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 string “ms”, 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 of G_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 string “a{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.