class GLib::VariantType

Overview

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:

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:

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.

Defined in:

auto/g_lib-2.0/variant_type.cr

Constructors

Class Method Summary

Instance Method Summary

Constructor Detail

def self.new(pointer : Pointer(Void), transfer : GICrystal::Transfer) #

def self.new(type_string : String) #

def self.new_array(element : GLib::VariantType) : self #

def self.new_dict_entry(key : GLib::VariantType, value : GLib::VariantType) : self #

def self.new_maybe(element : GLib::VariantType) : self #

def self.new_tuple(items : Enumerable(GLib::VariantType)) : self #

Class Method Detail

def self.checked_(arg0 : String) : GLib::VariantType #

def self.g_type : UInt64 #

Returns the type id (GType) registered in GLib type system.


def self.new_tuple(*items : GLib::VariantType) #

def self.string_get_depth_(type_string : String) : UInt64 #

def self.string_is_valid(type_string : String) : Bool #

def self.string_scan(string : String, limit : String?) : Bool #

Instance Method Detail

def ==(other : self) : Bool #
Description copied from class Reference

Returns true if this reference is the same as other. Invokes same?.


def _next : GLib::VariantType #

def copy : GLib::VariantType #

def dup_string : String #

def element : GLib::VariantType #

def equal(type2 : GLib::VariantType) : Bool #

def finalize #

def first : GLib::VariantType #

def free : Nil #

def hash : UInt32 #
Description copied from class Object

Generates an UInt64 hash value for this object.

This method must have the property that a == b implies a.hash == b.hash.

The hash value is used along with #== by the Hash class to determine if two objects reference the same hash key.

Subclasses must not override this method. Instead, they must define hash(hasher), though usually the macro def_hash can be used to generate this method.


def is_array : Bool #

def is_basic : Bool #

def is_container : Bool #

def is_definite : Bool #

def is_dict_entry : Bool #

def is_maybe : Bool #

def is_subtype_of(supertype : GLib::VariantType) : Bool #

def is_tuple : Bool #

def is_variant : Bool #

def key : GLib::VariantType #

def n_items : UInt64 #

def string_length : UInt64 #

def to_unsafe : Pointer(Void) #

def value : GLib::VariantType #