### Struct

# GLib VariantType

since: 2.24

#### Description [src]

    struct GVariantType {
      /* No available fields */
    }

A type in the `GVariant` type system.

`GVariant` types are represented as strings, but have a strict syntax described below. All `GVariantType`s passed to GLib must be valid, and they are typically expected to be static (i.e. not provided by user input) as they determine how binary `GVariant` data is interpreted.

To convert a static string to a `GVariantType` in C, use the `G_VARIANT_TYPE()` casting macro. When GLib is compiled with checks enabled, it will validate the type. To check if an arbitrary string is a valid `GVariantType`, use `g_variant_type_string_is_valid()`.

## 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` 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.

Available since: 2.24

#### Constructors

###### g_variant_type_new

Creates a new `GVariantType` corresponding to the type string given by `type_string`.

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`.

###### g_variant_type_new_tuple

Constructs a new tuple type, from `items`.

#### Functions

###### g_variant_type_checked_

No description available.

###### g_variant_type_string_get_depth_

No description available.

###### g_variant_type_string_is_valid

Checks if `type_string` is a valid GVariant type string.

###### g_variant_type_string_scan

Scan for a single complete and valid GVariant type string in `string`.

since: 2.24

#### Instance methods

###### g_variant_type_copy

Makes a copy of a `GVariantType`.

###### g_variant_type_dup_string

Returns a newly-allocated copy of the type string corresponding to `type`.

###### g_variant_type_element

Determines the element type of an array or ‘maybe’ type.

###### g_variant_type_equal

Compares `type1` and `type2` for equality.

###### g_variant_type_first

Determines the first item type of a tuple or dictionary entry type.

###### 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`.

###### g_variant_type_hash

Hashes `type`.

###### g_variant_type_is_array

Determines if the given `type` is an array type.

###### g_variant_type_is_basic

Determines if the given `type` is a basic type.

###### g_variant_type_is_container

Determines if the given `type` is a container type.

###### g_variant_type_is_definite

Determines if the given `type` is definite (ie: not indefinite).

###### g_variant_type_is_dict_entry

Determines if the given `type` is a dictionary entry type.

###### g_variant_type_is_maybe

Determines if the given `type` is a ‘maybe’ type.

###### g_variant_type_is_subtype_of

Checks if `type` is a subtype of `supertype`.

###### g_variant_type_is_tuple

Determines if the given `type` is a tuple type.

###### g_variant_type_is_variant

Determines if the given `type` is the variant type.

###### g_variant_type_key

Determines the key type of a dictionary entry type.

###### g_variant_type_n_items

Determines the number of items contained in a tuple or dictionary entry type.

###### g_variant_type_next

Determines the next item type of a tuple or dictionary entry type.

###### g_variant_type_peek_string

Returns the type string corresponding to the given `type`.

###### g_variant_type_value

Determines the value type of a dictionary entry type.