Variant

class Variant(pointer: <Error class: unknown class><<Error class: unknown class>>) : ProxyInstance

GVariant is a variant datatype; it can contain one or more values along with information about the type of the values.

A GVariant may contain simple types, like an integer, or a boolean value; or complex types, like an array of two strings, or a dictionary of key value pairs. A GVariant is also immutable: once it’s been created neither its type nor its content can be modified further.

GVariant is useful whenever data needs to be serialized, for example when sending method parameters in D-Bus, or when saving settings using ../gio/class.Settings.html.

When creating a new GVariant, you pass the data you want to store in it along with a string representing the type of data you wish to pass to it.

For instance, if you want to create a GVariant holding an integer value you can use:

GVariant *v = g_variant_new ("u", 40);

The string u in the first argument tells GVariant that the data passed to the constructor (40) is going to be an unsigned integer.

More advanced examples of GVariant in use can be found in documentation for gvariant-format-strings.html#pointers.

The range of possible values is determined by the type.

The type system used by GVariant is type@GLib.VariantType.

GVariant instances always have a type and a value (which are given at construction time). The type and value of a GVariant instance can never change other than by the GVariant itself being destroyed. A GVariant cannot contain a pointer.

GVariant is reference counted using method@GLib.Variant.ref and method@GLib.Variant.unref. GVariant also has floating reference counts — see method@GLib.Variant.ref_sink.

GVariant is completely threadsafe. A GVariant instance can be concurrently accessed in any way from any number of threads without problems.

GVariant is heavily optimised for dealing with data in serialized form. It works particularly well with data located in memory-mapped files. It can perform nearly all deserialization operations in a small constant time, usually touching only a single memory page. Serialized GVariant data can also be sent over the network.

GVariant is largely compatible with D-Bus. Almost all types of GVariant instances can be sent over D-Bus. See type@GLib.VariantType for exceptions. (However, GVariant’s serialization format is not the same as the serialization format of a D-Bus message body: use ../gio/class.DBusMessage.html, in the GIO library, for those.)

For space-efficiency, the GVariant serialization format does not automatically include the variant’s length, type or endianness, which must either be implied from context (such as knowledge that a particular file format always contains a little-endian G_VARIANT_TYPE_VARIANT which occupies the whole length of the file) or supplied out-of-band (for instance, a length, type and/or endianness indicator could be placed at the beginning of a file, network message or network stream).

A GVariant’s size is limited mainly by any lower level operating system constraints, such as the number of bits in gsize. For example, it is reasonable to have a 2GB file mapped into memory with struct@GLib.MappedFile, and call ctor@GLib.Variant.new_from_data on it.

For convenience to C programmers, GVariant features powerful varargs-based value construction and destruction. This feature is designed to be embedded in other libraries.

There is a Python-inspired text language for describing GVariant values. GVariant includes a printer for this language and a parser with type inferencing.

Memory Use

GVariant tries to be quite efficient with respect to memory use. This section gives a rough idea of how much memory is used by the current implementation. The information here is subject to change in the future.

The memory allocated by GVariant can be grouped into 4 broad purposes: memory for serialized data, memory for the type information cache, buffer management memory and memory for the GVariant structure itself.

Serialized Data Memory

This is the memory that is used for storing GVariant data in serialized form. This is what would be sent over the network or what would end up on disk, not counting any indicator of the endianness, or of the length or type of the top-level variant.

The amount of memory required to store a boolean is 1 byte. 16, 32 and 64 bit integers and double precision floating point numbers use their ‘natural’ size. Strings (including object path and signature strings) are stored with a nul terminator, and as such use the length of the string plus 1 byte.

‘Maybe’ types use no space at all to represent the null value and use the same amount of space (sometimes plus one byte) as the equivalent non-maybe-typed value to represent the non-null case.

Arrays use the amount of space required to store each of their members, concatenated. Additionally, if the items stored in an array are not of a fixed-size (ie: strings, other arrays, etc) then an additional framing offset is stored for each item. The size of this offset is either 1, 2 or 4 bytes depending on the overall size of the container. Additionally, extra padding bytes are added as required for alignment of child values.

Tuples (including dictionary entries) use the amount of space required to store each of their members, concatenated, plus one framing offset (as per arrays) for each non-fixed-sized item in the tuple, except for the last one. Additionally, extra padding bytes are added as required for alignment of child values.

Variants use the same amount of space as the item inside of the variant, plus 1 byte, plus the length of the type string for the item inside the variant.

As an example, consider a dictionary mapping strings to variants. In the case that the dictionary is empty, 0 bytes are required for the serialization.

If we add an item ‘width’ that maps to the int32 value of 500 then we will use 4 bytes to store the int32 (so 6 for the variant containing it) and 6 bytes for the string. The variant must be aligned to 8 after the 6 bytes of the string, so that’s 2 extra bytes. 6 (string) + 2 (padding) + 6 (variant) is 14 bytes used for the dictionary entry. An additional 1 byte is added to the array as a framing offset making a total of 15 bytes.

If we add another entry, ‘title’ that maps to a nullable string that happens to have a value of null, then we use 0 bytes for the null value (and 3 bytes for the variant to contain it along with its type string) plus 6 bytes for the string. Again, we need 2 padding bytes. That makes a total of 6 + 2 + 3 = 11 bytes.

We now require extra padding between the two items in the array. After the 14 bytes of the first item, that’s 2 bytes required. We now require 2 framing offsets for an extra two bytes. 14 + 2 + 11 + 2 = 29 bytes to encode the entire two-item dictionary.

Type Information Cache

For each GVariant type that currently exists in the program a type information structure is kept in the type information cache. The type information structure is required for rapid deserialization.

Continuing with the above example, if a GVariant exists with the type a{sv} then a type information struct will exist for a{sv}, {sv}, s, and v. Multiple uses of the same type will share the same type information. Additionally, all single-digit types are stored in read-only static memory and do not contribute to the writable memory footprint of a program using GVariant.

Aside from the type information structures stored in read-only memory, there are two forms of type information. One is used for container types where there is a single element type: arrays and maybe types. The other is used for container types where there are multiple element types: tuples and dictionary entries.

Array type info structures are 6 * sizeof (void *), plus the memory required to store the type string itself. This means that on 32-bit systems, the cache entry for a{sv} would require 30 bytes of memory (plus allocation overhead).

Tuple type info structures are 6 * sizeof (void *), plus 4 * sizeof (void *) for each item in the tuple, plus the memory required to store the type string itself. A 2-item tuple, for example, would have a type information structure that consumed writable memory in the size of 14 * sizeof (void *) (plus type string) This means that on 32-bit systems, the cache entry for {sv} would require 61 bytes of memory (plus allocation overhead).

This means that in total, for our a{sv} example, 91 bytes of type information would be allocated.

The type information cache, additionally, uses a struct@GLib.HashTable to store and look up the cached items and stores a pointer to this hash table in static storage. The hash table is freed when there are zero items in the type cache.

Although these sizes may seem large it is important to remember that a program will probably only have a very small number of different types of values in it and that only one type information structure is required for many different values of the same type.

Buffer Management Memory

GVariant uses an internal buffer management structure to deal with the various different possible sources of serialized data that it uses. The buffer is responsible for ensuring that the correct call is made when the data is no longer in use by GVariant. This may involve a func@GLib.free or even method@GLib.MappedFile.unref.

One buffer management structure is used for each chunk of serialized data. The size of the buffer management structure is 4 * (void *). On 32-bit systems, that’s 16 bytes.

GVariant structure

The size of a GVariant structure is 6 * (void *). On 32-bit systems, that’s 24 bytes.

GVariant structures only exist if they are explicitly created with API calls. For example, if a GVariant is constructed out of serialized data for the example given above (with the dictionary) then although there are 9 individual values that comprise the entire dictionary (two keys, two values, two variants containing the values, two dictionary entries, plus the dictionary itself), only 1 GVariant instance exists — the one referring to the dictionary.

If calls are made to start accessing the other values then GVariant instances will exist for those values only for as long as they are in use (ie: until you call method@GLib.Variant.unref). The type information is shared. The serialized data and the buffer management structure for that serialized data is shared by the child.

Summary

To put the entire example together, for our dictionary mapping strings to variants (with two entries, as given above), we are using 91 bytes of memory for type information, 29 bytes of memory for the serialized data, 16 bytes for buffer management and 24 bytes for the GVariant instance, or a total of 160 bytes, plus allocation overhead. If we were to use method@GLib.Variant.get_child_value to access the two dictionary entries, we would use an additional 48 bytes. If we were to have other dictionaries of the same type, we would use more memory for the serialized data and buffer management for those dictionaries, but the type information would be shared.

Skipped during bindings generation

  • parameter two: Variant

  • parameter length: length: Out parameter is not supported

  • parameter length: length: Out parameter is not supported

  • parameter length: length: Out parameter is not supported

  • parameter length: length: Out parameter is not supported

  • parameter length: length: Out parameter is not supported

  • parameter two: Variant

  • method get: Varargs parameter is not supported

  • method get_bytestring: Array parameter of type guint8 is not supported

  • parameter length: length: Out parameter is not supported

  • method get_child: Varargs parameter is not supported

  • parameter n_elements: n_elements: Out parameter is not supported

  • parameter length: length: Out parameter is not supported

  • parameter length: length: Out parameter is not supported

  • parameter length: length: Out parameter is not supported

  • parameter endptr: Unsupported string with cType const gchar**

  • method lookup: Varargs parameter is not supported

  • constructor new: Varargs parameter is not supported

  • parameter children: Array parameter of type Variant is not supported

  • parameter string: Array parameter of type guint8 is not supported

  • parameter data: Array parameter of type guint8 is not supported

  • constructor new_parsed: Varargs parameter is not supported

  • parameter app: va_list

  • constructor new_printf: Varargs parameter is not supported

  • parameter children: Array parameter of type Variant is not supported

  • parameter endptr: Unsupported string with cType const gchar**

  • parameter endptr: Unsupported string with cType const gchar**

Since

2.24

Constructors

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constructor(pointer: <Error class: unknown class><<Error class: unknown class>>)

Types

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object Companion

Properties

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val glibVariantPointer: <Error class: unknown class><<Error class: unknown class>>
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open override val handle: <Error class: unknown class>?

Functions

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Performs a byteswapping operation on the contents of @value. The result is that all multi-byte numeric data contained in @value is byteswapped. That includes 16, 32, and 64bit signed and unsigned integers as well as file handles and double precision floating point values.

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fun checkFormatString(formatString: String, copyOnly: Boolean): Boolean

Checks if calling g_variant_get() with @format_string on @value would be valid from a type-compatibility standpoint. @format_string is assumed to be a valid format string (from a syntactic standpoint).

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Classifies @value according to its top-level type.

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open operator override fun equals(other: Any?): Boolean

Compare two proxy instances for equality. This will compare both the type of the instances, and their memory addresses.

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Returns the boolean value of @value.

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fun getByte(): <Error class: unknown class>

Returns the byte value of @value.

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fun getChildValue(index: <Error class: unknown class>): Variant

Reads a child item out of a container #GVariant instance. This includes variants, maybes, arrays, tuples and dictionary entries. It is an error to call this function on any other type of #GVariant.

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fun getData(): <Error class: unknown class>?

Returns a pointer to the serialized form of a #GVariant instance. The returned data may not be in fully-normalised form if read from an untrusted source. The returned data must not be freed; it remains valid for as long as @value exists.

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Returns a pointer to the serialized form of a #GVariant instance. The semantics of this function are exactly the same as g_variant_get_data(), except that the returned #GBytes holds a reference to the variant data.

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fun getDouble(): <Error class: unknown class>

Returns the double precision floating point value of @value.

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fun getHandle(): <Error class: unknown class>

Returns the 32-bit signed integer value of @value.

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Returns the 16-bit signed integer value of @value.

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fun getInt32(): <Error class: unknown class>

Returns the 32-bit signed integer value of @value.

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fun getInt64(): <Error class: unknown class>

Returns the 64-bit signed integer value of @value.

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Given a maybe-typed #GVariant instance, extract its value. If the value is Nothing, then this function returns null.

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Gets a #GVariant instance that has the same value as @value and is trusted to be in normal form.

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fun getSize(): <Error class: unknown class>

Determines the number of bytes that would be required to store @value with g_variant_store().

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Determines the type of @value.

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Returns the type string of @value. Unlike the result of calling g_variant_type_peek_string(), this string is nul-terminated. This string belongs to #GVariant and must not be freed.

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fun getUint16(): <Error class: unknown class>

Returns the 16-bit unsigned integer value of @value.

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fun getUint32(): <Error class: unknown class>

Returns the 32-bit unsigned integer value of @value.

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fun getUint64(): <Error class: unknown class>

Returns the 64-bit unsigned integer value of @value.

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Unboxes @value. The result is the #GVariant instance that was contained in @value.

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fun hash(): <Error class: unknown class>

Generates a hash value for a #GVariant instance.

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open override fun hashCode(): Int
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Checks if @value is a container.

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Checks whether @value has a floating reference count.

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Checks if @value is in normal form.

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Checks if a value has a type matching the provided type.

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Creates a heap-allocated #GVariantIter for iterating over the items in @value.

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fun lookupValue(key: String, expectedType: VariantType? = null): Variant

Looks up a value in a dictionary #GVariant.

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fun nChildren(): <Error class: unknown class>

Determines the number of children in a container #GVariant instance. This includes variants, maybes, arrays, tuples and dictionary entries. It is an error to call this function on any other type of #GVariant.

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fun print(typeAnnotate: Boolean): String

Pretty-prints @value in the format understood by g_variant_parse().

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fun printString(string: String? = null, typeAnnotate: Boolean): String

Behaves as g_variant_print(), but operates on a #GString.

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fun ref(): Variant

Increases the reference count of @value.

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#GVariant uses a floating reference count system. All functions with names starting with g_variant_new_ return floating references.

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fun store(data: <Error class: unknown class>)

Stores the serialized form of @value at @data. @data should be large enough. See g_variant_get_size().

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If @value is floating, sink it. Otherwise, do nothing.

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fun unref()

Decreases the reference count of @value. When its reference count drops to 0, the memory used by the variant is freed.