Gnome::Glib::Variant

Strongly typed value datatype

Description

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

A Gnome::Glib::Variant 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 Gnome::Glib::Variant is also immutable: once it’s been created neither its type nor its content can be modified further.

Gnome::Glib::Variant is useful whenever data needs to be serialized, for example when sending method parameters in DBus, or when saving settings using Gnome::Glib::Settings.

When creating a new Gnome::Glib::Variant, 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 Gnome::Glib::Variant holding an integer value you can use:

my Gnome::Glib::Variant $v .= new(
  :type-string<u>, :value(42)
);

The string “u” in the first argument tells Gnome::Glib::Variant that the data passed to the constructor (40) is going to be an unsigned 32 bit integer.

As an alternative you can write

my Gnome::Glib::Variant $v .= new(:parse('-42'));

where the default used type is a signed 32 bit integer. To use an other integer type, write the type with it.

my Gnome::Glib::Variant $v .= new(:parse('uint64 42'));

More advanced examples of Gnome::Glib::Variant in use can be found in documentation for GVariant format strings.

The range of possible values is determined by the type.

The type system used by Gnome::Glib::Variant is Gnome::Glib::VariantType.

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

Gnome::Glib::Variant is completely threadsafe. A Gnome::Glib::Variant instance can be concurrently accessed in any way from any number of threads without problems.

Gnome::Glib::Variant is heavily optimised for dealing with data in serialised form. It works particularly well with data located in memory-mapped files. It can perform nearly all deserialisation operations in a small constant time, usually touching only a single memory page. Serialised Gnome::Glib::Variant data can also be sent over the network.

Gnome::Glib::Variant is largely compatible with D-Bus. Almost all types of Gnome::Glib::Variant instances can be sent over D-Bus. See Gnome::Glib::VariantType for exceptions. (However, Gnome::Glib::Variant’s serialisation format is not the same as the serialisation format of a D-Bus message body: use GDBusMessage, in the gio library, for those.)

For space-efficiency, the Gnome::Glib::Variant serialisation 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 Gnome::Glib::Variant’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 GMappedFile, and call g_variant_new_from_data() on it.

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

Memory Use

Gnome::Glib::Variant 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 Gnome::Glib::Variant can be grouped into 4 broad purposes: memory for serialised data, memory for the type information cache, buffer management memory and memory for the Gnome::Glib::Variant structure itself.

Serialised Data Memory

This is the memory that is used for storing GVariant data in serialised 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 serialisation.

If we add an item “width” that maps to the int32 value of 500 then we will use 4 byte 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 deserialisation.

Continuing with the above example, if a Gnome::Glib::Variant 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 Gnome::Glib::Variant.

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 malloc 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 malloc 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 GHashTable to store and lookup 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

Gnome::Glib::Variant uses an internal buffer management structure to deal with the various different possible sources of serialised 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 Gnome::Glib::Variant. This may involve a g_free() or a g_slice_free() or even g_mapped_file_unref().

One buffer management structure is used for each chunk of serialised 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 Gnome::Glib::Variant structure is 6 * (void *). On 32-bit systems, that’s 24 bytes.

Gnome::Glib::Variant structures only exist if they are explicitly created with API calls. For example, if a Gnome::Glib::Variant is constructed out of serialised 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 Gnome::Glib::Variant instance exists – the one referring to the dictionary.

If calls are made to start accessing the other values then Gnome::Glib::Variant instances will exist for those values only for as long as they are in use (ie: until you call g_variant_unref()). The type information is shared. The serialised data and the buffer management structure for that serialised 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 serialised data, 16 bytes for buffer management and 24 bytes for the Gnome::Glib::Variant instance, or a total of 160 bytes, plus malloc overhead. If we were to use g_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 serialised data and buffer management for those dictionaries, but the type information would be shared.

Synopsis

Declaration

unit class Gnome::Glib::Variant;
also is Gnome::N::TopLevelClassSupport;

Types

GVariantClass

The range of possible top-level types of GVariant instances.

G_VARIANT_CLASS_BOOLEAN; The GVariant is a boolean.
G_VARIANT_CLASS_BYTE; The GVariant is a byte.
G_VARIANT_CLASS_INT16; The GVariant is a signed 16 bit integer.
G_VARIANT_CLASS_UINT16; The GVariant is an unsigned 16 bit integer.
G_VARIANT_CLASS_INT32; The GVariant is a signed 32 bit integer.
G_VARIANT_CLASS_UINT32; The GVariant is an unsigned 32 bit integer.
G_VARIANT_CLASS_INT64; The GVariant is a signed 64 bit integer.
G_VARIANT_CLASS_UINT64; The GVariant is an unsigned 64 bit integer.
G_VARIANT_CLASS_HANDLE; The GVariant is a file handle index.
G_VARIANT_CLASS_DOUBLE; The GVariant is a double precision floating point value.
G_VARIANT_CLASS_STRING; The GVariant is a normal string.
G_VARIANT_CLASS_OBJECT_PATH; The GVariant is a D-Bus object path string.
G_VARIANT_CLASS_SIGNATURE; The GVariant is a D-Bus signature string.
G_VARIANT_CLASS_VARIANT; The GVariant is a variant.
G_VARIANT_CLASS_MAYBE; The GVariant is a maybe-typed value.
G_VARIANT_CLASS_ARRAY; The GVariant is an array.
G_VARIANT_CLASS_TUPLE; The GVariant is a tuple.
G_VARIANT_CLASS_DICT_ENTRY; The GVariant is a dictionary entry.

Methods

new

:array

Create a new Variant object. The type of the array elements is taken from the first element.

multi method new ( Array :$array! )

Example

Create a Variant array type containing integers;

my Array $array = [];
for 40, 41, 42 -> $value {
  $array.push: Gnome::Glib::Variant.new( :type-string<i>, :$value);
}
my Gnome::Glib::Variant $v .= new(:$array);
say $v.get-type-string;      # ai

:boolean

Creates a new boolean Variant – either True or False. Note that the value in the variant is stored as an integer. Its type becomes ‘b’.

multi method new ( Bool :$boolean! )

:byte

Creates a new byte Variant. Its type becomes ‘y’.

multi method new ( Int :$byte! )

:byte-string

Creates a new byte-string Variant. Its type becomes ‘ay’ which is essentially an array of bytes. This can be an ascii type of string which does not have to be UTF complient.

multi method new ( Str :$byte-string! )

:byte-string-array

Creates a new byte-string-array Variant. Its type becomes ‘aay’. which is essentially an array of an array of bytes.

multi method new ( Array :$byte-string-array! )

:dict

Creates a new dictionary Variant. Its type becomes ‘{}’.

multi method new ( List :$dict! )

The List $dict has two values, a key and a value and must both be valid Gnome::Glib::Variant objects. key must be a value of a basic type (ie: not a container). It will mostly be a string (variant type ‘s’).

Example

my Gnome::Glib::Variant $v .= new(
  :dict(
    Gnome::Glib::Variant.new(:parse<width>),
    Gnome::Glib::Variant.new(:parse<200>)
  )
);

say $v.print; #

:double

Creates a new double Variant. Its type becomes ‘d’.

multi method new ( Num :$double! )

:int16

Creates a new int16 Variant. Its type becomes ‘n’.

multi method new ( Int :$int16! )

:int32

Creates a new int32 Variant. Its type becomes ‘i’.

multi method new ( Int :$int32! )

:int64

Creates a new int64 Variant. Its type becomes ‘x’.

multi method new ( Int :$int64! )

:string

Creates a new string Variant. Its type becomes ‘s’.

multi method new ( Str :$string! )

:strv

Creates a new string array Variant. Its type becomes ‘as’.

multi method new ( Array :$strv! )

Example

my Gnome::Glib::Variant $v .= new(:string-array([<abc def ghi αβ ⓒ™⅔>]));
say $v.get-type-string; #    as

:tuple

Creates a new tuple Variant. Its type becomes ‘’.

multi method new ( Array :$tuple! )

Example

my Array $tuple = [];
$tuple.push: Gnome::Glib::Variant.new( :type-string<i>, :value(40));
$tuple.push: Gnome::Glib::Variant.new( :type-string<s>, :value<fourtyone>);
$tuple.push: Gnome::Glib::Variant.new( :type-string<x>, :value(42));
my Gnome::Glib::Variant $v .= new(:$tuple);
say $v.get-type-string; #    (isx)

:uint16

Creates a new uint16 Variant. Its type becomes ‘q’.

multi method new ( UInt :$uint16! )

:uint32

Creates a new uint32 Variant. Its type becomes ‘u’.

multi method new ( UInt :$uint32! )

:uint64

Creates a new uint64 Variant. Its type becomes ‘t’.

multi method new ( UInt :$uint64! )

:variant

Creates a new variant Variant. Its type becomes ‘v’.

multi method new ( N-GObject :$variant! )

Example

my Gnome::Glib::Variant $v .= new(
  :variant(Gnome::Glib::Variant.new( :type-string<i>, :value(40)))
);
say $v.get-type-string; #    v

:type-string, :parse

Create a new Variant object by parsing the type and data provided in strings. The format of the parse string is described here.

multi method new ( Str :$type-string?, Str :$parse! )

Example

Create a Variant tuple containing a string, an unsigned integer and a boolean (Note the lowercase ‘true’!);

my Gnome::Glib::Variant $v .= new(
  :type-string<(sub)>, :parse('("abc",20,true)')
);

Because the values in the :parse string take the default types you can also leave out the type string;

my Gnome::Glib::Variant $v .= new(:parse('("abc",20,true)'));

:type-string, :value

Create a new Variant object by parsing the type and a provided value. The type strings are simple like (unsigned) integer (‘u’ or ‘i’) but no arrays (‘a’) etc.

multi method new ( Str :$type-string!, Any :$value! )

:native-object

Create a Variant object using a native object from elsewhere. See also Gnome::N::TopLevelClassSupport.

multi method new ( N-GObject :$native-object! )

get-boolean

Returns the boolean value of value. It is an error to call this function with a value of any type other than G_VARIANT_TYPE_BOOLEAN.

Returns: True or False

method get-boolean ( --> Bool )

get-byte

Returns the byte value of value. It is an error to call this function with a value of any type other than G_VARIANT_TYPE_BYTE.

Returns: a guint8

method get-byte ( --> UInt )

get-bytestring

Returns the string value of a N-GObject instance with an array-of-bytes type. The string has no particular encoding.

method get-bytestring ( -->  Str  )

get-bytestring-array

Gets the contents of an array of array of bytes N-GObject.

method get-bytestring-array ( -->  Array[Str]  )

get-double

Returns the double precision floating point value of value. It is an error to call this function with a value of any type other than G_VARIANT_TYPE_DOUBLE.

Returns: a gdouble

method get-double ( --> Num )

get-int16

Returns the 16-bit signed integer value of value. It is an error to call this function with a value of any type other than G_VARIANT_TYPE_INT16.

Returns: a gint16

method get-int16 ( --> Int )

get-int32

Returns the 32-bit signed integer value of value. It is an error to call this function with a value of any type other than G_VARIANT_TYPE_INT32.

Returns: a gint32

method get-int32 ( --> Int )

get-int64

Returns the 64-bit signed integer value of value. It is an error to call this function with a value of any type other than G_VARIANT_TYPE_INT64.

Returns: a gint64

method get-int64 ( --> Int )

get-string

Returns the string value of a N-GObject instance with a string type.

method get-string ( -->  Str  )

get-strv

Gets the contents of an array of strings N-GObject. This call makes a shallow copy.

method get-strv ( --> Array[Str]  )

get-type

Determines the type of value. The return value is valid for the lifetime of value and must not be freed.

Returns: a GVariantType

method get-type ( --> Gnome::Glib::Variant )

get-type-string

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 N-GObject and must not be freed.

Returns: the type string for the type of value

method get-type-string ( -->  Str  )

get-uint16

Returns the 16-bit unsigned integer value of value. It is an error to call this function with a value of any type other than G_VARIANT_TYPE_UINT16.

Returns: a guint16

method get-uint16 ( --> UInt )

get-uint32

Returns the 32-bit unsigned integer value of value. It is an error to call this function with a value of any type other than G_VARIANT_TYPE_UINT32.

Returns: a guint32

method get-uint32 ( --> UInt )

get-uint64

Returns the 64-bit unsigned integer value of value. It is an error to call this function with a value of any type other than G_VARIANT_TYPE_UINT64.

Returns: a guint64

method get-uint64 ( --> UInt )

get-variant

Unboxes value. The result is the Gnome::Glib::Variant that was contained in value.

Returns: the item contained in the variant

method get-variant ( --> Gnome::Glib::Variant )

is-container

Checks if value is a container.

Returns: 1 if value is a container

method is-container ( --> Int )

is-of-type

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

Returns: True if the type of value matches type

method is-of-type ( N-GObject $type --> Bool )

N-GObject $type; a GVariantType

print

Pretty-prints value in the format understood by parse(). If $type_annotate is True, then type information is included in the output.

Returns: a newly-allocated string holding the result.

method print ( Bool $type_annotate = False --> Str )

Int $type_annotate; True if type information should be included in the output

Gnome::Glib::Variant

Description

Memory Use

Serialised Data Memory

Type Information Cache

Buffer Management Memory

GVariant structure

Summary

See Also

Synopsis

Declaration

Types

GVariantClass

Methods

new

:array

Example

:boolean

:byte

:byte-string

:byte-string-array

:dict

Example

:double

:int16

:int32

:int64

:string

:strv

Example

:tuple

Example

:uint16

:uint32

:uint64

:variant

Example

:type-string, :parse

Example

:type-string, :value

:native-object

get-boolean

get-byte

get-bytestring

get-bytestring-array

get-double

get-int16

get-int32

get-int64

get-string

get-strv

get-type

get-type-string

get-uint16

get-uint32

get-uint64

get-variant

is-container

is-of-type

print