Glib::VariantBase Class Reference

This is the base class for all Variant types. More...

#include <glibmm/variant.h>

Inheritance diagram for Glib::VariantBase:

Public Types
using	BoolExpr = const void *
	This typedef is just to make it more obvious that our operator const void* should be used like operator bool(). More...

Public Member Functions
	VariantBase ()
	Constructs an invalid object. More...
	VariantBase (GVariant * castitem, bool make_a_copy=false)
	VariantBase (const VariantBase & src)
VariantBase &	operator= (const VariantBase & src)
	VariantBase (VariantBase && other) noexcept
VariantBase &	operator= (VariantBase && other) noexcept
	~VariantBase () noexcept
void	swap (VariantBase & other) noexcept
GVariant *	gobj ()
const GVariant *	gobj () const
GVariant *	gobj_copy () const
	Provides access to the underlying C instance. The caller is responsible for freeing it. Use when directly setting fields in structs. More...
	operator BoolExpr () const
	Test whether the Variant has an underlying instance. More...
	operator bool () const
	Test whether the Variant has an underlying instance. More...
void	init (const GVariant * cobject, bool take_a_reference=false)
	Replace the underlying GVariant. More...
VariantType	get_type () const
	Determines the type of value. More...
std::string	get_type_string () const
	Returns the type string of value. More...
bool	is_floating () const
	Checks whether value has a floating reference count. More...
bool	is_of_type (const VariantType & type) const
	Checks if a value has a type matching the provided type. More...
bool	is_container () const
	Checks if value is a container. More...
GVariantClass	classify () const
	Classifies value according to its top-level type. More...
gsize	get_size () const
	Determines the number of bytes that would be required to store value with g_variant_store(). More...
gconstpointer	get_data ()
	Returns a pointer to the serialised form of a Variant instance. More...
gconstpointer	get_data () const
	Returns a pointer to the serialised form of a Variant instance. More...
Glib::RefPtr< const Glib::Bytes >	get_data_as_bytes () const
	Returns a pointer to the serialised form of a Variant instance. More...
void	store (gpointer data) const
	Stores the serialised form of value at data. More...
Glib::ustring	print (bool type_annotate=false) const
	Pretty-prints value in the format understood by g_variant_parse(). More...
guint	hash () const
	Generates a hash value for a Variant instance. More...
bool	equal (const VariantBase & other) const
	Checks if *this and other have the same type and value. More...
void	get_normal_form (VariantBase & result) const
	Gets a VariantBase instance that has the same value as this variant and is trusted to be in normal form. More...
bool	is_normal_form () const
	Checks if value is in normal form. More...
void	byteswap (VariantBase & result) const
	Performs a byteswapping operation on the contents of this variant. More...
bool	check_format_string (const std::string & format_string, bool copy_only=false) const
	Checks if calling g_variant_get() with format_string on value would be valid from a type-compatibility standpoint. More...

Static Public Member Functions
template<class V_CastTo >
static V_CastTo	cast_dynamic (const VariantBase & v)
	Cast to a specific variant type. More...
template<>
VariantContainerBase	cast_dynamic (const VariantBase & v)

Protected Attributes
GVariant *	gobject_

Related Functions
(Note that these are not member functions.)
void	swap (VariantBase & lhs, VariantBase & rhs) noexcept
Glib::VariantBase	wrap (GVariant * object, bool take_copy=false)
	A Glib::wrap() method for this object. More...

Detailed Description

This is the base class for all Variant types.

If the actual type is known at compile-time then you should use a specific Variant<>, such as Variant<int>. Otherwise, you may use get_type(), is_of_type(), or cast_dynamic().

Since glibmm 2.28:

Member Typedef Documentation

BoolExpr

using Glib::VariantBase::BoolExpr = const void*

This typedef is just to make it more obvious that our operator const void* should be used like operator bool().

Deprecated:

Use the explicit operator bool() instead.

Constructor & Destructor Documentation

VariantBase() [1/4]

Glib::VariantBase::VariantBase

(

)

Constructs an invalid object.

E.g. for output arguments to methods. There is not much you can do with the object before it has been assigned a valid value.

VariantBase() [2/4]

Glib::VariantBase::VariantBase ( GVariant *  castitem, bool  make_a_copy = false  )

explicit

VariantBase() [3/4]

Glib::VariantBase::VariantBase

(

const VariantBase &

src

)

VariantBase() [4/4]

Glib::VariantBase::VariantBase ( VariantBase &&  other )

noexcept

~VariantBase()

Glib::VariantBase::~VariantBase ( )

noexcept

Member Function Documentation

byteswap()

void Glib::VariantBase::byteswap

(

VariantBase &

result

)

const

Performs a byteswapping operation on the contents of this variant.

The result is that all multi-byte numeric data contained in the variant is byteswapped. That includes 16, 32, and 64bit signed and unsigned integers as well as file handles and double precision floating point values.

This function is an identity mapping on any value that does not contain multi-byte numeric data. That include strings, booleans, bytes and containers containing only these things (recursively).

The returned value is always in normal form and is marked as trusted.

Parameters

result

A location in which to store the byteswapped form of this variant.

Since glibmm 2.24:

cast_dynamic() [1/2]

template <class V_CastTo >

V_CastTo Glib::VariantBase::cast_dynamic ( const VariantBase &  v )

static

Cast to a specific variant type.

For instance:

Variant<std::string> derived = VariantBase::cast_dynamic< Variant<std::string> >(base);

Parameters

v	The variant to cast to a specific type.

Returns

The variant as a specific type.

Exceptions

std::bad_cast

if the Variant was not of the expected type.

cast_dynamic() [2/2]

template <>

VariantContainerBase Glib::VariantBase::cast_dynamic ( const VariantBase &  v )

static

check_format_string()

bool Glib::VariantBase::check_format_string	(	const std::string &	format_string,
		bool	copy_only = false
	)		const

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).

If copy_only is true then this function additionally checks that it would be safe to call g_variant_unref() on value immediately after the call to g_variant_get() without invalidating the result. This is only possible if deep copies are made (ie: there are no pointers to the data inside of the soon-to-be-freed Variant instance). If this check fails then a g_critical() is printed and false is returned.

This function is meant to be used by functions that wish to provide varargs accessors to Variant values of uncertain values (eg: g_variant_lookup() or Glib::menu_model_get_item_attribute()).

Since glibmm 2.34:

Parameters

format_string	A valid Variant format string.
copy_only	true to ensure the format string makes deep copies.

Returns

true if format_string is safe to use.

classify()

GVariantClass Glib::VariantBase::classify

(

)

const

Classifies value according to its top-level type.

Since glibmm 2.24:

Returns

The VariantClass of value.

equal()

bool Glib::VariantBase::equal

(

const VariantBase &

other

)

const

Checks if *this and other have the same type and value.

Since glibmm 2.24:

Parameters

other

The Variant to compare with.

Returns

true if *this and other are equal.

get_data() [1/2]

gconstpointer Glib::VariantBase::get_data

(

)

Returns a pointer to the serialised form of a Variant 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.

If value is a fixed-sized value that was deserialised from a corrupted serialised container then nullptr may be returned. In this case, the proper thing to do is typically to use the appropriate number of nul bytes in place of value. If value is not fixed-sized then nullptr is never returned.

In the case that value is already in serialised form, this function is O(1). If the value is not already in serialised form, serialisation occurs implicitly and is approximately O(n) in the size of the result.

To deserialise the data returned by this function, in addition to the serialised data, you must know the type of the Variant, and (if the machine might be different) the endianness of the machine that stored it. As a result, file formats or network messages that incorporate serialised Variants must include this information either implicitly (for instance "the file always contains a G_VARIANT_TYPE_VARIANT and it is always in little-endian order") or explicitly (by storing the type and/or endianness in addition to the serialised data).

Since glibmm 2.24:

Deprecated:

Use the const version instead.

Returns

The serialised form of value, or nullptr.

get_data() [2/2]

gconstpointer Glib::VariantBase::get_data

(

)

const

Returns a pointer to the serialised form of a Variant 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.

If value is a fixed-sized value that was deserialised from a corrupted serialised container then nullptr may be returned. In this case, the proper thing to do is typically to use the appropriate number of nul bytes in place of value. If value is not fixed-sized then nullptr is never returned.

In the case that value is already in serialised form, this function is O(1). If the value is not already in serialised form, serialisation occurs implicitly and is approximately O(n) in the size of the result.

To deserialise the data returned by this function, in addition to the serialised data, you must know the type of the Variant, and (if the machine might be different) the endianness of the machine that stored it. As a result, file formats or network messages that incorporate serialised Variants must include this information either implicitly (for instance "the file always contains a G_VARIANT_TYPE_VARIANT and it is always in little-endian order") or explicitly (by storing the type and/or endianness in addition to the serialised data).

Since glibmm 2.46:

Returns

The serialised form of value, or nullptr.

get_data_as_bytes()

Glib::RefPtr< const Glib::Bytes > Glib::VariantBase::get_data_as_bytes

(

)

const

Returns a pointer to the serialised form of a Variant instance.

The semantics of this function are exactly the same as g_variant_get_data(), except that the returned Bytes holds a reference to the variant data.

Since glibmm 2.46:

Returns

A new Bytes representing the variant data.

get_normal_form()

void Glib::VariantBase::get_normal_form

(

VariantBase &

result

)

const

Gets a VariantBase instance that has the same value as this variant and is trusted to be in normal form.

If this variant is already trusted to be in normal form then a new reference to the variant is returned.

If this variant is not already trusted, then it is scanned to check if it is in normal form. If it is found to be in normal form then it is marked as trusted and a new reference to it is returned.

If this variant is found not to be in normal form then a new trusted VariantBase is created with the same value as this variant.

It makes sense to call this function if you've received variant data from untrusted sources and you want to ensure your serialised output is definitely in normal form.

Parameters

result

A location in which to store the trusted VariantBase.

Since glibmm 2.24:

get_size()

gsize Glib::VariantBase::get_size

(

)

const

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

If value has a fixed-sized type then this function always returned that fixed size.

In the case that value is already in serialised form or the size has already been calculated (ie: this function has been called before) then this function is O(1). Otherwise, the size is calculated, an operation which is approximately O(n) in the number of values involved.

Since glibmm 2.24:

Returns

The serialised size of value.

get_type()

VariantType Glib::VariantBase::get_type

(

)

const

Determines the type of value.

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

Since glibmm 2.24:

Returns

A VariantType.

get_type_string()

std::string Glib::VariantBase::get_type_string

(

)

const

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 Variant and must not be freed.

Since glibmm 2.24:

Returns

The type string for the type of value.

gobj() [1/2]

GVariant * Glib::VariantBase::gobj ( )

inline

gobj() [2/2]

const GVariant * Glib::VariantBase::gobj ( ) const

inline

gobj_copy()

GVariant * Glib::VariantBase::gobj_copy

(

)

const

Provides access to the underlying C instance. The caller is responsible for freeing it. Use when directly setting fields in structs.

hash()

guint Glib::VariantBase::hash

(

)

const

Generates a hash value for a Variant instance.

The output of this function is guaranteed to be the same for a given value only per-process. It may change between different processor architectures or even different versions of GLib. Do not use this function as a basis for building protocols or file formats.

The type of value is #gconstpointer only to allow use of this function with HashTable. value must be a Variant.

Since glibmm 2.24:

Returns

A hash value corresponding to value.

init()

void Glib::VariantBase::init	(	const GVariant *	cobject,
		bool	take_a_reference = false
	)

Replace the underlying GVariant.

This is for use by methods that take a VariantBase& as an output parameter.

Parameters

cobject	The GVariant* obtained from a C function.
take_a_reference	Whether this method should take a reference, for instance if the C function has not given one.

is_container()

bool Glib::VariantBase::is_container

(

)

const

Checks if value is a container.

Since glibmm 2.24:

Returns

true if value is a container.

is_floating()

bool Glib::VariantBase::is_floating

(

)

const

Checks whether value has a floating reference count.

This function should only ever be used to assert that a given variant is or is not floating, or for debug purposes. To acquire a reference to a variant that might be floating, always use g_variant_ref_sink() or g_variant_take_ref().

See g_variant_ref_sink() for more information about floating reference counts.

Since glibmm 2.26:

Returns

Whether value is floating.

is_normal_form()

bool Glib::VariantBase::is_normal_form

(

)

const

Checks if value is in normal form.

The main reason to do this is to detect if a given chunk of serialised data is in normal form: load the data into a Variant using g_variant_new_from_data() and then use this function to check.

If value is found to be in normal form then it will be marked as being trusted. If the value was already marked as being trusted then this function will immediately return true.

There may be implementation specific restrictions on deeply nested values. GVariant is guaranteed to handle nesting up to at least 64 levels.

Since glibmm 2.24:

Returns

true if value is in normal form.

is_of_type()

bool Glib::VariantBase::is_of_type

(

const VariantType &

type

)

const

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

Since glibmm 2.24:

Parameters

type	A VariantType.

Returns

true if the type of value matches type.

operator bool()

Glib::VariantBase::operator bool ( ) const

explicit

Test whether the Variant has an underlying instance.

Since glibmm 2.50:

operator BoolExpr()

Glib::VariantBase::operator BoolExpr

(

)

const

Test whether the Variant has an underlying instance.

Mimics usage of pointers:

if (variant)
  do_something();

Deprecated:

Use the explicit operator bool() instead.

Since glibmm 2.36:

operator=() [1/2]

VariantBase & Glib::VariantBase::operator=

(

const VariantBase &

src

)

operator=() [2/2]

VariantBase & Glib::VariantBase::operator= ( VariantBase &&  other )

noexcept

print()

Glib::ustring Glib::VariantBase::print

(

bool

type_annotate = false

)

const

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

The format is described [here][gvariant-text].

If type_annotate is true, then type information is included in the output.

Since glibmm 2.24:

Parameters

type_annotate

true if type information should be included in the output.

Returns

A newly-allocated string holding the result.

store()

void Glib::VariantBase::store

(

gpointer

data

)

const

Stores the serialised form of value at data.

data should be large enough. See g_variant_get_size().

The stored data is in machine native byte order but may not be in fully-normalised form if read from an untrusted source. See g_variant_get_normal_form() for a solution.

As with g_variant_get_data(), to be able to deserialise the serialised variant successfully, its type and (if the destination machine might be different) its endianness must also be available.

This function is approximately O(n) in the size of data.

Since glibmm 2.24:

Parameters

data	The location to store the serialised data at.

swap()

void Glib::VariantBase::swap ( VariantBase &  other )

noexcept

Friends And Related Function Documentation

swap()

void swap ( VariantBase &  lhs, VariantBase &  rhs  )

related

Parameters

lhs	The left-hand side
rhs	The right-hand side

wrap()

Glib::VariantBase wrap ( GVariant *  object, bool  take_copy = false  )

related

A Glib::wrap() method for this object.

Parameters

object	The C instance.
take_copy	False if the result should take ownership of the C instance. True if it should take a new copy or ref.

Returns

A C++ instance that wraps this C instance.

Member Data Documentation

gobject_

GVariant* Glib::VariantBase::gobject_

protected