glibmm: Glib::VariantBase Class Reference

glibmm 2.38.1

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Glib::VariantBase Class Reference
Variant Data Types

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

`#include <glibmm/variant.h>`

Inheritance diagram for Glib::VariantBase:
[legend]

## Public Types
typedef const void* | BoolExpr
| 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 ()
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...
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...
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...
template<class V_CastTo >
V_CastTo | cast_dynamic (const VariantBase& v) throw (std::bad_cast)
template<>
VariantContainerBase | cast_dynamic (const VariantBase& v) throw (std::bad_cast)

## Static Public Member Functions
template<class V_CastTo >
static V_CastTo | cast_dynamic (const VariantBase& v) throw (std::bad_cast)
| Cast to a specific variant type. More...

## Protected Attributes
GVariant* | gobject_

## Related Functions
(Note that these are not member functions.)
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

typedef const void* Glib::VariantBase::BoolExpr

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

## Constructor & Destructor Documentation

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.

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

Glib::VariantBase::VariantBase | ( | const VariantBase& | src | ) |

Glib::VariantBase::~VariantBase | ( |  | ) |

## Member Function Documentation

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:

template <class V_CastTo >

static V_CastTo Glib::VariantBase::cast_dynamic | ( | const VariantBase& | v | ) | throw (std::bad_cast)
| 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.

template <class V_CastTo >

V_CastTo Glib::VariantBase::cast_dynamic | ( | const VariantBase& | v | ) | throw (std::bad_cast)

template <>

VariantContainerBase Glib::VariantBase::cast_dynamic | ( | const VariantBase& | v | ) | throw (std::bad_cast)

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 Glib::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.

GVariantClass Glib::VariantBase::classify | ( |  | ) | const

Classifies value according to its top-level type.

Since glibmm 2.24:

Returns
  The VariantClass of value.

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.

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 `0` 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 `0` 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 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:

Returns
  The serialised form of value, or `0`.

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:

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.

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.

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.

GVariant* Glib::VariantBase::gobj | ( |  | ) |
| inline

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

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.

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:

Parameters

  value | A basic Variant value as a #gconstpointer.

Returns
  A hash value corresponding to value.

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.

bool Glib::VariantBase::is_container | ( |  | ) | const

Checks if value is a container.

Since glibmm 2.24:

Returns
  `true` if value is a container.

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.

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`.

Since glibmm 2.24:

Returns
  `true` if value is in normal form.

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.

Glib::VariantBase::operator BoolExpr | ( |  | ) | const

Test whether the Variant has an underlying instance.

Mimics usage of pointers:

if (variant)
do_something();

Since glibmm 2.36:

VariantBase& Glib::VariantBase::operator= | ( | const VariantBase& | src | ) |

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.

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.

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.

## Friends And Related Function Documentation

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

GVariant* Glib::VariantBase::gobject_
| protected