std::variant_size, std::variant_size_v

C++

Language

Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

Utilities library

Type support (basic types, RTTI, type traits)
Dynamic memory management
Error handling
Program utilities
Variadic functions
Library feature-test macros
Date and time
Function objects
initializer_list (C++11)
bitset
hash (C++11)
integer_sequence (C++14)
Relational operators (deprecated)
rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=
Comparisons (C++20)
strong_order weak_order partial_order strong_equal weak_equal	strong_ordering weak_ordering partial_ordering strong_equality weak_equality	is_eqis_neqis_ltis_lteqis_gtis_gteq
common_comparison_category
Common vocabulary types
pair tuple (C++11) apply (C++17) make_from_tuple (C++17)	optional (C++17) any (C++17) variant (C++17)
Swap, forward and move
swap
exchange (C++14)
forward (C++11)
move (C++11)
move_if_noexcept (C++11)
Elementary string conversions
to_chars (C++17)
from_chars (C++17)
chars_format (C++17)
Type operations
declval (C++11)
as_const (C++17)

std::variant

Member functions

variant::variant

variant::~variant

variant::operator=

Observers

variant::index

variant::valueless_by_exception

Modifiers

variant::emplace

variant::swap

Non-member functions

visit

holds_alternative

get

get_if

operator==operator!=operator<operator<=operator>operator>=

swap

Helper classes

monostate

bad_variant_access

variant_size

variant_alternative

hash

Helper objects

variant_npos

Defined in header <variant>
template <class T> struct variant_size; /* undefined */	(1)	(since C++17)
template <class... Types> struct variant_size<std::variant<Types...>> : std::integral_constant<std::size_t, sizeof...(Types)> { };	(2)	(since C++17)
template <class T> class variant_size<const T>; template <class T> class variant_size<volatile T>; template <class T> class variant_size<const volatile T>;	(3)	(since C++17)

Provides access to the number of alternatives in a possibly cv-qualified variant as a compile-time constant expression.

Formally,

2) meets the UnaryTypeTrait requirements with a BaseCharacteristic of std::integral_constant<std::size_t, sizeof...(Types)>

3) meets the UnaryTypeTrait requirements with a BaseCharacteristic of std::integral_constant<std::size_t, variant_size<T>>

Helper variable template

template <class T> inline constexpr std::size_t variant_size_v = std::variant_size<T>::value;		(since C++17)

Inherited from std::integral_constant

Member constants

value [static]

sizeof...(Types) (public static member constant)

Member functions

operator std::size_t	converts the object to std::size_t, returns value (public member function)
operator() (C++14)	returns value (public member function)

Member types

Type	Definition
value_type	std::size_t
type	std::integral_constant<std::size_t, value>

Notes

All specializations of std::variant_size satisfy UnaryTypeTrait with BaseCharacteristic std::integral_constant<std::size_t, N> for some N.

See also

variant_alternativevariant_alternative_t (C++17)	obtains the type of the alternative specified by its index, at compile time (class template) (alias template)
tuple_size	obtains the size of tuple at compile time (class template specialization)