std::variant_size, std::variant_size_v

 

C++

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Freestanding and hosted

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Feature test macros(C++20)

Language support library

Concepts library(C++20)

Diagnostics library

Memory management library

Metaprogramming library(C++11)

General utilities library

Containers library

Iterators library

Ranges library(C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library(C++17)

Concurrency support library(C++11)

Execution control library(C++26)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros(C++20) Program utilities Variadic functions initializer_list (C++11) is_constant_evaluated (C++20) is_within_lifetime (C++26) source_location (C++20) Coroutine support(C++20) Contract support(C++26) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Function objects Bit manipulation(C++20) bitset hash (C++11) Relational operators(deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23)

 

std::variant

Member functions

variant::variant

variant::~variant

variant::operator=

Observers

variant::index

variant::valueless_by_exception

Modifiers

variant::emplace

variant::swap

Visitation

variant::visit (C++26)

Non-member functions

visit(std::variant)

holds_alternative

get(std::variant)

get_if

operator==operator!=operator<operator<=operator>operator>=operator<=> (C++20)

swap(std::variant)

Helper classes

monostate

bad_variant_access

variant_size

variant_alternative

hash<std::variant>

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>;	(3)	(since C++17)
template<class T > class variant_size<volatile T>;	(4)	(since C++17) (deprecated in C++20)
template<class T > class variant_size<constvolatile T>;	(5)	(since C++17) (deprecated in C++20)

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 base characteristic of std::integral_constant<std::size_t, sizeof...(Types)>

3-5) meets the UnaryTypeTrait requirements with a base characteristic of std::integral_constant<std::size_t, std::variant_size<T>::value>

Helper variable template

template<class T > constexprstd::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 base characteristicstd::integral_constant<std::size_t, N> for some N.

Example

Run this code

#include <any>#include <variant>   static_assert(std::variant_size_v<std::variant<>>==0); static_assert(std::variant_size_v<std::variant<int>>==1); static_assert(std::variant_size_v<std::variant<int, int>>==2); static_assert(std::variant_size_v<std::variant<int, int, int>>==3); static_assert(std::variant_size_v<std::variant<int, float, double>>==3); static_assert(std::variant_size_v<std::variant<std::monostate, void>>==2); static_assert(std::variant_size_v<std::variant<constint, constfloat>>==2); static_assert(std::variant_size_v<std::variant<std::variant<std::any>>>==1);   int main(){}

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)
std::tuple_size<std::tuple> (C++11)	obtains the size of a tuple (class template specialization)