std::variant_size,std::variant_size_v

From cppreference.com

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
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

[edit]

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)

(C++20)

(C++20)

(C++20)

(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)
C-style bit manipulation(C++26)
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 andtype 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)

[edit]

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

[edit]

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 theUnaryTypeTrait requirements with a base characteristic ofstd::integral_constant<std::size_t, sizeof...(Types)>

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

[edit]Helper variable template

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

Inherited fromstd::integral_constant

Member constants

value

[static]

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

Member functions

operator std::size_t	converts the object tostd::size_t, returnsvalue (public member function)
operator() (C++14)	returnsvalue (public member function)

Member types

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

[edit]Notes

All specializations ofstd::variant_size satisfyUnaryTypeTrait withbase characteristicstd::integral_constant<std::size_t, N> for someN.

[edit]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(){}

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