std::variant<Types...>::index

 

C++

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Symbols index

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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)

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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

 

constexpr std::size_t index() const noexcept;		(since C++17)

Returns the zero-based index of the alternative that is currently held by the variant.

If the variant is valueless_by_exception, returns variant_npos.

Example

Run this code

#include <iostream>
#include <string>
#include <variant>

int main()
{
    std::variant<int, std::string> v = "abc";
    std::cout << "v.index = " << v.index() << '\n';
    v = {};
    std::cout << "v.index = " << v.index() << '\n';
}

Output:

v.index = 1
v.index = 0

See also

holds_alternative (C++17)	checks if a variant currently holds a given type (function template)
get(std::variant) (C++17)	reads the value of the variant given the index or the type (if the type is unique), throws on error (function template)