std::variant::index

 

C++

Language

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Regular expressions library (C++11)

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Technical Specifications

 

Utilities library

Type support (basic types, RTTI, type traits)

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initializer_list (C++11)

bitset

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rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=

optional, any and variant (C++17)

optional

any

variant

in_placein_place_typein_place_indexin_place_tin_place_type_tin_place_index_t

Pairs and tuples

pair

tuple (C++11)

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swap

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Type operations

declval (C++11)

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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

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

uses_allocator

Helper objects

variant_npos

 

constexpr std::size_t index() const		(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

Exceptions

noexcept specification:  

noexcept

  

Example

Run this code

#include <variant>
#include <string>
#include <iostream>
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 given type appears exactly once in a variant (function template)
std::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)