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

From cppreference.net
C++
Utilities library
std::variant
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

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