std::variant::index (3) Linux Manual Page

std::variant<Types…>::index – std::variant<Types…>::index

Synopsis

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