std::variant::index

From cppreference.com
< cpplrm; | utilitylrm; | 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

#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

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