在c ++ stl中有任何三态类型吗？

template< class T >
class optional;

template< class... Types >
class variant;

#include<iostream>
#include<optional>
#include<variant>

int main() {
    std::variant<int, std::monostate, std::string> v1 = 42;
    std::variant<int, std::monostate, std::string> v2 = std::monostate{};
    std::variant<int, std::monostate, std::string> v3 = "hello";

    if (v1.index() == 0) {
        std::cout << "v1 is an int: "<< std::get<0>(v1) << '\n';
    } else if (v1.index() == 1) {
        std::cout << "v1 is empty\n";
    } else if (v1.index() == 2) {
        std::cout << "v1 is a string: "<< std::get<2>(v1) << '\n';
    }

    if (v2.index() == 0) {
        std::cout << "v2 is an int: "<< std::get<0>(v2) << '\n';
    } else if (v2.index() == 1) {
        std::cout << "v2 is empty\n";
    } else if (v2.index() == 2) {
        std::cout << "v2 is a string: "<< std::get<2>(v2) << '\n';
    }

    if (v3.index() == 0) {
        std::cout << "v3 is an int: "<< std::get<0>(v3) << '\n';
    } else if (v3.index() == 1) {
        std::cout << "v3 is empty\n";
    } else if (v3.index() == 2) {
        std::cout << "v3 is a string: "<< std::get<2>(v3) << '\n';
    }
}