Class visitor

tests/test_stl.cpp:515–525

                  "visitor::result_type is required by boost::variant in C++11 mode");

    struct visitor {
        using result_type = const char *;

        result_type operator()(int) { return "int"; }
        result_type operator()(const std::string &) { return "std::string"; }
        result_type operator()(double) { return "double"; }
        result_type operator()(std::nullptr_t) { return "std::nullptr_t"; }
#    if defined(PYBIND11_HAS_VARIANT)
        result_type operator()(std::monostate) { return "std::monostate"; }
#    endif
    };

    // test_variant
    m.def("load_variant", [](const variant<int, std::string, double, std::nullptr_t> &v) {