Function key

extern/json/json.hpp:6813–6821

    }

    bool key(string_t& val)
    {
        JSON_ASSERT(!ref_stack.empty());
        JSON_ASSERT(ref_stack.back()->is_object());

        // add null at given key and store the reference for later
        object_element = &(ref_stack.back()->m_value.object->operator[](val));
        return true;
    }

    bool end_object()
    {