diff --git a/.gitmodules b/.gitmodules
index e69de29b..d2e779d0 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -0,0 +1,3 @@
+[submodule "external/function2"]
+	path = external/function2
+	url = https://github.com/Naios/function2
diff --git a/external/function2 b/external/function2
new file mode 160000
index 00000000..2d3a878e
--- /dev/null
+++ b/external/function2
@@ -0,0 +1 @@
+Subproject commit 2d3a878ef19dd5d2fb188898513610fac0a48621
diff --git a/include/mqtt/callable_overlay.hpp b/include/mqtt/callable_overlay.hpp
index 0dbf03c6..fd76ca97 100644
--- a/include/mqtt/callable_overlay.hpp
+++ b/include/mqtt/callable_overlay.hpp
@@ -13,6 +13,7 @@
 #include <mqtt/attributes.hpp>
 #include <mqtt/endpoint.hpp>
 #include <mqtt/move.hpp>
+#include <mqtt/move_only_function.hpp>
 
 namespace MQTT_NS {
 template<typename Impl>
@@ -758,7 +759,7 @@ struct callable_overlay final : public Impl
      *        3.13 PINGREQ – PING request
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using pingreq_handler = std::function<bool()>;
+    using pingreq_handler = move_only_function<bool()>;
 
     /**
      * @brief Pingresp handler
@@ -766,7 +767,7 @@ struct callable_overlay final : public Impl
      *        3.13 PINGRESP – PING response
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using pingresp_handler = std::function<bool()>;
+    using pingresp_handler = move_only_function<bool()>;
 
 
     // MQTT v3_1_1 handlers
@@ -808,7 +809,7 @@ struct callable_overlay final : public Impl
      * @return if the handler returns true, then continue receiving, otherwise quit.
      *
      */
-    using connect_handler = std::function<
+    using connect_handler = move_only_function<
         bool(buffer client_id,
              optional<buffer> user_name,
              optional<buffer> password,
@@ -828,7 +829,7 @@ struct callable_overlay final : public Impl
      *        3.2.2.3 Connect Return code
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using connack_handler = std::function<bool(bool session_present, connect_return_code return_code)>;
+    using connack_handler = move_only_function<bool(bool session_present, connect_return_code return_code)>;
 
     /**
      * @brief Publish handler
@@ -847,7 +848,7 @@ struct callable_overlay final : public Impl
      *        Published contents
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using publish_handler = std::function<bool(optional<packet_id_t> packet_id,
+    using publish_handler = move_only_function<bool(optional<packet_id_t> packet_id,
                                                publish_options pubopts,
                                                buffer topic_name,
                                                buffer contents)>;
@@ -860,7 +861,7 @@ struct callable_overlay final : public Impl
      *        3.4.2 Variable header
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using puback_handler = std::function<bool(packet_id_t packet_id)>;
+    using puback_handler = move_only_function<bool(packet_id_t packet_id)>;
 
     /**
      * @brief Pubrec handler
@@ -870,7 +871,7 @@ struct callable_overlay final : public Impl
      *        3.5.2 Variable header
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using pubrec_handler = std::function<bool(packet_id_t packet_id)>;
+    using pubrec_handler = move_only_function<bool(packet_id_t packet_id)>;
 
     /**
      * @brief Pubrel handler
@@ -880,7 +881,7 @@ struct callable_overlay final : public Impl
      *        3.6.2 Variable header
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using pubrel_handler = std::function<bool(packet_id_t packet_id)>;
+    using pubrel_handler = move_only_function<bool(packet_id_t packet_id)>;
 
     /**
      * @brief Pubcomp handler
@@ -890,7 +891,7 @@ struct callable_overlay final : public Impl
      *        3.7.2 Variable header
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using pubcomp_handler = std::function<bool(packet_id_t packet_id)>;
+    using pubcomp_handler = move_only_function<bool(packet_id_t packet_id)>;
 
     /**
      * @brief Subscribe handler
@@ -902,7 +903,7 @@ struct callable_overlay final : public Impl
      *        See https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc385349802<BR>
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using subscribe_handler = std::function<bool(packet_id_t packet_id,
+    using subscribe_handler = move_only_function<bool(packet_id_t packet_id,
                                                  std::vector<subscribe_entry> entries)>;
 
     /**
@@ -916,7 +917,7 @@ struct callable_overlay final : public Impl
      *        See https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc398718071<BR>
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using suback_handler = std::function<bool(packet_id_t packet_id,
+    using suback_handler = move_only_function<bool(packet_id_t packet_id,
                                               std::vector<suback_return_code> qoss)>;
 
     /**
@@ -929,7 +930,7 @@ struct callable_overlay final : public Impl
      *        See https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc384800448<BR>
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using unsubscribe_handler = std::function<bool(packet_id_t packet_id,
+    using unsubscribe_handler = move_only_function<bool(packet_id_t packet_id,
                                                    std::vector<unsubscribe_entry> entries)>;
 
     /**
@@ -939,14 +940,14 @@ struct callable_overlay final : public Impl
      *        3.11.2 Variable header
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using unsuback_handler = std::function<bool(packet_id_t)>;
+    using unsuback_handler = move_only_function<bool(packet_id_t)>;
 
     /**
      * @brief Disconnect handler
      *        See https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc384800463<BR>
      *        3.14 DISCONNECT – Disconnect notification
      */
-    using disconnect_handler = std::function<void()>;
+    using disconnect_handler = move_only_function<void()>;
 
     // MQTT v5 handlers
 
@@ -993,7 +994,7 @@ struct callable_overlay final : public Impl
      * @return if the handler returns true, then continue receiving, otherwise quit.
      *
      */
-    using v5_connect_handler = std::function<
+    using v5_connect_handler = move_only_function<
         bool(buffer client_id,
              optional<buffer> user_name,
              optional<buffer> password,
@@ -1019,7 +1020,7 @@ struct callable_overlay final : public Impl
      *        3.2.2.3 CONNACK Properties
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using v5_connack_handler = std::function<
+    using v5_connack_handler = move_only_function<
         bool(bool session_present,
              v5::connect_reason_code reason_code,
              v5::properties props)
@@ -1050,7 +1051,7 @@ struct callable_overlay final : public Impl
      *        3.3.2.3 PUBLISH Properties
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using v5_publish_handler = std::function<
+    using v5_publish_handler = move_only_function<
         bool(optional<packet_id_t> packet_id,
              publish_options pubopts,
              buffer topic_name,
@@ -1074,7 +1075,7 @@ struct callable_overlay final : public Impl
      *        3.4.2.2 PUBACK Properties
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using v5_puback_handler = std::function<
+    using v5_puback_handler = move_only_function<
         bool(packet_id_t packet_id,
              v5::puback_reason_code reason_code,
              v5::properties props)
@@ -1096,7 +1097,7 @@ struct callable_overlay final : public Impl
      *        3.5.2.2 PUBREC Properties
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using v5_pubrec_handler = std::function<
+    using v5_pubrec_handler = move_only_function<
         bool(packet_id_t packet_id,
              v5::pubrec_reason_code reason_code,
              v5::properties props)
@@ -1118,7 +1119,7 @@ struct callable_overlay final : public Impl
      *        3.6.2.2 PUBREL Properties
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using v5_pubrel_handler = std::function<
+    using v5_pubrel_handler = move_only_function<
         bool(packet_id_t packet_id,
              v5::pubrel_reason_code reason_code,
              v5::properties props)
@@ -1140,7 +1141,7 @@ struct callable_overlay final : public Impl
      *        3.7.2.2 PUBCOMP Properties
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using v5_pubcomp_handler = std::function<
+    using v5_pubcomp_handler = move_only_function<
         bool(packet_id_t packet_id,
              v5::pubcomp_reason_code reason_code,
              v5::properties props)
@@ -1160,7 +1161,7 @@ struct callable_overlay final : public Impl
      *        3.8.2.1 SUBSCRIBE Properties
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using v5_subscribe_handler = std::function<
+    using v5_subscribe_handler = move_only_function<
         bool(packet_id_t packet_id,
              std::vector<subscribe_entry> entries,
              v5::properties props)
@@ -1181,7 +1182,7 @@ struct callable_overlay final : public Impl
      *        3.9.2.1 SUBACK Properties
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using v5_suback_handler = std::function<
+    using v5_suback_handler = move_only_function<
         bool(packet_id_t packet_id,
              std::vector<v5::suback_reason_code> reasons,
              v5::properties props)
@@ -1202,7 +1203,7 @@ struct callable_overlay final : public Impl
      *        3.10.2.1 UNSUBSCRIBE Properties
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using v5_unsubscribe_handler = std::function<
+    using v5_unsubscribe_handler = move_only_function<
         bool(packet_id_t packet_id,
              std::vector<unsubscribe_entry> entries,
              v5::properties props)
@@ -1223,7 +1224,7 @@ struct callable_overlay final : public Impl
      *        3.11.2.1 UNSUBACK Properties
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using v5_unsuback_handler = std::function<
+    using v5_unsuback_handler = move_only_function<
         bool(packet_id_t,
              std::vector<v5::unsuback_reason_code> reasons,
              v5::properties props)
@@ -1242,7 +1243,7 @@ struct callable_overlay final : public Impl
      *        See https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901209<BR>
      *        3.14.2.2 DISCONNECT Properties
      */
-    using v5_disconnect_handler = std::function<
+    using v5_disconnect_handler = move_only_function<
         void(v5::disconnect_reason_code reason_code,
              v5::properties props)
     >;
@@ -1261,7 +1262,7 @@ struct callable_overlay final : public Impl
      *        3.15.2.2 AUTH Properties
      * @return if the handler returns true, then continue receiving, otherwise quit.
      */
-    using v5_auth_handler = std::function<
+    using v5_auth_handler = move_only_function<
         bool(v5::auth_reason_code reason_code,
              v5::properties props)
     >;
@@ -1275,7 +1276,7 @@ struct callable_overlay final : public Impl
      * This handler is called if the client called `disconnect()` and the server closed the socket cleanly.
      * If the socket is closed by other reasons, error_handler is called.
      */
-    using close_handler = std::function<void()>;
+    using close_handler = move_only_function<void()>;
 
     /**
      * @brief Error handler
@@ -1284,7 +1285,7 @@ struct callable_overlay final : public Impl
      *
      * @param ec error code
      */
-    using error_handler = std::function<void(error_code ec)>;
+    using error_handler = move_only_function<void(error_code ec)>;
 
     /**
      * @brief Publish response sent handler
@@ -1294,7 +1295,7 @@ struct callable_overlay final : public Impl
      *        See https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901026<BR>
      *        2.2.1 Packet Identifier
      */
-    using pub_res_sent_handler = std::function<void(packet_id_t packet_id)>;
+    using pub_res_sent_handler = move_only_function<void(packet_id_t packet_id)>;
 
     /**
      * @brief Serialize publish handler
@@ -1302,7 +1303,7 @@ struct callable_overlay final : public Impl
      *        To restore the message, use restore_serialized_message().
      * @param msg publish message
      */
-    using serialize_publish_message_handler = std::function<void(basic_publish_message<sizeof(packet_id_t)> msg)>;
+    using serialize_publish_message_handler = move_only_function<void(basic_publish_message<sizeof(packet_id_t)> msg)>;
 
     /**
      * @brief Serialize publish handler
@@ -1310,7 +1311,7 @@ struct callable_overlay final : public Impl
      *        To restore the message, use restore_serialized_message().
      * @param msg v5::publish message
      */
-    using serialize_v5_publish_message_handler = std::function<void(v5::basic_publish_message<sizeof(packet_id_t)> msg)>;
+    using serialize_v5_publish_message_handler = move_only_function<void(v5::basic_publish_message<sizeof(packet_id_t)> msg)>;
 
     /**
      * @brief Serialize publish handler
@@ -1320,7 +1321,7 @@ struct callable_overlay final : public Impl
      * @param data      pointer to the serializing message
      * @param size      size of the serializing message
      */
-    using serialize_publish_handler = std::function<void(packet_id_t packet_id, char const* data, std::size_t size)>;
+    using serialize_publish_handler = move_only_function<void(packet_id_t packet_id, char const* data, std::size_t size)>;
 
     /**
      * @brief Serialize pubrel handler
@@ -1330,7 +1331,7 @@ struct callable_overlay final : public Impl
      *        To restore the message, use restore_serialized_message().
      * @param msg pubrel message
      */
-    using serialize_pubrel_message_handler = std::function<void(basic_pubrel_message<sizeof(packet_id_t)> msg)>;
+    using serialize_pubrel_message_handler = move_only_function<void(basic_pubrel_message<sizeof(packet_id_t)> msg)>;
 
     /**
      * @brief Serialize pubrel handler
@@ -1340,7 +1341,7 @@ struct callable_overlay final : public Impl
      *        To restore the message, use restore_serialized_message().
      * @param msg pubrel message
      */
-    using serialize_v5_pubrel_message_handler = std::function<void(v5::basic_pubrel_message<sizeof(packet_id_t)> msg)>;
+    using serialize_v5_pubrel_message_handler = move_only_function<void(v5::basic_pubrel_message<sizeof(packet_id_t)> msg)>;
 
     /**
      * @brief Serialize pubrel handler
@@ -1352,19 +1353,19 @@ struct callable_overlay final : public Impl
      * @param data      pointer to the serializing message
      * @param size      size of the serializing message
      */
-    using serialize_pubrel_handler = std::function<void(packet_id_t packet_id, char const* data, std::size_t size)>;
+    using serialize_pubrel_handler = move_only_function<void(packet_id_t packet_id, char const* data, std::size_t size)>;
 
     /**
      * @brief Remove serialized message
      * @param packet_id packet identifier of the removing message
      */
-    using serialize_remove_handler = std::function<void(packet_id_t packet_id)>;
+    using serialize_remove_handler = move_only_function<void(packet_id_t packet_id)>;
 
     /**
      * @brief Pre-send handler
      *        This handler is called when any mqtt control packet is decided to send.
      */
-    using pre_send_handler = std::function<void()>;
+    using pre_send_handler = move_only_function<void()>;
 
     /**
      * @brief is valid length handler
@@ -1374,7 +1375,7 @@ struct callable_overlay final : public Impl
      * @return true if check is success, otherwise false
      */
     using is_valid_length_handler =
-        std::function<bool(control_packet_type packet_type, std::size_t remaining_length)>;
+        move_only_function<bool(control_packet_type packet_type, std::size_t remaining_length)>;
 
     /**
      * @brief next read handler
@@ -1382,7 +1383,7 @@ struct callable_overlay final : public Impl
      * @param func A callback function that is called when async operation will finish.
      */
     using mqtt_message_processed_handler =
-        std::function<void(any session_life_keeper)>;
+        move_only_function<void(any session_life_keeper)>;
 
 
 
@@ -1879,7 +1880,7 @@ struct callable_overlay final : public Impl
         serialize_remove_handler h_remove) {
         h_serialize_publish_ =
             [h_publish = force_move(h_publish)]
-            (basic_publish_message<sizeof(packet_id_t)> msg) {
+            (basic_publish_message<sizeof(packet_id_t)> msg) mutable {
                 if (h_publish) {
                     auto buf = msg.continuous_buffer();
                     h_publish(msg.packet_id(), buf.data(), buf.size());
@@ -1887,7 +1888,7 @@ struct callable_overlay final : public Impl
             };
         h_serialize_pubrel_ =
             [h_pubrel = force_move(h_pubrel)]
-            (basic_pubrel_message<sizeof(packet_id_t)> msg) {
+            (basic_pubrel_message<sizeof(packet_id_t)> msg) mutable {
                 if (h_pubrel) {
                     auto buf = msg.continuous_buffer();
                     h_pubrel(msg.packet_id(), buf.data(), buf.size());
@@ -1908,7 +1909,7 @@ struct callable_overlay final : public Impl
         serialize_remove_handler h_remove) {
         h_serialize_v5_publish_ =
             [h_publish = force_move(h_publish)]
-            (v5::basic_publish_message<sizeof(packet_id_t)> msg) {
+            (v5::basic_publish_message<sizeof(packet_id_t)> msg) mutable {
                 if (h_publish) {
                     auto buf = msg.continuous_buffer();
                     h_publish(msg.packet_id(), buf.data(), buf.size());
@@ -1916,7 +1917,7 @@ struct callable_overlay final : public Impl
             };
         h_serialize_v5_pubrel_ =
             [h_pubrel = force_move(h_pubrel)]
-            (v5::basic_pubrel_message<sizeof(packet_id_t)> msg) {
+            (v5::basic_pubrel_message<sizeof(packet_id_t)> msg) mutable {
                 if (h_pubrel) {
                     auto buf = msg.continuous_buffer();
                     h_pubrel(msg.packet_id(), buf.data(), buf.size());
@@ -2042,7 +2043,7 @@ struct callable_overlay final : public Impl
      * @brief Get  mqtt_message_processed_handler.
      * @return mqtt_message_processed_handler.
      */
-    mqtt_message_processed_handler get_mqtt_message_processed_handler() const {
+    mqtt_message_processed_handler const& get_mqtt_message_processed_handler() const {
         return h_mqtt_message_processed_;
     }
 
@@ -2051,7 +2052,7 @@ struct callable_overlay final : public Impl
      * @param h handler
      */
     void set_close_handler(close_handler h = close_handler()) {
-        h_close_ = force_move(h);
+        // h_close_ = force_move(h);
     }
 
     /**
@@ -2066,7 +2067,7 @@ struct callable_overlay final : public Impl
      * @brief Get close handler
      * @return handler
      */
-    close_handler get_close_handler() const {
+    close_handler const& get_close_handler() const {
         return h_close_;
     }
 
@@ -2074,7 +2075,7 @@ struct callable_overlay final : public Impl
      * @brief Get error handler
      * @return handler
      */
-    error_handler get_error_handler() const {
+    error_handler const& get_error_handler() const {
         return h_error_;
     }
 
diff --git a/include/mqtt/endpoint.hpp b/include/mqtt/endpoint.hpp
index 179cfa2b..5b86acfd 100644
--- a/include/mqtt/endpoint.hpp
+++ b/include/mqtt/endpoint.hpp
@@ -71,6 +71,7 @@
 #include <mqtt/shared_subscriptions.hpp>
 #include <mqtt/packet_id_manager.hpp>
 #include <mqtt/store.hpp>
+#include <mqtt/move_only_function.hpp>
 
 #if defined(MQTT_USE_WS)
 #include <mqtt/ws_endpoint.hpp>
@@ -173,7 +174,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
     using this_type_sp = std::shared_ptr<this_type>;
 
 public:
-    using async_handler_t = std::function<void(error_code ec)>;
+    using async_handler_t = move_only_function<void(error_code ec)>;
     using packet_id_t = typename packet_id_type<PacketIdBytes>::type;
 
     /**
@@ -2229,7 +2230,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
         }
         else {
             socket_->post(
-                [func = force_move(func)] {
+                [func = force_move(func)] () mutable {
                     if (func) func(boost::system::errc::make_error_code(boost::system::errc::success));
                 }
             );
@@ -2268,7 +2269,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
         }
         else {
             socket_->post(
-                [func = force_move(func)] {
+                [func = force_move(func)] () mutable {
                     if (func) func(boost::system::errc::make_error_code(boost::system::errc::success));
                 }
             );
@@ -2756,7 +2757,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             packet_id,
             v5::properties{},
             [life_keeper = force_move(sp_topic_filter), func = force_move(func)]
-            (error_code ec) {
+            (error_code ec) mutable {
                 if(func) func(ec);
             }
         );
@@ -2807,7 +2808,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             packet_id,
             force_move(props),
             [life_keeper = force_move(sp_topic_filter), func = force_move(func)]
-            (error_code ec) {
+            (error_code ec) mutable {
                 if(func) func(ec);
             }
         );
@@ -2938,7 +2939,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             packet_id,
             v5::properties{},
             [life_keeper = force_move(topic_filter), func = force_move(func)]
-            (error_code ec) {
+            (error_code ec) mutable {
                 if(func) func(ec);
             }
         );
@@ -2987,7 +2988,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             packet_id,
             force_move(props),
             [life_keeper = force_move(topic_filter), func = force_move(func)]
-            (error_code ec) {
+            (error_code ec) mutable {
                 if(func) func(ec);
             }
         );
@@ -3034,7 +3035,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             packet_id,
             v5::properties{},
             [life_keeper = force_move(life_keepers), func = force_move(func)]
-            (error_code ec) {
+            (error_code ec) mutable {
                 if(func) func(ec);
             }
         );
@@ -3085,7 +3086,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             packet_id,
             force_move(props),
             [life_keeper = force_move(life_keepers), func = force_move(func)]
-            (error_code ec) {
+            (error_code ec) mutable {
                 if(func) func(ec);
             }
         );
@@ -3191,7 +3192,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             packet_id,
             v5::properties{},
             [life_keeper = force_move(params), func = force_move(func)]
-            (error_code ec) {
+            (error_code ec) mutable {
                 if(func) func(ec);
             }
         );
@@ -3237,7 +3238,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             packet_id,
             force_move(props),
             [life_keeper = force_move(params), func = force_move(func)]
-            (error_code ec) {
+            (error_code ec) mutable {
                 if(func) func(ec);
             }
         );
@@ -3272,7 +3273,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             packet_id,
             v5::properties{},
             [life_keeper = force_move(sp_topic_filter), func = force_move(func)]
-            (error_code ec) {
+            (error_code ec) mutable {
                 if(func) func(ec);
             }
         );
@@ -3331,7 +3332,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
                                packet_id,
                                v5::properties{},
                                [life_keeper = force_move(topic_filter), func = force_move(func)]
-                               (error_code ec) {
+                               (error_code ec) mutable {
                                    if(func) func(ec);
                                });
     }
@@ -3368,7 +3369,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
                                packet_id,
                                force_move(props),
                                [life_keeper = force_move(topic_filter), func = force_move(func)]
-                               (error_code ec) {
+                               (error_code ec) mutable {
                                    if(func) func(ec);
                                });
     }
@@ -3411,7 +3412,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             packet_id,
             v5::properties{},
             [life_keeper = force_move(life_keepers), func = force_move(func)]
-            (error_code ec) {
+            (error_code ec) mutable {
                 if(func) func(ec);
             }
         );
@@ -3465,7 +3466,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             packet_id,
             force_move(props),
             [life_keeper = force_move(life_keepers), func = force_move(func)]
-            (error_code ec) {
+            (error_code ec) mutable {
                 if(func) func(ec);
             }
         );
@@ -3571,7 +3572,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             packet_id,
             v5::properties{},
             [life_keeper = force_move(params), func = force_move(func)]
-            (error_code ec) {
+            (error_code ec) mutable {
                 if(func) func(ec);
             }
         );
@@ -3615,7 +3616,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             packet_id,
             force_move(props),
             [life_keeper = force_move(params), func = force_move(func)]
-            (error_code ec) {
+            (error_code ec) mutable {
                 if(func) func(ec);
             }
         );
@@ -4322,16 +4323,17 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
      * @brief Apply f to stored messages.
      * @param f applying function. f should be void(char const*, std::size_t)
      */
-    void for_each_store(std::function<void(char const*, std::size_t)> const& f) {
+    void for_each_store(move_only_function<void(char const*, std::size_t)> f) {
         MQTT_LOG("mqtt_api", info)
             << MQTT_ADD_VALUE(address, this)
             << "for_each_store(ptr, size)";
         LockGuard<Mutex> lck (store_mtx_);
         store_.for_each(
-            [f](
+            [f = force_move(f)]
+            (
                 basic_store_message_variant<PacketIdBytes> const& message,
                 any const& /*life_keeper*/
-            ) {
+            ) mutable {
                 auto cb = continuous_buffer(message);
                 f(cb.data(), cb.size());
                 return false; // no erase
@@ -4343,16 +4345,17 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
      * @brief Apply f to stored messages.
      * @param f applying function. f should be void(store_message_variant)
      */
-    void for_each_store(std::function<void(basic_store_message_variant<PacketIdBytes>)> const& f) {
+    void for_each_store(move_only_function<void(basic_store_message_variant<PacketIdBytes>)> f) {
         MQTT_LOG("mqtt_api", info)
             << MQTT_ADD_VALUE(address, this)
             << "for_each_store(store_message_variant)";
         LockGuard<Mutex> lck (store_mtx_);
         store_.for_each(
-            [f](
+            [f = force_move(f)]
+            (
                 basic_store_message_variant<PacketIdBytes> const& message,
                 any const& /*life_keeper*/
-            ) {
+            ) mutable {
                 f(message);
                 return false; // no erase
             }
@@ -4363,17 +4366,18 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
      * @brief Apply f to stored messages.
      * @param f applying function. f should be void(store_message_variant, any)
      */
-    void for_each_store_with_life_keeper(std::function<void(basic_store_message_variant<PacketIdBytes>, any)> const& f) {
+    void for_each_store_with_life_keeper(move_only_function<void(basic_store_message_variant<PacketIdBytes>, any)> f) {
         MQTT_LOG("mqtt_api", info)
             << MQTT_ADD_VALUE(address, this)
 
             << "for_each_store(store_message_variant, life_keeper)";
         LockGuard<Mutex> lck (store_mtx_);
         store_.for_each(
-            [f](
+            [f = force_move(f)]
+            (
                 basic_store_message_variant<PacketIdBytes> const& message,
                 any const& life_keeper
-            ) {
+            ) mutable {
                 f(message, life_keeper);
                 return false; // no erase
             }
@@ -4782,7 +4786,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
                 auto msg_lk = apply_topic_alias(msg, life_keeper);
                 if (maximum_packet_size_send_ < size<PacketIdBytes>(std::get<0>(msg_lk))) {
                     socket_->post(
-                        [func = force_move(func)] {
+                        [func = force_move(func)] () mutable {
                             if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                         }
                     );
@@ -4799,7 +4803,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
                     do_async_write(
                         force_move(std::get<0>(msg_lk)),
                         [func = force_move(func), life_keeper = force_move(std::get<1>(msg_lk))]
-                        (error_code ec) {
+                        (error_code ec) mutable {
                             if (func) func(ec);
                         }
                     );
@@ -4810,7 +4814,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             [&](auto msg, auto const& serialize) {
                 if (maximum_packet_size_send_ < size<PacketIdBytes>(msg)) {
                     socket_->post(
-                        [func = force_move(func)] {
+                        [func = force_move(func)] () mutable {
                             if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                         }
                     );
@@ -4859,14 +4863,14 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
                     publish_proc(
                         force_move(m),
                         &endpoint::on_serialize_v5_publish_message,
-                        [this, func] (v5::basic_publish_message<PacketIdBytes>&& msg) mutable {
+                        [this, func = force_move(func)] (v5::basic_publish_message<PacketIdBytes>&& msg) mutable {
                             if (publish_send_count_.load() == publish_send_max_) {
                                 {
                                     LockGuard<Mutex> lck (publish_send_queue_mtx_);
                                     publish_send_queue_.emplace_back(force_move(msg), true);
                                 }
                                 socket_->post(
-                                    [func = force_move(func)] {
+                                    [func = force_move(func)] () mutable {
                                         // message has already been stored so func should be called with success here
                                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::success));
                                     }
@@ -5365,7 +5369,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
                     std::back_inserter(handlers)
                 );
                 async_shutdown_handler_queue_.clear();
-                for (auto const& h : handlers) {
+                for (auto& h : handlers) {
                     if (h) h(boost::system::errc::make_error_code(boost::system::errc::success));
                 }
                 async_shutdown_handler_called_ = true;
@@ -5745,7 +5749,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
         v5::properties
     >;
     using parse_handler =
-        std::function<
+        move_only_function<
             void(
                 this_type_sp&& spep,
                 any&& session_life_keeper,
@@ -10492,7 +10496,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             );
             if (maximum_packet_size_send_ < size<PacketIdBytes>(msg)) {
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -10512,7 +10516,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             );
             if (maximum_packet_size_send_ < size<PacketIdBytes>(msg)) {
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -10551,7 +10555,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
                         pid_man_.release_id(packet_id);
                     }
                     socket_->post(
-                        [func = force_move(func)] {
+                        [func = force_move(func)] () mutable {
                             if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                         }
                     );
@@ -10566,7 +10570,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
                 ) {
                     do_async_write(
                         force_move(std::get<0>(msg_lk)),
-                        [life_keeper = force_move(std::get<1>(msg_lk)), func](error_code ec) {
+                        [life_keeper = force_move(std::get<1>(msg_lk)), func = force_move(func)](error_code ec) mutable {
                             if (func) func(ec);
                         }
                     );
@@ -10596,14 +10600,14 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
                     force_move(props)
                 ),
                 &endpoint::on_serialize_v5_publish_message,
-                [this, func] (v5::basic_publish_message<PacketIdBytes>&& msg) mutable {
+                [this, func = force_move(func)] (v5::basic_publish_message<PacketIdBytes>&& msg) mutable {
                     if (publish_send_count_.load() == publish_send_max_) {
                         {
                             LockGuard<Mutex> lck (publish_send_queue_mtx_);
                             publish_send_queue_.emplace_back(force_move(msg), true);
                         }
                         socket_->post(
-                            [func = force_move(func)] {
+                            [func = force_move(func)] () mutable {
                                 // message has already been stored so func should be called with success here
                                 if (func) func(boost::system::errc::make_error_code(boost::system::errc::success));
                             }
@@ -10635,7 +10639,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             auto msg = v3_1_1::basic_puback_message<PacketIdBytes>(packet_id);
             if (maximum_packet_size_send_ < size<PacketIdBytes>(msg)) {
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -10644,7 +10648,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             do_async_write(
                 force_move(msg),
                 [this, self = this->shared_from_this(), packet_id, func = force_move(func)]
-                (error_code ec) {
+                (error_code ec) mutable {
                     if (func) func(ec);
                     on_pub_res_sent(packet_id);
                 }
@@ -10654,7 +10658,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             auto msg = v5::basic_puback_message<PacketIdBytes>(packet_id, reason, force_move(props));
             if (maximum_packet_size_send_ < size<PacketIdBytes>(msg)) {
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -10663,7 +10667,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             do_async_write(
                 force_move(msg),
                 [this, self = this->shared_from_this(), packet_id, func = force_move(func)]
-                (error_code ec) {
+                (error_code ec) mutable {
                     erase_publish_received(packet_id);
                     if (func) func(ec);
                     on_pub_res_sent(packet_id);
@@ -10687,7 +10691,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             auto msg = v3_1_1::basic_pubrec_message<PacketIdBytes>(packet_id);
             if (maximum_packet_size_send_ < size<PacketIdBytes>(msg)) {
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -10702,7 +10706,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             auto msg = v5::basic_pubrec_message<PacketIdBytes>(packet_id, reason, force_move(props));
             if (maximum_packet_size_send_ < size<PacketIdBytes>(msg)) {
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -10711,7 +10715,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             do_async_write(
                 force_move(msg),
                 [this, self = this->shared_from_this(), packet_id, func = force_move(func)]
-                (error_code ec) {
+                (error_code ec) mutable {
                     erase_publish_received(packet_id);
                     if (func) func(ec);
                 }
@@ -10738,7 +10742,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
                 {
                     if (maximum_packet_size_send_ < size<PacketIdBytes>(msg)) {
                         socket_->post(
-                            [func = force_move(func)] {
+                            [func = force_move(func)] () mutable {
                                 if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                             }
                         );
@@ -10777,7 +10781,8 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
 
                 do_async_write(
                     force_move(msg),
-                    [life_keeper = force_move(life_keeper), func = force_move(func)](error_code ec) {
+                    [life_keeper = force_move(life_keeper), func = force_move(func)]
+                    (error_code ec) mutable {
                         if(func) func(ec);
                     }
                 );
@@ -10813,7 +10818,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             auto msg = v3_1_1::basic_pubcomp_message<PacketIdBytes>(packet_id);
             if (maximum_packet_size_send_ < size<PacketIdBytes>(msg)) {
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -10822,7 +10827,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             do_async_write(
                 force_move(msg),
                 [this, self = this->shared_from_this(), packet_id, func = force_move(func)]
-                (error_code ec) {
+                (error_code ec) mutable {
                     if (func) func(ec);
                     on_pub_res_sent(packet_id);
                 }
@@ -10832,7 +10837,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             auto msg = v5::basic_pubcomp_message<PacketIdBytes>(packet_id, reason, force_move(props));
             if (maximum_packet_size_send_ < size<PacketIdBytes>(msg)) {
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -10841,7 +10846,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             do_async_write(
                 force_move(msg),
                 [this, self = this->shared_from_this(), packet_id, func = force_move(func)]
-                (error_code ec) {
+                (error_code ec) mutable {
                     if (func) func(ec);
                     on_pub_res_sent(packet_id);
                 }
@@ -10868,7 +10873,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
                     pid_man_.release_id(packet_id);
                 }
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -10891,7 +10896,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
                     pid_man_.release_id(packet_id);
                 }
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -10926,7 +10931,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             );
             if (maximum_packet_size_send_ < size<PacketIdBytes>(msg)) {
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -10945,7 +10950,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             );
             if (maximum_packet_size_send_ < size<PacketIdBytes>(msg)) {
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -10977,7 +10982,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
                     pid_man_.release_id(packet_id);
                 }
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -11000,7 +11005,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
                     pid_man_.release_id(packet_id);
                 }
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -11030,7 +11035,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             auto msg = v3_1_1::basic_unsuback_message<PacketIdBytes>(packet_id);
             if (maximum_packet_size_send_ < size<PacketIdBytes>(msg)) {
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -11064,7 +11069,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             auto msg = v5::basic_unsuback_message<PacketIdBytes>(force_move(params), packet_id, force_move(props));
             if (maximum_packet_size_send_ < size<PacketIdBytes>(msg)) {
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -11087,7 +11092,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             auto msg = v3_1_1::pingreq_message();
             if (maximum_packet_size_send_ < size<PacketIdBytes>(msg)) {
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -11100,7 +11105,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             auto msg = v5::pingreq_message();
             if (maximum_packet_size_send_ < size<PacketIdBytes>(msg)) {
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -11121,7 +11126,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             auto msg = v3_1_1::pingresp_message();
             if (maximum_packet_size_send_ < size<PacketIdBytes>(msg)) {
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -11133,7 +11138,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             auto msg = v5::pingresp_message();
             if (maximum_packet_size_send_ < size<PacketIdBytes>(msg)) {
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -11160,7 +11165,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             auto msg = v5::auth_message(reason, force_move(props));
             if (maximum_packet_size_send_ < size<PacketIdBytes>(msg)) {
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -11184,7 +11189,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             auto msg = v3_1_1::disconnect_message();
             if (maximum_packet_size_send_ < size<PacketIdBytes>(msg)) {
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -11196,7 +11201,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             auto msg = v5::disconnect_message(reason, force_move(props));
             if (maximum_packet_size_send_ < size<PacketIdBytes>(msg)) {
                 socket_->post(
-                    [func = force_move(func)] {
+                    [func = force_move(func)] () mutable {
                         if (func) func(boost::system::errc::make_error_code(boost::system::errc::message_size));
                     }
                 );
@@ -11210,10 +11215,10 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
         }
     }
 
-    void async_send_store(std::function<void()> func) {
+    void async_send_store(move_only_function<void()> func) {
         // packet_id has already been registered
         auto g = shared_scope_guard(
-            [func = force_move(func)] {
+            [func = force_move(func)] () mutable {
                 func();
             }
         );
@@ -11359,7 +11364,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             // it's a bug if the handler is invalid when constructed.
             BOOST_ASSERT(func_);
         }
-        void operator()(error_code ec) const {
+        void operator()(error_code ec) {
             func_(ec);
             for (std::size_t i = 0; i != num_of_messages_; ++i) {
                 self_->queue_.pop_front();
@@ -11379,7 +11384,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
         }
         void operator()(
             error_code ec,
-            std::size_t bytes_transferred) const {
+            std::size_t bytes_transferred) {
             func_(ec);
             self_->total_bytes_sent_ += bytes_transferred;
             for (std::size_t i = 0; i != num_of_messages_; ++i) {
@@ -11418,7 +11423,7 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
         std::size_t iterator_count = (max_queue_send_count_ == 0)
                                 ? queue_.size()
                                 : std::min(max_queue_send_count_, queue_.size());
-        auto const& start = queue_.cbegin();
+        auto start = queue_.begin();
         auto end = std::next(start, boost::numeric_cast<difference_t>(iterator_count));
 
         // And further, only up to the specified maximum bytes
@@ -11446,11 +11451,11 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
         handlers.reserve(iterator_count);
 
         for (auto it = start; it != end; ++it) {
-            auto const& elem = *it;
+            auto& elem = *it;
             auto const& mv = elem.message();
             auto const& cbs = const_buffer_sequence(mv);
             std::copy(cbs.begin(), cbs.end(), std::back_inserter(buf));
-            handlers.emplace_back(elem.handler());
+            handlers.emplace_back(force_move(elem.handler()));
         }
 
         on_pre_send();
@@ -11460,8 +11465,8 @@ class endpoint : public std::enable_shared_from_this<endpoint<Mutex, LockGuard,
             write_completion_handler(
                 this->shared_from_this(),
                 [handlers = force_move(handlers)]
-                (error_code ec) {
-                    for (auto const& h : handlers) {
+                (error_code ec) mutable {
+                    for (auto& h : handlers) {
                         if (h) h(ec);
                     }
                 },
diff --git a/include/mqtt/move_only_function.hpp b/include/mqtt/move_only_function.hpp
new file mode 100644
index 00000000..6b4bebc2
--- /dev/null
+++ b/include/mqtt/move_only_function.hpp
@@ -0,0 +1,21 @@
+// Copyright Takatoshi Kondo 2022
+//
+// Distributed under the Boost Software License, Version 1.0.
+// (See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+
+#if !defined(MQTT_MOVE_ONLY_FUNCTION_HPP)
+#define MQTT_MOVE_ONLY_FUNCTION_HPP
+
+#include <mqtt/external/function2.hpp>
+
+#include <mqtt/namespace.hpp>
+
+namespace MQTT_NS {
+
+template <typename... Params>
+using move_only_function = fu2::unique_function<Params...>;
+
+} // namespace MQTT_NS
+
+#endif // MQTT_MOVE_ONLY_FUNCTION_HPP
diff --git a/include/mqtt/null_strand.hpp b/include/mqtt/null_strand.hpp
index a9f56b0c..f14fe124 100644
--- a/include/mqtt/null_strand.hpp
+++ b/include/mqtt/null_strand.hpp
@@ -17,9 +17,77 @@ namespace MQTT_NS {
 
 namespace as = boost::asio;
 
-// Using standard executor style null_strand / simple executor
-using null_strand = as::io_context::executor_type;
+namespace detail {
+
+struct null_strand {
+    using inner_executor_type = as::io_context::executor_type;
+    template <typename Executor>
+    explicit null_strand(Executor e) noexcept
+        : exe_{force_move(e)}
+    {}
+
+    template <typename Func, typename Allocator>
+    void defer(Func&& f, Allocator) const {
+        as::defer(
+            exe_,
+            [f = std::forward<Func>(f)] () mutable {
+                std::move(f)();
+            }
+        );
+    }
+    template <typename Func, typename Allocator>
+    void dispatch(Func&& f, Allocator) const {
+        as::dispatch(
+            exe_,
+            [f = std::forward<Func>(f)] () mutable {
+                std::move(f)();
+            }
+        );
+    }
+    template <typename Func, typename Allocator>
+    void post(Func&& f, Allocator) const {
+        as::post(
+            exe_,
+            [f = std::forward<Func>(f)] () mutable {
+                std::move(f)();
+            }
+        );
+    }
+    as::any_io_executor get_inner_executor() const {
+        return exe_;
+    }
+    void on_work_started() const noexcept {}
+    void on_work_finished() const noexcept {}
+    bool running_in_this_thread() const noexcept { return true; }
+    operator as::any_io_executor() const {
+        return exe_;
+    }
+private:
+    as::io_context::executor_type exe_;
+};
+
+} // namespace detail
+
+using null_strand = detail::null_strand;
+
+inline bool operator==(null_strand const& lhs, null_strand const& rhs) {
+    return std::addressof(lhs) == std::addressof(rhs);
+}
+
+inline bool operator!=(null_strand const& lhs, null_strand const& rhs) {
+    return !(lhs == rhs);
+}
 
 } // namespace MQTT_NS
 
+namespace boost {
+namespace asio {
+
+template<>
+struct is_executor<MQTT_NS::null_strand> : std::true_type {
+};
+
+} // namespace asio
+} // namespace boost
+
 #endif // MQTT_NULL_STRAND_HPP
diff --git a/include/mqtt/server.hpp b/include/mqtt/server.hpp
index 1cdc6b99..1e8c85fd 100644
--- a/include/mqtt/server.hpp
+++ b/include/mqtt/server.hpp
@@ -20,6 +20,7 @@
 #include <mqtt/callable_overlay.hpp>
 #include <mqtt/strand.hpp>
 #include <mqtt/null_strand.hpp>
+#include <mqtt/move_only_function.hpp>
 
 namespace MQTT_NS {
 
@@ -53,7 +54,7 @@ class server {
      *        After this handler called, the next accept will automatically start.
      * @param ep endpoint of the connecting client
      */
-    using accept_handler = std::function<void(std::shared_ptr<endpoint_t> ep)>;
+    using accept_handler = move_only_function<void(std::shared_ptr<endpoint_t> ep)>;
 
     /**
      * @brief Error handler during after accepted before connection established
@@ -61,7 +62,7 @@ class server {
      * @param ec error code
      * @param ioc_con io_context for incoming connection
      */
-    using connection_error_handler = std::function<void(error_code ec, as::io_context& ioc_con)>;
+    using connection_error_handler = move_only_function<void(error_code ec, as::io_context& ioc_con)>;
 
     /**
      * @brief Error handler for listen and accpet
@@ -69,7 +70,7 @@ class server {
      *        You need to call listen() again if you want to restart accepting.
      * @param ec error code
      */
-    using error_handler = std::function<void(error_code ec)>;
+    using error_handler = move_only_function<void(error_code ec)>;
 
     /**
      * @brief Error handler for listen and accpet
@@ -78,7 +79,7 @@ class server {
      * @param ec error code
      * @param ioc_con io_context for listen or accept
      */
-    using error_handler_with_ioc = std::function<void(error_code ec, as::io_context& ioc_accept)>;
+    using error_handler_with_ioc = move_only_function<void(error_code ec, as::io_context& ioc_accept)>;
 
     template <typename AsioEndpoint, typename AcceptorConfig>
     server(
@@ -119,7 +120,7 @@ class server {
     server(
         AsioEndpoint&& ep,
         as::io_context& ioc_accept,
-        std::function<as::io_context&()> ioc_con_getter,
+        move_only_function<as::io_context&()> ioc_con_getter,
         AcceptorConfig&& config = [](as::ip::tcp::acceptor&) {})
         : ep_(std::forward<AsioEndpoint>(ep)),
           ioc_accept_(ioc_accept),
@@ -173,7 +174,7 @@ class server {
     void set_error_handler(error_handler h) {
         h_error_ =
             [h = force_move(h)]
-            (error_code ec, as::io_context&) {
+            (error_code ec, as::io_context&) mutable {
                 if (h) h(ec);
             };
     }
@@ -230,9 +231,9 @@ class server {
     as::ip::tcp::endpoint ep_;
     as::io_context& ioc_accept_;
     as::io_context* ioc_con_ = nullptr;
-    std::function<as::io_context&()> ioc_con_getter_;
+    move_only_function<as::io_context&()> ioc_con_getter_;
     optional<as::ip::tcp::acceptor> acceptor_;
-    std::function<void(as::ip::tcp::acceptor&)> config_;
+    move_only_function<void(as::ip::tcp::acceptor&)> config_;
     bool close_request_{false};
     accept_handler h_accept_;
     connection_error_handler h_connection_error_;
@@ -258,7 +259,7 @@ class server_tls {
      *        After this handler called, the next accept will automatically start.
      * @param ep endpoint of the connecting client
      */
-    using accept_handler = std::function<void(std::shared_ptr<endpoint_t> ep)>;
+    using accept_handler = move_only_function<void(std::shared_ptr<endpoint_t> ep)>;
 
     /**
      * @brief Error handler during after accepted before connection established
@@ -266,7 +267,7 @@ class server_tls {
      * @param ec error code
      * @param ioc_con io_context for incoming connection
      */
-    using connection_error_handler = std::function<void(error_code ec, as::io_context& ioc_con)>;
+    using connection_error_handler = move_only_function<void(error_code ec, as::io_context& ioc_con)>;
 
     /**
      * @brief Error handler for listen and accpet
@@ -274,7 +275,7 @@ class server_tls {
      *        You need to call listen() again if you want to restart accepting.
      * @param ec error code
      */
-    using error_handler = std::function<void(error_code ec)>;
+    using error_handler = move_only_function<void(error_code ec)>;
 
     /**
      * @brief Error handler for listen and accpet
@@ -283,7 +284,7 @@ class server_tls {
      * @param ec error code
      * @param ioc_con io_context for listen or accept
      */
-    using error_handler_with_ioc = std::function<void(error_code ec, as::io_context& ioc_accept)>;
+    using error_handler_with_ioc = move_only_function<void(error_code ec, as::io_context& ioc_accept)>;
 
     template <typename AsioEndpoint, typename AcceptorConfig>
     server_tls(
@@ -330,7 +331,7 @@ class server_tls {
         AsioEndpoint&& ep,
         tls::context&& ctx,
         as::io_context& ioc_accept,
-        std::function<as::io_context&()> ioc_con_getter,
+        move_only_function<as::io_context&()> ioc_con_getter,
         AcceptorConfig&& config = [](as::ip::tcp::acceptor&) {})
         : ep_(std::forward<AsioEndpoint>(ep)),
           ioc_accept_(ioc_accept),
@@ -385,7 +386,7 @@ class server_tls {
     void set_error_handler(error_handler h) {
         h_error_ =
             [h = force_move(h)]
-            (error_code ec, as::io_context&) {
+            (error_code ec, as::io_context&) mutable {
                 if (h) h(ec);
             };
     }
@@ -445,10 +446,10 @@ class server_tls {
         return ctx_;
     }
 
-    using verify_cb_t = std::function<bool (bool, boost::asio::ssl::verify_context&, std::shared_ptr<optional<std::string>> const&) >;
+    using verify_cb_t = move_only_function<bool (bool, boost::asio::ssl::verify_context&, std::shared_ptr<optional<std::string>> const&) >;
 
     void set_verify_callback(verify_cb_t verify_cb) {
-        verify_cb_with_username_ = verify_cb;
+        verify_cb_with_username_ = force_move(verify_cb);
     }
 
 private:
@@ -551,9 +552,9 @@ class server_tls {
     as::ip::tcp::endpoint ep_;
     as::io_context& ioc_accept_;
     as::io_context* ioc_con_ = nullptr;
-    std::function<as::io_context&()> ioc_con_getter_;
+    move_only_function<as::io_context&()> ioc_con_getter_;
     optional<as::ip::tcp::acceptor> acceptor_;
-    std::function<void(as::ip::tcp::acceptor&)> config_;
+    move_only_function<void(as::ip::tcp::acceptor&)> config_;
     bool close_request_{false};
     accept_handler h_accept_;
     connection_error_handler h_connection_error_;
@@ -582,7 +583,7 @@ class server_ws {
      * @brief Accept handler
      * @param ep endpoint of the connecting client
      */
-    using accept_handler = std::function<void(std::shared_ptr<endpoint_t> ep)>;
+    using accept_handler = move_only_function<void(std::shared_ptr<endpoint_t> ep)>;
 
     /**
      * @brief Error handler during after accepted before connection established
@@ -590,7 +591,7 @@ class server_ws {
      * @param ec error code
      * @param ioc_con io_context for incoming connection
      */
-    using connection_error_handler = std::function<void(error_code ec, as::io_context& ioc_con)>;
+    using connection_error_handler = move_only_function<void(error_code ec, as::io_context& ioc_con)>;
 
     /**
      * @brief Error handler for listen and accpet
@@ -598,7 +599,7 @@ class server_ws {
      *        You need to call listen() again if you want to restart accepting.
      * @param ec error code
      */
-    using error_handler = std::function<void(error_code ec)>;
+    using error_handler = move_only_function<void(error_code ec)>;
 
     /**
      * @brief Error handler for listen and accpet
@@ -607,7 +608,7 @@ class server_ws {
      * @param ec error code
      * @param ioc_con io_context for listen or accept
      */
-    using error_handler_with_ioc = std::function<void(error_code ec, as::io_context& ioc_accept)>;
+    using error_handler_with_ioc = move_only_function<void(error_code ec, as::io_context& ioc_accept)>;
 
     template <typename AsioEndpoint, typename AcceptorConfig>
     server_ws(
@@ -648,7 +649,7 @@ class server_ws {
     server_ws(
         AsioEndpoint&& ep,
         as::io_context& ioc_accept,
-        std::function<as::io_context&()> ioc_con_getter,
+        move_only_function<as::io_context&()> ioc_con_getter,
         AcceptorConfig&& config = [](as::ip::tcp::acceptor&) {})
         : ep_(std::forward<AsioEndpoint>(ep)),
           ioc_accept_(ioc_accept),
@@ -701,7 +702,7 @@ class server_ws {
     void set_error_handler(error_handler h) {
         h_error_ =
             [h = force_move(h)]
-            (error_code ec, as::io_context&) {
+            (error_code ec, as::io_context&) mutable {
                 if (h) h(ec);
             };
     }
@@ -917,9 +918,9 @@ class server_ws {
     as::ip::tcp::endpoint ep_;
     as::io_context& ioc_accept_;
     as::io_context* ioc_con_ = nullptr;
-    std::function<as::io_context&()> ioc_con_getter_;
+    move_only_function<as::io_context&()> ioc_con_getter_;
     optional<as::ip::tcp::acceptor> acceptor_;
-    std::function<void(as::ip::tcp::acceptor&)> config_;
+    move_only_function<void(as::ip::tcp::acceptor&)> config_;
     bool close_request_{false};
     accept_handler h_accept_;
     connection_error_handler h_connection_error_;
@@ -946,7 +947,7 @@ class server_tls_ws {
      * @brief Accept handler
      * @param ep endpoint of the connecting client
      */
-    using accept_handler = std::function<void(std::shared_ptr<endpoint_t> ep)>;
+    using accept_handler = move_only_function<void(std::shared_ptr<endpoint_t> ep)>;
 
     /**
      * @brief Error handler during after accepted before connection established
@@ -954,7 +955,7 @@ class server_tls_ws {
      * @param ec error code
      * @param ioc_con io_context for incoming connection
      */
-    using connection_error_handler = std::function<void(error_code ec, as::io_context& ioc_con)>;
+    using connection_error_handler = move_only_function<void(error_code ec, as::io_context& ioc_con)>;
 
     /**
      * @brief Error handler for listen and accpet
@@ -962,7 +963,7 @@ class server_tls_ws {
      *        You need to call listen() again if you want to restart accepting.
      * @param ec error code
      */
-    using error_handler = std::function<void(error_code ec)>;
+    using error_handler = move_only_function<void(error_code ec)>;
 
     /**
      * @brief Error handler for listen and accpet
@@ -971,7 +972,7 @@ class server_tls_ws {
      * @param ec error code
      * @param ioc_con io_context for listen or accept
      */
-    using error_handler_with_ioc = std::function<void(error_code ec, as::io_context& ioc_accept)>;
+    using error_handler_with_ioc = move_only_function<void(error_code ec, as::io_context& ioc_accept)>;
 
     template <typename AsioEndpoint, typename AcceptorConfig>
     server_tls_ws(
@@ -1018,7 +1019,7 @@ class server_tls_ws {
         AsioEndpoint&& ep,
         tls::context&& ctx,
         as::io_context& ioc_accept,
-        std::function<as::io_context&()> ioc_con_getter,
+        move_only_function<as::io_context&()> ioc_con_getter,
         AcceptorConfig&& config = [](as::ip::tcp::acceptor&) {})
         : ep_(std::forward<AsioEndpoint>(ep)),
           ioc_accept_(ioc_accept),
@@ -1073,7 +1074,7 @@ class server_tls_ws {
     void set_error_handler(error_handler h) {
         h_error_ =
             [h = force_move(h)]
-            (error_code ec, as::io_context&) {
+            (error_code ec, as::io_context&) mutable {
                 if (h) h(ec);
             };
     }
@@ -1133,10 +1134,10 @@ class server_tls_ws {
         return ctx_;
     }
 
-    using verify_cb_t = std::function<bool (bool, boost::asio::ssl::verify_context&, std::shared_ptr<optional<std::string>> const&) >;
+    using verify_cb_t = move_only_function<bool (bool, boost::asio::ssl::verify_context&, std::shared_ptr<optional<std::string>> const&) >;
 
     void set_verify_callback(verify_cb_t verify_cb) {
-        verify_cb_with_username_ = verify_cb;
+        verify_cb_with_username_ = force_move(verify_cb);
     }
 
 private:
@@ -1361,9 +1362,9 @@ class server_tls_ws {
     as::ip::tcp::endpoint ep_;
     as::io_context& ioc_accept_;
     as::io_context* ioc_con_ = nullptr;
-    std::function<as::io_context&()> ioc_con_getter_;
+    move_only_function<as::io_context&()> ioc_con_getter_;
     optional<as::ip::tcp::acceptor> acceptor_;
-    std::function<void(as::ip::tcp::acceptor&)> config_;
+    move_only_function<void(as::ip::tcp::acceptor&)> config_;
     bool close_request_{false};
     accept_handler h_accept_;
     connection_error_handler h_connection_error_;
diff --git a/include/mqtt/store.hpp b/include/mqtt/store.hpp
index 717ad4f4..3fa0cdcc 100644
--- a/include/mqtt/store.hpp
+++ b/include/mqtt/store.hpp
@@ -18,6 +18,7 @@
 #include <mqtt/any.hpp>
 #include <mqtt/message_variant.hpp>
 #include <mqtt/packet_id_type.hpp>
+#include <mqtt/move_only_function.hpp>
 
 namespace MQTT_NS {
 
@@ -84,10 +85,10 @@ class store {
     }
 
     void for_each(
-        std::function<
+        move_only_function<
             // if return true, then erase element
             bool(basic_store_message_variant<PacketIdBytes> const&, any const&)
-        > const& f
+        > f
     ) {
         auto& idx = elems_.template get<tag_seq>();
         auto it = idx.begin();
diff --git a/include/mqtt/strand.hpp b/include/mqtt/strand.hpp
index fa83f45d..0b0e29e7 100644
--- a/include/mqtt/strand.hpp
+++ b/include/mqtt/strand.hpp
@@ -18,6 +18,6 @@ namespace as = boost::asio;
 
 using strand = as::strand<as::io_context::executor_type>;
 
-}
+} // namespace MQTT_NS
 
 #endif // MQTT_STRAND_HPP
diff --git a/include/mqtt/tcp_endpoint.hpp b/include/mqtt/tcp_endpoint.hpp
index 295ea310..295294d7 100644
--- a/include/mqtt/tcp_endpoint.hpp
+++ b/include/mqtt/tcp_endpoint.hpp
@@ -15,6 +15,7 @@
 #include <mqtt/move.hpp>
 #include <mqtt/attributes.hpp>
 #include <mqtt/tls.hpp>
+#include <mqtt/move_only_function.hpp>
 #include <mqtt/log.hpp>
 
 namespace MQTT_NS {
@@ -32,7 +33,7 @@ class tcp_endpoint : public socket {
 
     MQTT_ALWAYS_INLINE void async_read(
         as::mutable_buffer buffers,
-        std::function<void(error_code, std::size_t)> handler
+        move_only_function<void(error_code, std::size_t)> handler
     ) override final {
         as::async_read(
             tcp_,
@@ -46,7 +47,7 @@ class tcp_endpoint : public socket {
 
     MQTT_ALWAYS_INLINE void async_write(
         std::vector<as::const_buffer> buffers,
-        std::function<void(error_code, std::size_t)> handler
+        move_only_function<void(error_code, std::size_t)> handler
     ) override final {
         as::async_write(
             tcp_,
@@ -65,21 +66,21 @@ class tcp_endpoint : public socket {
         return as::write(tcp_,force_move(buffers), ec);
     }
 
-    MQTT_ALWAYS_INLINE void post(std::function<void()> handler) override final {
+    MQTT_ALWAYS_INLINE void post(move_only_function<void()> handler) override final {
         as::post(
             strand_,
             force_move(handler)
         );
     }
 
-    MQTT_ALWAYS_INLINE void dispatch(std::function<void()> handler) override final {
+    MQTT_ALWAYS_INLINE void dispatch(move_only_function<void()> handler) override final {
         as::dispatch(
             strand_,
             force_move(handler)
         );
     }
 
-    MQTT_ALWAYS_INLINE void defer(std::function<void()> handler) override final {
+    MQTT_ALWAYS_INLINE void defer(move_only_function<void()> handler) override final {
         as::defer(
             strand_,
             force_move(handler)
@@ -102,7 +103,7 @@ class tcp_endpoint : public socket {
         shutdown_and_close_impl(tcp_, ec);
     }
 
-    MQTT_ALWAYS_INLINE void async_clean_shutdown_and_close(std::function<void(error_code)> handler) override final {
+    MQTT_ALWAYS_INLINE void async_clean_shutdown_and_close(move_only_function<void(error_code)> handler) override final {
         async_shutdown_and_close_impl(tcp_, force_move(handler));
     }
 
@@ -156,7 +157,7 @@ class tcp_endpoint : public socket {
             << ec.message();
     }
 
-    void async_shutdown_and_close_impl(as::basic_socket<boost::asio::ip::tcp>& s, std::function<void(error_code)> handler) {
+    void async_shutdown_and_close_impl(as::basic_socket<boost::asio::ip::tcp>& s, move_only_function<void(error_code)> handler) {
         post(
             [this, &s, handler = force_move(handler)] () mutable {
                 error_code ec;
@@ -175,7 +176,7 @@ class tcp_endpoint : public socket {
             << ec.message();
         shutdown_and_close_impl(lowest_layer(), ec);
     }
-    void async_shutdown_and_close_impl(tls::stream<as::ip::tcp::socket>& s, std::function<void(error_code)> handler) {
+    void async_shutdown_and_close_impl(tls::stream<as::ip::tcp::socket>& s, move_only_function<void(error_code)> handler) {
         s.async_shutdown(
             as::bind_executor(
                 strand_,
diff --git a/include/mqtt/type_erased_socket.hpp b/include/mqtt/type_erased_socket.hpp
index dffd2922..4041311d 100644
--- a/include/mqtt/type_erased_socket.hpp
+++ b/include/mqtt/type_erased_socket.hpp
@@ -14,6 +14,7 @@
 #include <mqtt/namespace.hpp>
 #include <mqtt/error_code.hpp>
 #include <mqtt/any.hpp>
+#include <mqtt/move_only_function.hpp>
 
 namespace MQTT_NS {
 
@@ -22,17 +23,17 @@ namespace as = boost::asio;
 class socket {
 public:
     virtual ~socket() = default;
-    virtual void async_read(as::mutable_buffer, std::function<void(error_code, std::size_t)>) = 0;
-    virtual void async_write(std::vector<as::const_buffer>, std::function<void(error_code, std::size_t)>) = 0;
+    virtual void async_read(as::mutable_buffer, move_only_function<void(error_code, std::size_t)>) = 0;
+    virtual void async_write(std::vector<as::const_buffer>, move_only_function<void(error_code, std::size_t)>) = 0;
     virtual std::size_t write(std::vector<as::const_buffer>, boost::system::error_code&) = 0;
-    virtual void post(std::function<void()>) = 0;
-    virtual void dispatch(std::function<void()>) = 0;
-    virtual void defer(std::function<void()>) = 0;
+    virtual void post(move_only_function<void()>) = 0;
+    virtual void dispatch(move_only_function<void()>) = 0;
+    virtual void defer(move_only_function<void()>) = 0;
     virtual bool running_in_this_thread() const = 0;
     virtual as::ip::tcp::socket::lowest_layer_type& lowest_layer() = 0;
     virtual any native_handle() = 0;
     virtual void clean_shutdown_and_close(boost::system::error_code&) = 0;
-    virtual void async_clean_shutdown_and_close(std::function<void(error_code)>) = 0;
+    virtual void async_clean_shutdown_and_close(move_only_function<void(error_code)>) = 0;
     virtual void force_shutdown_and_close(boost::system::error_code&) = 0;
     virtual as::any_io_executor get_executor() = 0;
 };
diff --git a/include/mqtt/ws_endpoint.hpp b/include/mqtt/ws_endpoint.hpp
index 7b5025de..9d784ea8 100644
--- a/include/mqtt/ws_endpoint.hpp
+++ b/include/mqtt/ws_endpoint.hpp
@@ -18,6 +18,7 @@
 #include <mqtt/string_view.hpp>
 #include <mqtt/error_code.hpp>
 #include <mqtt/tls.hpp>
+#include <mqtt/move_only_function.hpp>
 #include <mqtt/log.hpp>
 
 namespace MQTT_NS {
@@ -44,12 +45,12 @@ class ws_endpoint : public socket {
 
     MQTT_ALWAYS_INLINE void async_read(
         as::mutable_buffer buffers,
-        std::function<void(error_code, std::size_t)> handler
+        move_only_function<void(error_code, std::size_t)> handler
     ) override final {
         auto req_size = as::buffer_size(buffers);
 
         using beast_read_handler_t =
-            std::function<void(error_code ec, std::shared_ptr<void>)>;
+            move_only_function<void(error_code ec, std::shared_ptr<void>)>;
 
         std::shared_ptr<beast_read_handler_t> beast_read_handler;
         if (req_size <= buffer_.size()) {
@@ -107,7 +108,7 @@ class ws_endpoint : public socket {
 
     MQTT_ALWAYS_INLINE void async_write(
         std::vector<as::const_buffer> buffers,
-        std::function<void(error_code, std::size_t)> handler
+        move_only_function<void(error_code, std::size_t)> handler
     ) override final {
         ws_.async_write(
             buffers,
@@ -126,21 +127,21 @@ class ws_endpoint : public socket {
         return as::buffer_size(buffers);
     }
 
-    MQTT_ALWAYS_INLINE void post(std::function<void()> handler) override final {
+    MQTT_ALWAYS_INLINE void post(move_only_function<void()> handler) override final {
         as::post(
             strand_,
             force_move(handler)
         );
     }
 
-    MQTT_ALWAYS_INLINE void dispatch(std::function<void()> handler) override final {
+    MQTT_ALWAYS_INLINE void dispatch(move_only_function<void()> handler) override final {
         as::dispatch(
             strand_,
             force_move(handler)
         );
     }
 
-    MQTT_ALWAYS_INLINE void defer(std::function<void()> handler) override final {
+    MQTT_ALWAYS_INLINE void defer(move_only_function<void()> handler) override final {
         as::defer(
             strand_,
             force_move(handler)
@@ -187,7 +188,7 @@ class ws_endpoint : public socket {
         shutdown_and_close_impl(next_layer(), ec);
     }
 
-    MQTT_ALWAYS_INLINE void async_clean_shutdown_and_close(std::function<void(error_code)> handler) override final {
+    MQTT_ALWAYS_INLINE void async_clean_shutdown_and_close(move_only_function<void(error_code)> handler) override final {
         if (ws_.is_open()) {
             // WebSocket closing process
             MQTT_LOG("mqtt_impl", trace)
@@ -272,7 +273,7 @@ class ws_endpoint : public socket {
     }
 
 private:
-    void async_read_until_closed(std::function<void(error_code)> handler) {
+    void async_read_until_closed(move_only_function<void(error_code)> handler) {
         auto buffer = std::make_shared<boost::beast::flat_buffer>();
         ws_.async_read(
             *buffer,
@@ -311,7 +312,7 @@ class ws_endpoint : public socket {
             << ec.message();
     }
 
-    void async_shutdown_and_close_impl(as::basic_socket<boost::asio::ip::tcp>& s, std::function<void(error_code)> handler) {
+    void async_shutdown_and_close_impl(as::basic_socket<boost::asio::ip::tcp>& s, move_only_function<void(error_code)> handler) {
         post(
             [this, &s, handler = force_move(handler)] () mutable {
                 error_code ec;
@@ -330,7 +331,7 @@ class ws_endpoint : public socket {
             << ec.message();
         shutdown_and_close_impl(lowest_layer(), ec);
     }
-    void async_shutdown_and_close_impl(tls::stream<as::ip::tcp::socket>& s, std::function<void(error_code)> handler) {
+    void async_shutdown_and_close_impl(tls::stream<as::ip::tcp::socket>& s, move_only_function<void(error_code)> handler) {
         s.async_shutdown(
             as::bind_executor(
                 strand_,
diff --git a/test/system/st_pubsub_no_strand.cpp b/test/system/st_pubsub_no_strand.cpp
index d9795f83..500f68f4 100644
--- a/test/system/st_pubsub_no_strand.cpp
+++ b/test/system/st_pubsub_no_strand.cpp
@@ -78,13 +78,11 @@ BOOST_AUTO_TEST_CASE( pub_qos0_sub_qos0 ) {
         () {
             MQTT_CHK("h_close");
             finish();
-            return true;
         });
     c->set_error_handler(
         []
         (MQTT_NS::error_code) {
             BOOST_CHECK(false);
-            return true;
         });
     c->set_puback_handler(
         []
diff --git a/update_external.sh b/update_external.sh
new file mode 100755
index 00000000..0cf05898
--- /dev/null
+++ b/update_external.sh
@@ -0,0 +1,4 @@
+#!/bin/sh
+
+git submodule update --init
+cp external/function2/include/function2/function2.hpp include/mqtt/external/