Compromise with c++17 and c++20 (just in MSVC).

examples/echo_server/echo_server.cpp

@@ -67,7 +67,11 @@ using namespace ascs::ext;
 //under the default behavior, each tcp::socket has their own packer, and cause memory waste
 //at here, we make each echo_socket use the same global packer for memory saving
 //notice: do not do this for unpacker, because unpacker has member variables and can't share each other
+#if _MSVC_LANG > 201703L
+auto global_packer = std::make_shared<ASCS_DEFAULT_PACKER>();
+#else
 auto global_packer(std::make_shared<ASCS_DEFAULT_PACKER>());
+#endif
 
 //demonstrate how to control the type of tcp::server_socket_base::server from template parameter
 class i_echo_server : public i_server

include/ascs/object_pool.h

@@ -257,7 +257,11 @@ public:
 	{
 		std::lock_guard<std::mutex> lock(invalid_object_can_mutex);
 		auto iter = std::find_if(std::begin(invalid_object_can), std::end(invalid_object_can), [&](object_ctype& item) {return item->is_equal_to(id);});
-		if (iter != std::end(invalid_object_can) && (*iter).unique() && (*iter)->obsoleted())
+#if _MSVC_LANG >= 201703L
+		if (iter != std::end(invalid_object_can) && 1 == iter->use_count() && (*iter)->obsoleted())
+#else
+		if (iter != std::end(invalid_object_can) && iter->unique() && (*iter)->obsoleted())
+#endif
 		{
 			auto object_ptr(std::move(*iter));
 			invalid_object_can.erase(iter);
@@ -271,7 +275,11 @@ public:
 	{
 		std::lock_guard<std::mutex> lock(invalid_object_can_mutex);
 		for (auto iter = std::begin(invalid_object_can); iter != std::end(invalid_object_can); ++iter)
-			if ((*iter).unique() && (*iter)->obsoleted())
+#if _MSVC_LANG >= 201703L
+			if (iter->use_count() && (*iter)->obsoleted())
+#else
+			if (iter->unique() && (*iter)->obsoleted())
+#endif
 			{
 				auto object_ptr(std::move(*iter));
 				invalid_object_can.erase(iter);
@@ -332,7 +340,11 @@ public:
 			//	socket_ptr->set_timer(...);
 			//}
 			//then in the future, when invoke the timer handler, the socket has been freed and its this pointer already became wild.
-			if ((*iter).unique() && (*iter)->obsoleted())
+#if _MSVC_LANG >= 201703L
+			if (1 == iter->use_count() && (*iter)->obsoleted())
+#else
+			if (iter->unique() && (*iter)->obsoleted())
+#endif
 			{
 				--num;
 				++num_affected;

include/ascs/tracked_executor.h

@@ -69,7 +69,11 @@ public:
 		{auto ref_holder(aci); return [=](const boost::system::error_code& ec, size_t bytes_transferred) {(void) ref_holder; handler(ec, bytes_transferred);};}
 #endif
 
+#if _MSVC_LANG >= 201703L
+	bool is_async_calling() const {return aci.use_count() > 1;}
+#else
 	bool is_async_calling() const {return !aci.unique();}
+#endif
 	bool is_last_async_call() const {return aci.use_count() <= 2;} //can only be called in callbacks
 	inline void set_async_calling(bool) {}