Knowing if the connection is open

[redboltz]

Knowing if the connection is open

[redboltz]

You can know MQTT level status as follows:

First, call recv(), then you can get packet_variant as the CompletionToken of the recv().

async_mqtt/include/async_mqtt/packet/packet_variant.hpp

Lines 184 to 215 in 9a9160c

	using variant_t = variant<
	system_error,
	v3_1_1::connect_packet,
	v3_1_1::connack_packet,
	v3_1_1::basic_publish_packet<PacketIdBytes>,
	v3_1_1::basic_puback_packet<PacketIdBytes>,
	v3_1_1::basic_pubrec_packet<PacketIdBytes>,
	v3_1_1::basic_pubrel_packet<PacketIdBytes>,
	v3_1_1::basic_pubcomp_packet<PacketIdBytes>,
	v3_1_1::basic_subscribe_packet<PacketIdBytes>,
	v3_1_1::basic_suback_packet<PacketIdBytes>,
	v3_1_1::basic_unsubscribe_packet<PacketIdBytes>,
	v3_1_1::basic_unsuback_packet<PacketIdBytes>,
	v3_1_1::pingreq_packet,
	v3_1_1::pingresp_packet,
	v3_1_1::disconnect_packet,
	v5::connect_packet,
	v5::connack_packet,
	v5::basic_publish_packet<PacketIdBytes>,
	v5::basic_puback_packet<PacketIdBytes>,
	v5::basic_pubrec_packet<PacketIdBytes>,
	v5::basic_pubrel_packet<PacketIdBytes>,
	v5::basic_pubcomp_packet<PacketIdBytes>,
	v5::basic_subscribe_packet<PacketIdBytes>,
	v5::basic_suback_packet<PacketIdBytes>,
	v5::basic_unsubscribe_packet<PacketIdBytes>,
	v5::basic_unsuback_packet<PacketIdBytes>,
	v5::pingreq_packet,
	v5::pingresp_packet,
	v5::disconnect_packet,
	v5::auth_packet
	>;

The index 0 of the packet_variant is system_error.
If connection is lost, then the system_error is set.

For convinience, packet_variant has operator bool().

async_mqtt/include/async_mqtt/packet/packet_variant.hpp

Lines 179 to 181 in 9a9160c

	operator bool() {
	return var_.index() != 0;
	}

If you want to know the reason of connection lost, you can get system_error from the packet_variant, then call .what() function.

[magni-mar]

My problem is that I want to check weather the connection is open while I am actively receiving in another place in the program. This causes me to not be able to initiate another receive.

[redboltz]

You can store the result of recv() in your application as a flag.

I guess that you would expect something like bool endpoint::connected(). But there is no such function because it is not a good idea.

async_mqtt doesn't own io_context. Just refer it.
So how to call io_context::run() is client's responsibility. It could call from different thread or multiple threads.

In this case the return valur of connected() is unreliable.

if (ep.connected()) {
    // could have already been disconnected here
}

When the status flag is reliable ? Only application developper knows it.
So it should be implemented by application side, not in the library.

[magni-mar]

I understand, thanks for the response.