packet_variant (including system_error) as recv() 's CompletionToken parameter

[redboltz]

async_mqtt/include/async_mqtt/endpoint.hpp

Lines 367 to 377 in 8d3ef84

	/**
	* @brief receive packet
	* users CANNOT call recv() before the previous recv()'s CompletionToken is invoked
	* @param token the param is packet_variant_type
	* @return deduced by token
	*/
	template <typename CompletionToken>
	typename as::async_result<std::decay_t<CompletionToken>, void(packet_variant_type)>::return_type
	recv(
	CompletionToken&& token
	) {

endpoint::recv is the most commonly used API. Its parameter is packet_variant.
The entry types are

async_mqtt/include/async_mqtt/packet/packet_variant.hpp

Lines 184 to 215 in 8d3ef84

	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
	>;

It contains all packet types and system_error.
I don't want to two parameters as system_error and packet_variant (except system_error), because the parameter packet_variant is enabled only system_error is succees. Such parameter's dependency is not elegant.
optional cannot explain the kind of errors.
I think that perhaps Boost.Outcome is appropriate choice. Now, I am not familiar to Boost.Outcome so I use variant. But I can replace if it is needed to propose async_mqtt to the Boost.Libraries.

[redboltz]

Conclusion

Use void(error_code, packet_variant) Completion handler.
It works well with asio.
If error_code contains error, packet_variant has std::monostate.