mqtt5/doc/qbk/reference/properties/publish_props.qbk

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    Copyright (c) 2023 Mireo
   
    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)
]

[section:publish_props PUBLISH properties]
The last field in the Variable header of __PUBLISH__ packet is a set of Properties.
A set contains a Property Length followed by the Properties.
A Property consists of an Identifier and a value.

This section lists all possible __PUBLISH__ Properties and describes their usage:

[table:publish_props PUBLISH properties
	[[Identifier] [Value type] [Description]]
	[[payload_format_indicator] [`uint8_t`] [Value of 0 indicates that the Payload is in unspecified bytes. Value of 1 indicates that the Payload is UTF-8 Encoded Character Data.]]
	[[message_expiry_interval] [`int32_t`] [The lifetime of the Application Message in seconds.]]
	[[topic_alias] [`int16_t`] [Two Byte integer representing the Topic Alias, an integer value that is used to identify the Topic instead of using the Topic Name.]]
	[[response_topic] [`std::string`] [A UTF-8 Encoded String which is used as the Topic Name for a response message.]]
	[[correlation_data] [`std::string`] [Binary Data used by the sender of the Request Message to identify which request the Response Message is for when it is received.]]
	[[user_property] [`std::vector<std::string>`] [A list of name, value pairs (__UTF8_STRING_PAIR__) defining User Properties. 
		The meaning of these properties is not defined by the specification.]]
	[[subscription_identifier] [`uint32_t`] [Identifier of the Subscription in range of 1 to 268,435,455.]]
	[[content_type] [`std::string`] [A UTF-8 Encoded String describing the content of the Application Message.]]
]

[h4 Usage]
After obtaining an instance of `async_mqtt5::publish_props`, the subscript operator can be used to access a Property.

The Identifiers listed in the table above are available within the `async_mqtt5::prop` namespace for Property access.

[h4 Example]

[!c++]
	async_mqtt5::publish_props props;
	props[async_mqtt5::prop::payload_format_indicator] = uint8_t(1);
	props[async_mqtt5::prop::topic_alias] = uint16_t(12);
	props[async_mqtt5::prop::response_topic] = "response_topic";

[endsect]