Will properties

The Will Properties consist of the properties that determine when to publish the Will Message and the Application Message properties to be sent with the Will Message. The Will Properties consists of a Property Length and the Properties. A Property consists of an Identifier and a value.

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

Table 1.4. Will properties

Identifier	Value type	Description
will_delay_interval	`uint32_t`	The delay in seconds that need to pass before Server publishes the Client's Will Message.
payload_format_indicator	`uint8_t`	Value of 0 indicates that the Will Message is in unspecified bytes. Value of 1 indicates that the Will Message is UTF-8 Encoded Character Data.
message_expiry_interval	`uint32_t`	The lifetime of the Will Message in seconds. It is send as Publication Expiry Interval when it is published.
content_type	`std::string`	A UTF-8 Encoded String describing the content of the Will Message.
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::pair<std::string, std::string>`	Name, value pair (`UTF-8 String Pair`) defining User Property. There can be multiple pairs in one packet. The meaning of these properties is not defined by the specification.

Usage

After creating an instance of async_mqtt5::will, 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.

	Note
	When accessing a property value, the subscript operator will return a `std::optional` of the value type for all properties, except for `async_mqtt5::prop::user_property`, where it will return an instance of `std::vector<std::pair<std::string, std::string>>`.

Example

The following example shows how to set a Property value:

async_mqtt5::will will;
will[async_mqtt5::prop::message_expiry_interval] = 90;
will[async_mqtt5::prop::content_type] = "Notification";
props[async_mqtt5::prop::user_property].emplace_back("name", "value");

The following example shows how to retrieve a Property value:

std::optional<std::string> c_type = will[async_mqtt5::prop::content_type];
if (c_type.has_value())
	// content type property was previously set
else
	// content type property was not set

std::vector<std::pair<std::string, std::string>>& user_props = props[async_mqtt5::prop::user_property];
if (!user_props.empty())
	// user property was previously set
else
	// user property was not set