3.3.6. Creating a DataWriter¶

A DataWriter always belongs to a Publisher. Creation of a DataWriter is done with the create_datawriter() member function on the Publisher instance, that acts as a factory for the DataWriter.

Mandatory arguments are:

A Topic bound to the data type that will be transmitted.

The DataWriterQos describing the behavior of the DataWriter. If the provided value is DATAWRITER_QOS_DEFAULT, the value of the Default DataWriterQos is used.

Optional arguments are:

A Listener derived from DataWriterListener, implementing the callbacks that will be triggered in response to events and state changes on the DataWriter. By default empty callbacks are used.

A StatusMask that activates or deactivates triggering of individual callbacks on the DataWriterListener. By default all events are enabled.

create_datawriter() will return a null pointer if there was an error during the operation, e.g. if the provided QoS is not compatible or is not supported. It is advisable to check that the returned value is a valid pointer.

// Create a DataWriter with default DataWriterQos and no Listener
// The value DATAWRITER_QOS_DEFAULT is used to denote the default QoS.
DataWriter* data_writer_with_default_qos =
        publisher->create_datawriter(topic, DATAWRITER_QOS_DEFAULT);
if (nullptr == data_writer_with_default_qos)
{
    // Error
    return;
}

// A custom DataWriterQos can be provided to the creation method
DataWriterQos custom_qos;

// Modify QoS attributes
// (...)

DataWriter* data_writer_with_custom_qos =
        publisher->create_datawriter(topic, custom_qos);
if (nullptr == data_writer_with_custom_qos)
{
    // Error
    return;
}

// Create a DataWriter with default QoS and a custom Listener.
// CustomDataWriterListener inherits from DataWriterListener.
// The value DATAWRITER_QOS_DEFAULT is used to denote the default QoS.
CustomDataWriterListener custom_listener;
DataWriter* data_writer_with_default_qos_and_custom_listener =
        publisher->create_datawriter(topic, DATAWRITER_QOS_DEFAULT, &custom_listener);
if (nullptr == data_writer_with_default_qos_and_custom_listener)
{
    // Error
    return;
}

3.3.6.1. Profile based creation of a DataWriter¶

Instead of using a DataWriterQos, the name of a profile can be used to create a DataWriter with the create_datawriter_with_profile() member function on the Publisher instance.

Mandatory arguments are:

A Topic bound to the data type that will be transmitted.

A string with the name that identifies the DataWriter.

Optional arguments are:

A Listener derived from DataWriterListener, implementing the callbacks that will be triggered in response to events and state changes on the DataWriter. By default empty callbacks are used.

A StatusMask that activates or deactivates triggering of individual callbacks on the DataWriterListener. By default all events are enabled.

create_datawriter_with_profile() will return a null pointer if there was an error during the operation, e.g. if the provided QoS is not compatible or is not supported. It is advisable to check that the returned value is a valid pointer.

Note

XML profiles must have been loaded previously. See Loading profiles from an XML file.

// First load the XML with the profiles
DomainParticipantFactory::get_instance()->load_XML_profiles_file("profiles.xml");

// Create a DataWriter using a profile and no Listener
DataWriter* data_writer_with_profile =
        publisher->create_datawriter_with_profile(topic, "data_writer_profile");
if (nullptr == data_writer_with_profile)
{
    // Error
    return;
}

// Create a DataWriter using a profile and a custom Listener.
// CustomDataWriterListener inherits from DataWriterListener.
CustomDataWriterListener custom_listener;
DataWriter* data_writer_with_profile_and_custom_listener =
        publisher->create_datawriter_with_profile(topic, "data_writer_profile", &custom_listener);
if (nullptr == data_writer_with_profile_and_custom_listener)
{
    // Error
    return;
}

3.3.6.2. Creating a DataWriter with a custom PayloadPool¶

A custom PayloadPool can be passed as an argument during the creation of a DataWriter. This allows for customizing the management of the information exchanged between DataWriters and DataReaders. The same configuration can be set in the opposite endpoint.

// A DataWriterQos must be provided to the creation method
DataWriterQos qos;

// Create PayloadPool
std::shared_ptr<eprosima::fastrtps::rtps::IPayloadPool> payload_pool =
        std::dynamic_pointer_cast<eprosima::fastrtps::rtps::IPayloadPool>(std::make_shared<CustomPayloadPool>());

DataWriter* data_writer = publisher->create_datawriter(topic, qos, nullptr, StatusMask::all(), payload_pool);
if (nullptr == data_writer)
{
    // Error
    return;
}

This configuration can be performed also in the RTPS layer. The customization example applies both layers.

3.3.6.3. Deleting a DataWriter¶

A DataWriter can be deleted with the delete_datawriter() member function on the Publisher instance where the DataWriter was created.

// Create a DataWriter
DataWriter* data_writer =
        publisher->create_datawriter(topic, DATAWRITER_QOS_DEFAULT);
if (nullptr == data_writer)
{
    // Error
    return;
}

// Use the DataWriter to communicate
// (...)

// Delete the DataWriter
if (publisher->delete_datawriter(data_writer) != ReturnCode_t::RETCODE_OK)
{
    // Error
    return;
}