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 ty...-DPDK中文网

The provided text describes the process of creating a DataWriter in a typical publish-subscribe system, often associated with middleware like DDS (Data Distribution Service). Here’s a breakdown of the key points and concepts mentioned:

Key Concepts

DataWriter and Publisher Relationship:
- A DataWriter is an entity that publishes data. It is always associated with a Publisher, which serves as its parent or factory.
Creating a DataWriter:
- The method to create a DataWriter is called create_datawriter() and it is invoked on an instance of the Publisher.
Mandatory Arguments:
- Topic: This argument specifies the type of data that will be transmitted. It must be bound to a specific data type.
- DataWriterQos: This Quality of Service (QoS) setting controls the behavior of the DataWriter. The QoS can determine aspects such as reliability, durability, lifespan, etc.
  - If you pass DATAWRITER_QOS_DEFAULT, it uses default settings for the DataWriter.
  - If you use DATAWRITER_QOS_USE_TOPIC_QOS, it combines defaults with any specific settings from the topic’s QoS.
Optional Arguments:
- Listener: An optional parameter derived from DataWriterListener. Implementing this allows for handling callbacks triggered by various events related to the DataWriter (e.g., deadline missed, publication matched).
  - By default, empty callback functions are used if no listener is provided.
- StatusMask: This parameter allows selective activation or deactivation of specific event notifications on the listener. All events are enabled by default.
Return Value and Error Handling:
- The function returns a pointer to the newly created DataWriter. If there was an error during creation (such as incompatible QoS), it will return a null pointer.
- It’s essential to check whether this pointer is valid before using it to avoid dereferencing null pointers, which can lead to runtime errors.

Example Usage

Here’s an example usage based on your description:

// Assuming necessary includes and namespace usage

// Create Topic
Topic* my_topic = publisher->create_topic("MyTopic", MyType::get_type_name(), TOPIC_QOS_DEFAULT);

// Create Data Writer Qos
DataWriterQos qos;
publisher->get_default_datawriter_qos(qos);
qos.reliability.kind = RELIABLE_RELIABILITY_QOS; // Customize QoS as needed

// Create Data Writer
DataWriter* my_data_writer = publisher->create_datawriter(
    my_topic,
    qos,
    nullptr, // No listener provided
    STATUS_MASK_ALL // Enable all status notifications
);

// Check for errors
if (my_data_writer == nullptr) {
    // Handle error case (e.g., log or throw exception)
}

Summary

This explanation outlines how to effectively create and manage a DataWriter, ensuring proper configuration through mandatory and optional parameters while also emphasizing error handling practices in systems utilizing publish-subscribe models like DDS.

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