3.4.8. Accessing received data

The application can access and consume the data values received on the DataReader by reading or taking.

When there is no data in the DataReader matching the required conditions, all the operations will return NO_DATA and output parameter will remain unchanged.

In addition to the data values, the data access operations also provide SampleInfo instances with additional information that help interpreting the returned data values, like the originating DataWriter or the publication time stamp. Please, refer to the SampleInfo section for an extensive description of its contents.

3.4.8.1. Loaning and Returning Data and SampleInfo Sequences

The DataReader::read() and DataReader::take() operations (and their variants) return information to the application in two sequences:

  • Received DDS data samples in a sequence of the data type

  • Corresponding information about each DDS sample in a SampleInfo sequence

These sequences are parameters that are passed by the application code into the DataReader::read() and DataReader::take() operations. When the passed sequences are empty (they are initialized but have a maximum length of 0), the middleware will fill those sequences with memory directly loaned from the receive queue itself. There is no copying of the data or SampleInfo when the contents of the sequences are loaned. This is certainly the most efficient way for the application code to retrieve the data.

When doing so, however, the code must return the loaned sequences back to the middleware, so that they can be reused by the receive queue. If the application does not return the loan by calling the DataReader::return_loan() operation, then Fast DDS will eventually run out of memory to store DDS data samples received from the network for that DataReader. See the code below for an example of borrowing and returning loaned sequences.

    // Sequences are automatically initialized to be empty (maximum == 0)
    FooSeq data_seq;
    SampleInfoSeq info_seq;

    // with empty sequences, a take() or read() will return loaned
    // sequence elements
    ReturnCode_t ret_code = data_reader->take(data_seq, info_seq,
                    LENGTH_UNLIMITED, ANY_SAMPLE_STATE,
                    ANY_VIEW_STATE, ANY_INSTANCE_STATE);

    // process the returned data

    // must return the loaned sequences when done processing
    data_reader->return_loan(data_seq, info_seq);

3.4.8.2. Processing returned data

After calling the DataReader::read() or DataReader::take() operations, accessing the data on the returned sequences is quite easy. The sequences API provides a length() operation returning the number of elements in the collections. The application code just needs to check this value and use the [] operator to access the corresponding elements. Elements on the DDS data sequence should only be accessed when the corresponding element on the SampleInfo sequence indicate that valid data is present.

    // Sequences are automatically initialized to be empty (maximum == 0)
    FooSeq data_seq;
    SampleInfoSeq info_seq;

    // with empty sequences, a take() or read() will return loaned
    // sequence elements
    ReturnCode_t ret_code = data_reader->take(data_seq, info_seq,
                    LENGTH_UNLIMITED, ANY_SAMPLE_STATE,
                    ANY_VIEW_STATE, ANY_INSTANCE_STATE);

    // process the returned data
    if (ret_code == ReturnCode_t::RETCODE_OK)
    {
        // Both info_seq.length() and data_seq.length() will have the number of samples returned
        for (FooSeq::size_type n = 0; n < info_seq.length(); ++n)
        {
            // Only samples for which valid_data is true should be accessed
            if (info_seq[n].valid_data)
            {
                // Do something with data_seq[n]
            }
        }

        // must return the loaned sequences when done processing
        data_reader->return_loan(data_seq, info_seq);
    }

3.4.8.3. Accessing data on callbacks

When the DataReader receives new data values from any matching DataWriter, it informs the application through two Listener callbacks:

These callbacks can be used to retrieve the newly arrived data, as in the following example.

class CustomizedDataReaderListener : public DataReaderListener
{

public:

    CustomizedDataReaderListener()
        : DataReaderListener()
    {
    }

    virtual ~CustomizedDataReaderListener()
    {
    }

    virtual void on_data_available(
            DataReader* reader)
    {
        // Create a data and SampleInfo instance
        Foo data;
        SampleInfo info;

        // Keep taking data until there is nothing to take
        while (reader->take_next_sample(&data, &info) == ReturnCode_t::RETCODE_OK)
        {
            if (info.valid_data)
            {
                // Do something with the data
                std::cout << "Received new data value for topic "
                          << reader->get_topicdescription()->get_name()
                          << std::endl;
            }
            else
            {
                std::cout << "Remote writer for topic "
                          << reader->get_topicdescription()->get_name()
                          << " is dead" << std::endl;
            }
        }
    }

};

Note

If several new data changes are received at once, the callbacks may be triggered just once, instead of once per change. The application must keep reading or taking until no new changes are available.

3.4.8.4. Accessing data with a waiting thread

Instead of relying on the Listener to try and get new data values, the application can also dedicate a thread to wait until any new data is available on the DataReader. This can be done with the wait_for_unread_message() member function, that blocks until a new data sample is available or the given timeout expires. If no new data was available after the timeout expired, it will return with value false. This function returning with value true means there is new data available on the DataReader ready for the application to retrieve.

// Create a DataReader
DataReader* data_reader =
        subscriber->create_datareader(topic, DATAREADER_QOS_DEFAULT);
if (nullptr == data_reader)
{
    // Error
    return;
}

// Create a data and SampleInfo instance
Foo data;
SampleInfo info;

//Define a timeout of 5 seconds
eprosima::fastrtps::Duration_t timeout (5, 0);

// Loop reading data as it arrives
// This will make the current threat to be dedicated exclusively to
// waiting and reading data until the remote DataWriter dies
while (true)
{
    if (data_reader->wait_for_unread_message(timeout))
    {
        if (data_reader->take_next_sample(&data, &info) == ReturnCode_t::RETCODE_OK)
        {
            if (info.valid_data)
            {
                // Do something with the data
                std::cout << "Received new data value for topic "
                          << topic->get_name()
                          << std::endl;
            }
            else
            {
                // If the remote writer is not alive, we exit the reading loop
                std::cout << "Remote writer for topic "
                          << topic->get_name()
                          << " is dead" << std::endl;
                break;
            }
        }
    }
    else
    {
        std::cout << "No data this time" << std::endl;
    }
}