# 15.1.3.1. DataWriter¶

class eprosima::fastdds::dds::DataWriter : public eprosima::fastdds::dds::DomainEntity

Class DataWriter, contains the actual implementation of the behaviour of the DataWriter.

Public Types

enum LoanInitializationKind

How to initialize samples loaned with loan_sample

Values:

enumerator NO_LOAN_INITIALIZATION

Do not perform initialization of sample.

This is the default initialization scheme of loaned samples. It is the fastest scheme, but implies the user should take care of writing every field on the data type before calling write on the loaned sample.

enumerator ZERO_LOAN_INITIALIZATION

Initialize all memory with zero-valued bytes.

The contents of the loaned sample will be zero-initialized upon return of loan_sample.

enumerator CONSTRUCTED_LOAN_INITIALIZATION

Use in-place constructor initialization.

This will call the constructor of the data type over the memory space being returned by loan_sample.

Public Functions

ReturnCode_t enable() override

This operation enables the DataWriter.

Return

RETCODE_OK is successfully enabled. RETCODE_PRECONDITION_NOT_MET if the Publisher creating this DataWriter is not enabled.

bool write(void *data)

Write data to the topic.

Return

True if correct, false otherwise

Parameters
• data: Pointer to the data

bool write(void *data, fastrtps::rtps::WriteParams &params)

Write data with params to the topic.

Return

True if correct, false otherwise

Parameters
• data: Pointer to the data

• params: Extra write parameters.

ReturnCode_t write(void *data, const InstanceHandle_t &handle)

Write data with handle.

The special value HANDLE_NIL can be used for the parameter handle.This indicates that the identity of the instance should be automatically deduced from the instance_data (by means of the key).

Return

RETCODE_PRECONDITION_NOT_MET if the handle introduced does not match with the one associated to the data, RETCODE_OK if the data is correctly sent and RETCODE_ERROR otherwise.

Parameters
• data: Pointer to the data

• handle: InstanceHandle_t.

ReturnCode_t write_w_timestamp(void *data, const InstanceHandle_t &handle, const fastrtps::rtps::Time_t &timestamp)

This operation performs the same function as write except that it also provides the value for the source_timestamp that is made available to DataReader objects by means of the source_timestamp attribute inside the SampleInfo. The constraints on the values of the handle parameter and the corresponding error behavior are the same specified for the write operation. This operation may block and return RETCODE_TIMEOUT under the same circumstances described for the write operation. This operation may return RETCODE_OUT_OF_RESOURCES, RETCODE_PRECONDITION_NOT_MET or RETCODE_BAD_PARAMETER under the same circumstances described for the write operation.

NOT YET IMPLEMENTED

Return

Any of the standard return codes.

Parameters
• data: Pointer to the data

• handle: InstanceHandle_t

• timestamp: Time_t used to set the source_timestamp.

InstanceHandle_t register_instance(void *instance)

Informs that the application will be modifying a particular instance.

It gives an opportunity to the middleware to pre-configure itself to improve performance.

Return

Handle containing the instance’s key. This handle could be used in successive write or dispose operations. In case of error, HANDLE_NIL will be returned.

Parameters
• [in] instance: Sample used to get the instance’s key.

InstanceHandle_t register_instance_w_timestamp(void *instance, const fastrtps::rtps::Time_t &timestamp)

This operation performs the same function as register_instance and can be used instead of register_instance in the cases where the application desires to specify the value for the source_timestamp. The source_timestamp potentially affects the relative order in which readers observe events from multiple writers. See the QoS policy DESTINATION_ORDER. This operation may block and return RETCODE_TIMEOUT under the same circumstances described for the write operation. This operation may return RETCODE_OUT_OF_RESOURCES under the same circumstances described for the write operation.

NOT YET IMPLEMENTED

Return

Handle containing the instance’s key.

Parameters
• instance: Sample used to get the instance’s key.

• timestamp: Time_t used to set the source_timestamp.

ReturnCode_t unregister_instance(void *instance, const InstanceHandle_t &handle)

This operation reverses the action of register_instance.

It should only be called on an instance that is currently registered. Informs the middleware that the DataWriter is not intending to modify any more of that data instance. Also indicates that the middleware can locally remove all information regarding that instance.

Return

Returns the operation’s result. If the operation finishes successfully, ReturnCode_t::RETCODE_OK is returned.

Parameters
• [in] instance: Sample used to deduce instance’s key in case of handle parameter is HANDLE_NIL.

• [in] handle: Instance’s key to be unregistered.

ReturnCode_t unregister_instance_w_timestamp(void *instance, const InstanceHandle_t &handle, const fastrtps::rtps::Time_t &timestamp)

This operation performs the same function as unregister_instance and can be used instead of unregister_instance in the cases where the application desires to specify the value for the source_timestamp. The source_timestamp potentially affects the relative order in which readers observe events from multiple writers. RETCODE_DESTINATION_ORDER). The constraints on the values of the handle parameter and the corresponding error behavior are the same specified for the unregister_instance operation. This operation may block and return RETCODE_TIMEOUT under the same circumstances described for the write operation.

NOT YET IMPLEMENTED

Return

Handle containing the instance’s key.

Parameters
• instance: Sample used to deduce instance’s key in case of handle parameter is HANDLE_NIL.

• handle: Instance’s key to be unregistered.

• timestamp: Time_t used to set the source_timestamp.

ReturnCode_t get_key_value(void *key_holder, const InstanceHandle_t &handle)

NOT YET IMPLEMENTED This operation can be used to retrieve the instance key that corresponds to an instance_handle. The operation will only fill the fields that form the key inside the key_holder instance.

This operation may return BAD_PARAMETER if the InstanceHandle_t a_handle does not correspond to an existing data-object known to the DataWriter. If the implementation is not able to check invalid handles then the result in this situation is unspecified.

Return

Any of the standard return codes.

Parameters
• [inout] key:

• [in] handle:

InstanceHandle_t lookup_instance(const void *instance) const

NOT YET IMPLEMENTED Takes as a parameter an instance and returns a handle that can be used in subsequent operations that accept an instance handle as an argument. The instance parameter is only used for the purpose of examining the fields that define the key.

Return

handle of the given instance

Parameters
• [in] instance: Data pointer to the sample

const fastrtps::rtps::GUID_t &guid() const

Returns the DataWriter’s GUID

Return

Reference to the DataWriter GUID

InstanceHandle_t get_instance_handle() const

Returns the DataWriter’s InstanceHandle

Return

Copy of the DataWriter InstanceHandle

TypeSupport get_type() const

Get data type associated to the DataWriter

Return

Copy of the TypeSupport

ReturnCode_t wait_for_acknowledgments(const fastrtps::Duration_t &max_wait)

Return

RETCODE_OK if the DataWriter receive the acknowledgments before the time expires and RETCODE_ERROR otherwise

Parameters
• max_wait: Maximum blocking time for this operation

ReturnCode_t get_offered_deadline_missed_status(OfferedDeadlineMissedStatus &status)

Returns the offered deadline missed status.

Return

RETCODE_OK

Parameters
• [out] status: Deadline missed status struct

ReturnCode_t get_offered_incompatible_qos_status(OfferedIncompatibleQosStatus &status)

Returns the offered incompatible qos status.

Return

RETCODE_OK

Parameters
• [out] status: Offered incompatible qos status struct

ReturnCode_t get_publication_matched_status(PublicationMatchedStatus &status) const

Returns the publication matched status.

Return

RETCODE_OK

Parameters
• [out] status: publication matched status struct

ReturnCode_t set_qos(const DataWriterQos &qos)

Establishes the DataWriterQos for this DataWriter.

Return

RETCODE_IMMUTABLE_POLICY if any of the Qos cannot be changed, RETCODE_INCONSISTENT_POLICY if the Qos is not self consistent and RETCODE_OK if the qos is changed correctly.

Parameters

const DataWriterQos &get_qos() const

Retrieves the DataWriterQos for this DataWriter.

Return

Reference to the current DataWriterQos

ReturnCode_t get_qos(DataWriterQos &qos) const

Fills the DataWriterQos with the values of this DataWriter.

Return

RETCODE_OK

Parameters
• qos: DataWriterQos object where the qos is returned.

Topic *get_topic() const

Retrieves the topic for this DataWriter.

Return

Pointer to the associated Topic

const DataWriterListener *get_listener() const

Retrieves the listener for this DataWriter.

Return

Pointer to the DataWriterListener

ReturnCode_t set_listener(DataWriterListener *listener)

Return

RETCODE_OK

Parameters

ReturnCode_t set_listener(DataWriterListener *listener, const StatusMask &mask)

Modifies the DataWriterListener.

Return

RETCODE_OK

Parameters
• listener: new value for the DataWriterListener

• mask: StatusMask that holds statuses the listener responds to (default: all).

ReturnCode_t dispose(void *data, const InstanceHandle_t &handle)

This operation requests the middleware to delete the data (the actual deletion is postponed until there is no more use for that data in the whole system). In general, applications are made aware of the deletion by means of operations on the DataReader objects that already knew that instance. This operation does not modify the value of the instance. The instance parameter is passed just for the purposes of identifying the instance. When this operation is used, the Service will automatically supply the value of the source_timestamp that is made available to DataReader objects by means of the source_timestamp attribute inside the SampleInfo. The constraints on the values of the handle parameter and the corresponding error behavior are the same specified for the unregister_instance operation.

Return

RETCODE_PRECONDITION_NOT_MET if the handle introduced does not match with the one associated to the data, RETCODE_OK if the data is correctly sent and RETCODE_ERROR otherwise.

Parameters
• [in] data: Sample used to deduce instance’s key in case of handle parameter is HANDLE_NIL.

• [in] handle: InstanceHandle of the data

ReturnCode_t dispose_w_timestamp(void *data, const InstanceHandle_t &handle)

This operation performs the same functions as dispose except that the application provides the value for the source_timestamp that is made available to DataReader objects by means of the source_timestamp attribute inside the SampleInfo. The constraints on the values of the handle parameter and the corresponding error behavior are the same specified for the dispose operation. This operation may return RETCODE_PRECONDITION_NOT_MET and RETCODE_BAD_PARAMETER under the same circumstances described for the dispose operation. This operation may return RETCODE_TIMEOUT and RETCODE_OUT_OF_RESOURCES under the same circumstances described for the write operation.

Return

RTPS_DllAPI

Parameters
• data: Pointer to the data.

• handle: InstanceHandle_t

ReturnCode_t get_liveliness_lost_status(LivelinessLostStatus &status)

Returns the liveliness lost status.

Return

RETCODE_OK

Parameters
• status: Liveliness lost status struct

const Publisher *get_publisher() const

Getter for the Publisher that creates this DataWriter.

Return

Pointer to the Publisher

ReturnCode_t assert_liveliness()

This operation manually asserts the liveliness of the DataWriter. This is used in combination with the LivelinessQosPolicy to indicate to the Service that the entity remains active. This operation need only be used if the LIVELINESS setting is either MANUAL_BY_PARTICIPANT or MANUAL_BY_TOPIC. Otherwise, it has no effect.

Note

Writing data via the write operation on a DataWriter asserts liveliness on the DataWriter itself and its DomainParticipant. Consequently the use of assert_liveliness is only needed if the application is not writing data regularly.

Return

RETCODE_OK if asserted, RETCODE_ERROR otherwise

ReturnCode_t get_matched_subscription_data(builtin::SubscriptionBuiltinTopicData &subscription_data, const fastrtps::rtps::InstanceHandle_t &subscription_handle) const

Retrieves in a subscription associated with the DataWriter.

Return

RETCODE_OK

Parameters
• [out] subscription_data: subscription data struct

• subscription_handle: InstanceHandle_t of the subscription

ReturnCode_t get_matched_subscriptions(std::vector<fastrtps::rtps::InstanceHandle_t*> &subscription_handles) const

Fills the given vector with the InstanceHandle_t of matched DataReaders.

Return

RETCODE_OK

Parameters
• [out] subscription_handles: Vector where the InstanceHandle_t are returned

ReturnCode_t clear_history(size_t *removed)

Clears the DataWriter history.

Return

RETCODE_OK if the samples are removed and RETCODE_ERROR otherwise

Parameters
• removed: size_t pointer to return the size of the data removed

ReturnCode_t loan_sample(void *&sample, LoanInitializationKind initialization = LoanInitializationKind::NO_LOAN_INITIALIZATION)

Get a pointer to the internal pool where the user could directly write.

This method can only be used on a DataWriter for a plain data type. It will provide the user with a pointer to an internal buffer where the data type can be prepared for sending.

When using NO_LOAN_INITIALIZATION on the initialization parameter, which is the default, no assumptions should be made on the contents where the pointer points to, as it may be an old pointer being reused. See LoanInitializationKind for more details.

Once the sample has been prepared, it can then be published by calling write. After a successful call to write, the middleware takes ownership of the loaned pointer again, and the user should not access that memory again.

If, for whatever reason, the sample is not published, the loan can be returned by calling discard_loan.

Return

ReturnCode_t::RETCODE_ILLEGAL_OPERATION when the data type does not support loans.

Return

ReturnCode_t::RETCODE_NOT_ENABLED if the writer has not been enabled.

Return

ReturnCode_t::RETCODE_OUT_OF_RESOURCES if the pool has been exhausted.

Return

ReturnCode_t::RETCODE_OK if a pointer to a sample is successfully obtained.

Parameters
• [out] sample: Pointer to the sample on the internal pool.

• [in] initialization: How to initialize the loaned sample.

ReturnCode_t discard_loan(void *&sample)