QDataStream Class

  QFile file("file.dat");
  file.open(QIODevice::WriteOnly);
  QDataStream out(&file);   // we will serialize the data into the file
  out << QString("the answer is");   // serialize a string
  out << (qint32)42;        // serialize an integer

  QFile file("file.dat");
  file.open(QIODevice::ReadOnly);
  QDataStream in(&file);    // read the data serialized from the file
  QString str;
  qint32 a;
  in >> str >> a;           // extract "the answer is" and 42

  stream.setVersion(QDataStream::Qt_4_0);

  QFile file("file.xxx");
  file.open(QIODevice::WriteOnly);
  QDataStream out(&file);

  // Write a header with a "magic number" and a version
  out << (quint32)0xA0B0C0D0;
  out << (qint32)123;

  out.setVersion(QDataStream::Qt_4_0);

  // Write the data
  out << lots_of_interesting_data;

  QFile file("file.xxx");
  file.open(QIODevice::ReadOnly);
  QDataStream in(&file);

  // Read and check the header
  quint32 magic;
  in >> magic;
  if (magic != 0xA0B0C0D0)
      return XXX_BAD_FILE_FORMAT;

  // Read the version
  qint32 version;
  in >> version;
  if (version < 100)
      return XXX_BAD_FILE_TOO_OLD;
  if (version > 123)
      return XXX_BAD_FILE_TOO_NEW;

  if (version <= 110)
      in.setVersion(QDataStream::Qt_3_2);
  else
      in.setVersion(QDataStream::Qt_4_0);

  // Read the data
  in >> lots_of_interesting_data;
  if (version >= 120)
      in >> data_new_in_XXX_version_1_2;
  in >> other_interesting_data;

  QDataStream &operator<<(QDataStream &, const QXxx &);
  QDataStream &operator>>(QDataStream &, QXxx &);

  QDataStream & operator<< (QDataStream& stream, const QImage& image);
  QDataStream & operator>> (QDataStream& stream, QImage& image);

  in.startTransaction();
  QString str;
  qint32 a;
  in >> str >> a; // try to read packet atomically

  if (!in.commitTransaction())
      return;     // wait for more data

Member Function Documentation

QDataStream::QDataStream()

Constructs a data stream that has no I/O device.

See also setDevice().

QDataStream::QDataStream(QIODevice *d)

Constructs a data stream that uses the I/O device d.

See also setDevice() and device().

QDataStream::QDataStream(QByteArray *a, QIODevice::OpenMode mode)

Constructs a data stream that operates on a byte array, a. The mode describes how the device is to be used.

Alternatively, you can use QDataStream(const QByteArray &) if you just want to read from a byte array.

Since QByteArray is not a QIODevice subclass, internally a QBuffer is created to wrap the byte array.

QDataStream::QDataStream(const QByteArray &a)

Constructs a read-only data stream that operates on byte array a. Use QDataStream(QByteArray*, int) if you want to write to a byte array.

Since QByteArray is not a QIODevice subclass, internally a QBuffer is created to wrap the byte array.

QDataStream::~QDataStream()

Destroys the data stream.

The destructor will not affect the current I/O device, unless it is an internal I/O device (e.g. a QBuffer) processing a QByteArray passed in the constructor, in which case the internal I/O device is destroyed.

void QDataStream::abortTransaction()

Aborts a read transaction.

This function is commonly used to discard the transaction after higher-level protocol errors or loss of stream synchronization.

If called on an inner transaction, aborting is delegated to the outermost transaction, and subsequently started inner transactions are forced to fail.

For the outermost transaction, discards the restoration point and any internally duplicated data of the stream. Will not affect the current read position of the stream.

Sets the status of the data stream to

This function was introduced in Qt 5.7.

See also startTransaction(), commitTransaction(), and rollbackTransaction().

bool QDataStream::atEnd() const

Returns true if the I/O device has reached the end position (end of the stream or file) or if there is no I/O device set; otherwise returns false.

See also QIODevice::atEnd().

ByteOrder QDataStream::byteOrder() const

Returns the current byte order setting -- either BigEndian or LittleEndian.

See also setByteOrder().

bool QDataStream::commitTransaction()

Completes a read transaction. Returns true if no read errors have occurred during the transaction; otherwise returns false.

If called on an inner transaction, committing will be postponed until the outermost commitTransaction(), rollbackTransaction(), or abortTransaction() call occurs.

Otherwise, if the stream status indicates reading past the end of the data, this function restores the stream data to the point of the startTransaction() call. When this situation occurs, you need to wait for more data to arrive, after which you start a new transaction. If the data stream has read corrupt data or any of the inner transactions was aborted, this function aborts the transaction.

This function was introduced in Qt 5.7.

See also startTransaction(), rollbackTransaction(), and abortTransaction().

QIODevice *QDataStream::device() const

Returns the I/O device currently set, or 0 if no device is currently set.

See also setDevice().

FloatingPointPrecision QDataStream::floatingPointPrecision() const

Returns the floating point precision of the data stream.

This function was introduced in Qt 4.6.

See also FloatingPointPrecision and setFloatingPointPrecision().

QDataStream &QDataStream::readBytes(char *&s, uint &l)

Reads the buffer s from the stream and returns a reference to the stream.

The buffer s is allocated using new []. Destroy it with the delete [] operator.

The l parameter is set to the length of the buffer. If the string read is empty, l is set to 0 and s is set to a null pointer.

The serialization format is a quint32 length specifier first, then l bytes of data.

See also readRawData() and writeBytes().

int QDataStream::readRawData(char *s, int len)

Reads at most len bytes from the stream into s and returns the number of bytes read. If an error occurs, this function returns -1.

The buffer s must be preallocated. The data is not encoded.

See also readBytes(), QIODevice::read(), and writeRawData().

void QDataStream::resetStatus()

Resets the status of the data stream.

See also Status, status(), and setStatus().

void QDataStream::rollbackTransaction()

Reverts a read transaction.

This function is commonly used to rollback the transaction when an incomplete read was detected prior to committing the transaction.

If called on an inner transaction, reverting is delegated to the outermost transaction, and subsequently started inner transactions are forced to fail.

For the outermost transaction, restores the stream data to the point of the startTransaction() call. If the data stream has read corrupt data or any of the inner transactions was aborted, this function aborts the transaction.

If the preceding stream operations were successful, sets the status of the data stream to

This function was introduced in Qt 5.7.

See also startTransaction(), commitTransaction(), and abortTransaction().

void QDataStream::setByteOrder(ByteOrder bo)

Sets the serialization byte order to bo.

The bo parameter can be QDataStream::BigEndian or QDataStream::LittleEndian.

The default setting is big endian. We recommend leaving this setting unless you have special requirements.

See also byteOrder().

void QDataStream::setDevice(QIODevice *d)

void QDataStream::setDevice(QIODevice *d)

Sets the I/O device to d, which can be 0 to unset to current I/O device.

See also device().

void QDataStream::setFloatingPointPrecision(FloatingPointPrecision precision)

Sets the floating point precision of the data stream to precision. If the floating point precision is DoublePrecision and the version of the data stream is Qt_4_6 or higher, all floating point numbers will be written and read with 64-bit precision. If the floating point precision is SinglePrecision and the version is Qt_4_6 or higher, all floating point numbers will be written and read with 32-bit precision.

For versions prior to Qt_4_6, the precision of floating point numbers in the data stream depends on the stream operator called.

The default is DoublePrecision.

Warning: This property must be set to the same value on the object that writes and the object that reads the data stream.

This function was introduced in Qt 4.6.

See also floatingPointPrecision().

void QDataStream::setStatus(Status status)

Sets the status of the data stream to the status given.

Subsequent calls to setStatus() are ignored until resetStatus() is called.

See also Status, status(), and resetStatus().

void QDataStream::setVersion(int v)

Sets the version number of the data serialization format to v, a value of the Version enum.

You don't have to set a version if you are using the current version of Qt, but for your own custom binary formats we recommend that you do; see Versioning in the Detailed Description.

To accommodate new functionality, the datastream serialization format of some Qt classes has changed in some versions of Qt. If you want to read data that was created by an earlier version of Qt, or write data that can be read by a program that was compiled with an earlier version of Qt, use this function to modify the serialization format used by QDataStream.

The Version enum provides symbolic constants for the different versions of Qt. For example:

  QDataStream out(file);
  out.setVersion(QDataStream::Qt_4_0);

See also version() and Version.

int QDataStream::skipRawData(int len)

Skips len bytes from the device. Returns the number of bytes actually skipped, or -1 on error.

This is equivalent to calling readRawData() on a buffer of length len and ignoring the buffer.

This function was introduced in Qt 4.1.

See also QIODevice::seek().

void QDataStream::startTransaction()

Starts a new read transaction on the stream.

Defines a restorable point within the sequence of read operations. For sequential devices, read data will be duplicated internally to allow recovery in case of incomplete reads. For random-access devices, this function saves the current position of the stream. Call commitTransaction(), rollbackTransaction(), or abortTransaction() to finish the current transaction.

Once a transaction is started, subsequent calls to this function will make the transaction recursive. Inner transactions act as agents of the outermost transaction (i.e., report the status of read operations to the outermost transaction, which can restore the position of the stream).

Note: Restoring to the point of the nested startTransaction() call is not supported.

When an error occurs during a transaction (including an inner transaction failing), reading from the data stream is suspended (all subsequent read operations return empty/zero values) and subsequent inner transactions are forced to fail. Starting a new outermost transaction recovers from this state. This behavior makes it unnecessary to error-check every read operation separately.

This function was introduced in Qt 5.7.

See also commitTransaction(), rollbackTransaction(), and abortTransaction().

Status QDataStream::status() const

Returns the status of the data stream.

See also Status, setStatus(), and resetStatus().

int QDataStream::version() const

Returns the version number of the data serialization format.

See also setVersion() and Version.

QDataStream &QDataStream::writeBytes(const char *s, uint len)

Writes the length specifier len and the buffer s to the stream and returns a reference to the stream.

The len is serialized as a quint32, followed by len bytes from s. Note that the data is not encoded.

See also writeRawData() and readBytes().

int QDataStream::writeRawData(const char *s, int len)

Writes len bytes from s to the stream. Returns the number of bytes actually written, or -1 on error. The data is not encoded.

See also writeBytes(), QIODevice::write(), and readRawData().

QDataStream &QDataStream::operator<<(qint8 i)

Writes a signed byte, i, to the stream and returns a reference to the stream.

QDataStream &QDataStream::operator<<(quint8 i)

This is an overloaded function.

Writes an unsigned byte, i, to the stream and returns a reference to the stream.

QDataStream &QDataStream::operator<<(qint16 i)

This is an overloaded function.

Writes a signed 16-bit integer, i, to the stream and returns a reference to the stream.

QDataStream &QDataStream::operator<<(quint16 i)

This is an overloaded function.

Writes an unsigned 16-bit integer, i, to the stream and returns a reference to the stream.

QDataStream &QDataStream::operator<<(qint32 i)

This is an overloaded function.

Writes a signed 32-bit integer, i, to the stream and returns a reference to the stream.

QDataStream &QDataStream::operator<<(quint32 i)

This is an overloaded function.

Writes an unsigned integer, i, to the stream as a 32-bit unsigned integer (quint32). Returns a reference to the stream.

QDataStream &QDataStream::operator<<(qint64 i)

This is an overloaded function.

Writes a signed 64-bit integer, i, to the stream and returns a reference to the stream.

QDataStream &QDataStream::operator<<(quint64 i)

This is an overloaded function.

Writes an unsigned 64-bit integer, i, to the stream and returns a reference to the stream.

QDataStream &QDataStream::operator<<(bool i)

Writes a boolean value, i, to the stream. Returns a reference to the stream.

QDataStream &QDataStream::operator<<(float f)

This is an overloaded function.

Writes a floating point number, f, to the stream using the standard IEEE 754 format. Returns a reference to the stream.

See also setFloatingPointPrecision().

QDataStream &QDataStream::operator<<(double f)

This is an overloaded function.

Writes a floating point number, f, to the stream using the standard IEEE 754 format. Returns a reference to the stream.

See also setFloatingPointPrecision().

QDataStream &QDataStream::operator<<(const char *s)

This is an overloaded function.

Writes the '\0'-terminated string s to the stream and returns a reference to the stream.

The string is serialized using writeBytes().

See also writeBytes() and writeRawData().

QDataStream &QDataStream::operator>>(qint8 &i)

Reads a signed byte from the stream into i, and returns a reference to the stream.

QDataStream &QDataStream::operator>>(quint8 &i)

This is an overloaded function.

Reads an unsigned byte from the stream into i, and returns a reference to the stream.

QDataStream &QDataStream::operator>>(qint16 &i)

This is an overloaded function.

Reads a signed 16-bit integer from the stream into i, and returns a reference to the stream.

QDataStream &QDataStream::operator>>(quint16 &i)

This is an overloaded function.

Reads an unsigned 16-bit integer from the stream into i, and returns a reference to the stream.

QDataStream &QDataStream::operator>>(qint32 &i)

This is an overloaded function.

Reads a signed 32-bit integer from the stream into i, and returns a reference to the stream.

QDataStream &QDataStream::operator>>(quint32 &i)

This is an overloaded function.

Reads an unsigned 32-bit integer from the stream into i, and returns a reference to the stream.

QDataStream &QDataStream::operator>>(qint64 &i)

This is an overloaded function.

Reads a signed 64-bit integer from the stream into i, and returns a reference to the stream.

QDataStream &QDataStream::operator>>(quint64 &i)

This is an overloaded function.

Reads an unsigned 64-bit integer from the stream, into i, and returns a reference to the stream.

QDataStream &QDataStream::operator>>(bool &i)

Reads a boolean value from the stream into i. Returns a reference to the stream.

QDataStream &QDataStream::operator>>(float &f)

This is an overloaded function.

Reads a floating point number from the stream into f, using the standard IEEE 754 format. Returns a reference to the stream.

See also setFloatingPointPrecision().

QDataStream &QDataStream::operator>>(double &f)

This is an overloaded function.

Reads a floating point number from the stream into f, using the standard IEEE 754 format. Returns a reference to the stream.

See also setFloatingPointPrecision().

QDataStream &QDataStream::operator>>(char *&s)

This is an overloaded function.

Reads the '\0'-terminated string s from the stream and returns a reference to the stream.

The string is deserialized using readBytes().

Space for the string is allocated using new [] -- the caller must destroy it with delete [].

See also readBytes() and readRawData().