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().

[explicit] QDataStream::QDataStream(QIODevice *d)

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

See also setDevice() and device().

QDataStream::QDataStream(QByteArray *a, QIODeviceBase::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.

[noexcept] 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

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().

QDataStream::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.

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

QIODevice *QDataStream::device() const

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

See also setDevice().

QDataStream::FloatingPointPrecision QDataStream::floatingPointPrecision() const

Returns the floating point precision of the data stream.

See also FloatingPointPrecision and setFloatingPointPrecision().

[since 6.7] QDataStream &QDataStream::readBytes(char *&s, qint64 &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 nullptr.

The serialization format is a length specifier first, then l bytes of data. The length specifier is one quint32 if the version is less than 6.7 or if the number of elements is less than 0xfffffffe (2^32 -2), otherwise there is an extend value 0xfffffffe followed by one quint64 with the actual value. In addition for containers that support isNull(), it is encoded as a single quint32 with all bits set and no data.

This function was introduced in Qt 6.7.

See also readRawData() and writeBytes().

qint64 QDataStream::readRawData(char *s, qint64 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 decoded.

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

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

void QDataStream::setByteOrder(QDataStream::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 nullptr to unset to current I/O device.

See also device().

void QDataStream::setFloatingPointPrecision(QDataStream::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.

Note that this property does not affect the serialization or deserialization of qfloat16 instances.

See also floatingPointPrecision().

void QDataStream::setStatus(QDataStream::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.

qint64 QDataStream::skipRawData(qint64 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.

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).

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.

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

QDataStream::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, qint64 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 and an optional quint64, followed by len bytes from s. Note that the data is not encoded.

See also writeRawData() and readBytes().

qint64 QDataStream::writeRawData(const char *s, qint64 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<<(std::nullptr_t ptr)

This is an overloaded function.

Simulates writing a std::nullptr_t, ptr, to the stream and returns a reference to the stream. This function does not actually write anything to the stream, as std::nullptr_t values are stored as 0 bytes.

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().

[since 6.0] QDataStream &QDataStream::operator<<(char16_t c)

This is an overloaded function.

Writes a character, c, to the stream. Returns a reference to the stream

This function was introduced in Qt 6.0.

[since 6.0] QDataStream &QDataStream::operator<<(char32_t c)

This is an overloaded function.

Writes a character, c, to the stream. Returns a reference to the stream

This function was introduced in Qt 6.0.

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>>(std::nullptr_t &ptr)

This is an overloaded function.

Simulates reading a std::nullptr_t from the stream into ptr and returns a reference to the stream. This function does not actually read anything from the stream, as std::nullptr_t values are stored as 0 bytes.

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 string s from the stream and returns a reference to the stream.

The string is deserialized using readBytes() where the serialization format is a quint32 length specifier first, followed by that many bytes of data. The resulting string is always '\0'-terminated.

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

See also readBytes() and readRawData().

[since 6.0] QDataStream &QDataStream::operator>>(char16_t &c)

This is an overloaded function.

Reads a 16bit wide char from the stream into c and returns a reference to the stream.

This function was introduced in Qt 6.0.

[since 6.0] QDataStream &QDataStream::operator>>(char32_t &c)

This is an overloaded function.

Reads a 32bit wide character from the stream into c and returns a reference to the stream.

This function was introduced in Qt 6.0.