class QDataStream#

The QDataStream class provides serialization of binary data to a QIODevice . More…

Synopsis#

Methods#

def __init__()
def abortTransaction()
def atEnd()
def byteOrder()
def commitTransaction()
def device()
def floatingPointPrecision()
def isDeviceTransactionStarted()
def __lshift__()
def __rshift__()
def readBool()
def readBytes()
def readDouble()
def readFloat()
def readInt16()
def readInt32()
def readInt64()
def readInt8()
def readQChar()
def readQString()
def readQStringList()
def readQVariant()
def readRawData()
def readString()
def readUInt16()
def readUInt32()
def readUInt64()
def readUInt8()
def resetStatus()
def rollbackTransaction()
def setByteOrder()
def setDevice()
def setFloatingPointPrecision()
def setStatus()
def setVersion()
def skipRawData()
def startTransaction()
def status()
def version()
def writeBool()
def writeBytes()
def writeDouble()
def writeFloat()
def writeInt16()
def writeInt32()
def writeInt64()
def writeInt8()
def writeQChar()
def writeQString()
def writeQStringList()
def writeQVariant()
def writeRawData()
def writeString()
def writeUInt16()
def writeUInt32()
def writeUInt64()
def writeUInt8()

Note

This documentation may contain snippets that were automatically translated from C++ to Python. We always welcome contributions to the snippet translation. If you see an issue with the translation, you can also let us know by creating a ticket on https:/bugreports.qt.io/projects/PYSIDE

Detailed Description#

Warning

This section contains snippets that were automatically translated from C++ to Python and may contain errors.

A data stream is a binary stream of encoded information which is 100% independent of the host computer’s operating system, CPU or byte order. For example, a data stream that is written by a PC under Windows can be read by a Sun SPARC running Solaris.

You can also use a data stream to read/write raw unencoded binary data . If you want a “parsing” input stream, see QTextStream .

The QDataStream class implements the serialization of C++’s basic data types, like char, short, int, char *, etc. Serialization of more complex data is accomplished by breaking up the data into primitive units.

A data stream cooperates closely with a QIODevice . A QIODevice represents an input/output medium one can read data from and write data to. The QFile class is an example of an I/O device.

Example (write binary data to a stream):

file = QFile("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

Example (read binary data from a stream):

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

Each item written to the stream is written in a predefined binary format that varies depending on the item’s type. Supported Qt types include QBrush, QColor, QDateTime , QFont, QPixmap, QString , QVariant and many others. For the complete list of all Qt types supporting data streaming see Serializing Qt Data Types .

For integers it is best to always cast to a Qt integer type for writing, and to read back into the same Qt integer type. This ensures that you get integers of the size you want and insulates you from compiler and platform differences.

Enumerations can be serialized through QDataStream without the need of manually defining streaming operators. Enum classes are serialized using the declared size.

To take one example, a char * string is written as a 32-bit integer equal to the length of the string including the ‘\0’ byte, followed by all the characters of the string including the ‘\0’ byte. When reading a char * string, 4 bytes are read to create the 32-bit length value, then that many characters for the char * string including the ‘\0’ terminator are read.

The initial I/O device is usually set in the constructor, but can be changed with setDevice() . If you’ve reached the end of the data (or if there is no I/O device set) atEnd() will return true.

Versioning#

QDataStream ‘s binary format has evolved since Qt 1.0, and is likely to continue evolving to reflect changes done in Qt. When inputting or outputting complex types, it’s very important to make sure that the same version of the stream ( version() ) is used for reading and writing. If you need both forward and backward compatibility, you can hardcode the version number in the application:

stream.setVersion(QDataStream.Qt_4_0)

If you are producing a new binary data format, such as a file format for documents created by your application, you could use a QDataStream to write the data in a portable format. Typically, you would write a brief header containing a magic string and a version number to give yourself room for future expansion. For example:

file = QFile("file.xxx")
file.open(QIODevice.WriteOnly)
out = QDataStream(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

Then read it in with:

file = QFile("file.xxx")
file.open(QIODevice.ReadOnly)
in = QDataStream(file)
# Read and check the header
magic = quint32()
in >> magic
if magic != 0xA0B0C0D0:
    return XXX_BAD_FILE_FORMAT
# Read the version
version = qint32()
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

You can select which byte order to use when serializing data. The default setting is big-endian (MSB first). Changing it to little-endian breaks the portability (unless the reader also changes to little-endian). We recommend keeping this setting unless you have special requirements.

Reading and Writing Raw Binary Data#

You may wish to read/write your own raw binary data to/from the data stream directly. Data may be read from the stream into a preallocated char * using readRawData() . Similarly data can be written to the stream using writeRawData() . Note that any encoding/decoding of the data must be done by you.

A similar pair of functions is readBytes() and writeBytes() . These differ from their raw counterparts as follows: readBytes() reads a quint32 which is taken to be the length of the data to be read, then that number of bytes is read into the preallocated char *; writeBytes() writes a quint32 containing the length of the data, followed by the data. Note that any encoding/decoding of the data (apart from the length quint32) must be done by you.

Reading and Writing Qt Collection Classes#

The Qt container classes can also be serialized to a QDataStream . These include QList , QSet , QHash , and QMap . The stream operators are declared as non-members of the classes.

Reading and Writing Other Qt Classes#

In addition to the overloaded stream operators documented here, any Qt classes that you might want to serialize to a QDataStream will have appropriate stream operators declared as non-member of the class:

operator<< = QDataStream(QDataStream , QXxx )
operator>> = QDataStream(QDataStream , QXxx )

For example, here are the stream operators declared as non-members of the QImage class:

operator<< = QDataStream(QDataStream stream, QImage image)
operator>> = QDataStream(QDataStream stream, QImage image)

To see if your favorite Qt class has similar stream operators defined, check the Related Non-Members section of the class’s documentation page.

Using Read Transactions#

When a data stream operates on an asynchronous device, the chunks of data can arrive at arbitrary points in time. The QDataStream class implements a transaction mechanism that provides the ability to read the data atomically with a series of stream operators. As an example, you can handle incomplete reads from a socket by using a transaction in a slot connected to the readyRead() signal:

in.startTransaction()
str = QString()
a = qint32()
in >> str >> a # try to read packet atomically
if not in.commitTransaction():
    return # wait for more data

If no full packet is received, this code restores the stream to the initial position, after which you need to wait for more data to arrive.

Corruption and Security#

QDataStream is not resilient against corrupted data inputs and should therefore not be used for security-sensitive situations, even when using transactions. Transactions will help determine if a valid input can currently be decoded with the data currently available on an asynchronous device, but will assume that the data that is available is correctly formed.

Additionally, many QDataStream demarshalling operators will allocate memory based on information found in the stream. Those operators perform no verification on whether the requested amount of memory is reasonable or if it is compatible with the amount of data available in the stream (example: demarshalling a QByteArray or QString may see the request for allocation of several gigabytes of data).

QDataStream should not be used on content whose provenance cannot be trusted. Applications should be designed to attempt to decode only streams whose provenance is at least as trustworthy as that of the application itself or its plugins.

See also

QTextStream QVariant

class Version#

(inherits enum.IntEnum) This enum provides symbolic synonyms for the data serialization format version numbers.

Constant

Description

QDataStream.Qt_1_0

Version 1 (Qt 1.x)

QDataStream.Qt_2_0

Version 2 (Qt 2.0)

QDataStream.Qt_2_1

Version 3 (Qt 2.1, 2.2, 2.3)

QDataStream.Qt_3_0

Version 4 (Qt 3.0)

QDataStream.Qt_3_1

Version 5 (Qt 3.1, 3.2)

QDataStream.Qt_3_3

Version 6 (Qt 3.3)

QDataStream.Qt_4_0

Version 7 (Qt 4.0, Qt 4.1)

QDataStream.Qt_4_1

Version 7 (Qt 4.0, Qt 4.1)

QDataStream.Qt_4_2

Version 8 (Qt 4.2)

QDataStream.Qt_4_3

Version 9 (Qt 4.3)

QDataStream.Qt_4_4

Version 10 (Qt 4.4)

QDataStream.Qt_4_5

Version 11 (Qt 4.5)

QDataStream.Qt_4_6

Version 12 (Qt 4.6, Qt 4.7, Qt 4.8)

QDataStream.Qt_4_7

Same as Qt_4_6.

QDataStream.Qt_4_8

Same as Qt_4_6.

QDataStream.Qt_4_9

Same as Qt_4_6.

QDataStream.Qt_5_0

Version 13 (Qt 5.0)

QDataStream.Qt_5_1

Version 14 (Qt 5.1)

QDataStream.Qt_5_2

Version 15 (Qt 5.2)

QDataStream.Qt_5_3

Same as Qt_5_2

QDataStream.Qt_5_4

Version 16 (Qt 5.4)

QDataStream.Qt_5_5

Same as Qt_5_4

QDataStream.Qt_5_6

Version 17 (Qt 5.6)

QDataStream.Qt_5_7

Same as Qt_5_6

QDataStream.Qt_5_8

Same as Qt_5_6

QDataStream.Qt_5_9

Same as Qt_5_6

QDataStream.Qt_5_10

Same as Qt_5_6

QDataStream.Qt_5_11

Same as Qt_5_6

QDataStream.Qt_5_12

Version 18 (Qt 5.12)

QDataStream.Qt_5_13

Version 19 (Qt 5.13)

QDataStream.Qt_5_14

Same as Qt_5_13

QDataStream.Qt_5_15

Same as Qt_5_13

QDataStream.Qt_6_0

Version 20 (Qt 6.0)

QDataStream.Qt_6_1

Same as Qt_6_0

QDataStream.Qt_6_2

Same as Qt_6_0

QDataStream.Qt_6_3

Same as Qt_6_0

QDataStream.Qt_6_4

Same as Qt_6_0

QDataStream.Qt_6_5

Same as Qt_6_0

QDataStream.Qt_6_6

Version 21 (Qt 6.6)

QDataStream.Qt_6_7

Version 22 (Qt 6.7)

See also

setVersion() version()

Constant	Description
QDataStream.Qt_1_0	Version 1 (Qt 1.x)
QDataStream.Qt_2_0	Version 2 (Qt 2.0)
QDataStream.Qt_2_1	Version 3 (Qt 2.1, 2.2, 2.3)
QDataStream.Qt_3_0	Version 4 (Qt 3.0)
QDataStream.Qt_3_1	Version 5 (Qt 3.1, 3.2)
QDataStream.Qt_3_3	Version 6 (Qt 3.3)
QDataStream.Qt_4_0	Version 7 (Qt 4.0, Qt 4.1)
QDataStream.Qt_4_1	Version 7 (Qt 4.0, Qt 4.1)
QDataStream.Qt_4_2	Version 8 (Qt 4.2)
QDataStream.Qt_4_3	Version 9 (Qt 4.3)
QDataStream.Qt_4_4	Version 10 (Qt 4.4)
QDataStream.Qt_4_5	Version 11 (Qt 4.5)
QDataStream.Qt_4_6	Version 12 (Qt 4.6, Qt 4.7, Qt 4.8)
QDataStream.Qt_4_7	Same as Qt_4_6.
QDataStream.Qt_4_8	Same as Qt_4_6.
QDataStream.Qt_4_9	Same as Qt_4_6.
QDataStream.Qt_5_0	Version 13 (Qt 5.0)
QDataStream.Qt_5_1	Version 14 (Qt 5.1)
QDataStream.Qt_5_2	Version 15 (Qt 5.2)
QDataStream.Qt_5_3	Same as Qt_5_2
QDataStream.Qt_5_4	Version 16 (Qt 5.4)
QDataStream.Qt_5_5	Same as Qt_5_4
QDataStream.Qt_5_6	Version 17 (Qt 5.6)
QDataStream.Qt_5_7	Same as Qt_5_6
QDataStream.Qt_5_8	Same as Qt_5_6
QDataStream.Qt_5_9	Same as Qt_5_6
QDataStream.Qt_5_10	Same as Qt_5_6
QDataStream.Qt_5_11	Same as Qt_5_6
QDataStream.Qt_5_12	Version 18 (Qt 5.12)
QDataStream.Qt_5_13	Version 19 (Qt 5.13)
QDataStream.Qt_5_14	Same as Qt_5_13
QDataStream.Qt_5_15	Same as Qt_5_13
QDataStream.Qt_6_0	Version 20 (Qt 6.0)
QDataStream.Qt_6_1	Same as Qt_6_0
QDataStream.Qt_6_2	Same as Qt_6_0
QDataStream.Qt_6_3	Same as Qt_6_0
QDataStream.Qt_6_4	Same as Qt_6_0
QDataStream.Qt_6_5	Same as Qt_6_0
QDataStream.Qt_6_6	Version 21 (Qt 6.6)
QDataStream.Qt_6_7	Version 22 (Qt 6.7)

class ByteOrder#

The byte order used for reading/writing the data.

Constant

Description

QDataStream.BigEndian

Most significant byte first (the default)

QDataStream.LittleEndian

Least significant byte first

Constant	Description
QDataStream.BigEndian	Most significant byte first (the default)
QDataStream.LittleEndian	Least significant byte first

class Status#

This enum describes the current status of the data stream.

Constant

Description

QDataStream.Ok

The data stream is operating normally.

QDataStream.ReadPastEnd

The data stream has read past the end of the data in the underlying device.

QDataStream.ReadCorruptData

The data stream has read corrupt data.

QDataStream.WriteFailed

The data stream cannot write to the underlying device.

QDataStream.SizeLimitExceeded

The data stream cannot read or write the data because its size is larger than supported by the current platform. This can happen, for example, when trying to read more that 2 GiB of data on a 32-bit platform.

Constant	Description
QDataStream.Ok	The data stream is operating normally.
QDataStream.ReadPastEnd	The data stream has read past the end of the data in the underlying device.
QDataStream.ReadCorruptData	The data stream has read corrupt data.
QDataStream.WriteFailed	The data stream cannot write to the underlying device.
QDataStream.SizeLimitExceeded	The data stream cannot read or write the data because its size is larger than supported by the current platform. This can happen, for example, when trying to read more that 2 GiB of data on a 32-bit platform.

class FloatingPointPrecision#

The precision of floating point numbers used for reading/writing the data. This will only have an effect if the version of the data stream is Qt_4_6 or higher.

Warning

The floating point precision must be set to the same value on the object that writes and the object that reads the data stream.

Constant	Description
QDataStream.SinglePrecision	All floating point numbers in the data stream have 32-bit precision.
QDataStream.DoublePrecision	All floating point numbers in the data stream have 64-bit precision.