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QDataStream Class Reference
[QtCore module]

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

 #include <QDataStream>

Note: All functions in this class are reentrant.

• List of all members, including inherited members
• Obsolete members
• Qt 3 support members

Public Types

Public Functions

Detailed Description

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

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):

 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

Example (read binary data from a stream):

 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

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 the Format of the QDataStream operators.

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.

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:

 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;

Then read it in with:

 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;

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 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:

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

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

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

See also QTextStream and QVariant.

Member Type Documentation

enum QDataStream::ByteOrder

The byte order used for reading/writing the data.

enum QDataStream::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.

See also setFloatingPointPrecision() and floatingPointPrecision().

enum QDataStream::Status

This enum describes the current status of the data stream.

enum QDataStream::Version

This enum provides symbolic synonyms for the data serialization format version numbers.

See also setVersion() and version().

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.

Warning: If you use QSocket or QSocketDevice as the I/O device d for reading data, you must make sure that enough data is available on the socket for the operation to successfully proceed; QDataStream does not have any means to handle or recover from short-reads.

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 ()   [virtual]

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.

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().

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::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.

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

void QDataStream::setVersion ( int v )

Sets the version number of the data serialization format to v.

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.

In order 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().

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<< ( bool i )

Writes a boolean value, i, to the stream. 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<< ( 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<< ( 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<< ( 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<< ( 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<< ( 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<< ( 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<< ( 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().

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

Reads a signed byte 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>> ( 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>> ( 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>> ( 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>> ( 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>> ( 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>> ( 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>> ( 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>> ( 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.

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