QDataStream

Section: Misc. Reference Manual Pages (3qt)
Updated: 2 February 2007
Index Return to Main Contents
 

NAME

QDataStream - Serialization of binary data to a QIODevice  

SYNOPSIS

All the functions in this class are reentrant when Qt is built with thread support.</p>

#include <qdatastream.h>

 

Public Members


QDataStream ()

QDataStream ( QIODevice * d )

QDataStream ( QByteArray a, int mode )

virtual ~QDataStream ()

QIODevice * device () const

void setDevice ( QIODevice * d )

void unsetDevice ()

bool atEnd () const

bool eof () const (obsolete)

enum ByteOrder { BigEndian, LittleEndian }

int byteOrder () const

void setByteOrder ( int bo )

bool isPrintableData () const

void setPrintableData ( bool enable )

int version () const

void setVersion ( int v )

QDataStream & operator>> ( Q_INT8 & i )

QDataStream & operator>> ( Q_UINT8 & i )

QDataStream & operator>> ( Q_INT16 & i )

QDataStream & operator>> ( Q_UINT16 & i )

QDataStream & operator>> ( Q_INT32 & i )

QDataStream & operator>> ( Q_UINT32 & i )

QDataStream & operator>> ( Q_INT64 & i )

QDataStream & operator>> ( Q_UINT64 & i )

QDataStream & operator>> ( Q_LONG & i )

QDataStream & operator>> ( Q_ULONG & i )

QDataStream & operator>> ( float & f )

QDataStream & operator>> ( double & f )

QDataStream & operator>> ( char *& s )

QDataStream & operator<< ( Q_INT8 i )

QDataStream & operator<< ( Q_UINT8 i )

QDataStream & operator<< ( Q_INT16 i )

QDataStream & operator<< ( Q_UINT16 i )

QDataStream & operator<< ( Q_INT32 i )

QDataStream & operator<< ( Q_UINT32 i )

QDataStream & operator<< ( Q_INT64 i )

QDataStream & operator<< ( Q_UINT64 i )

QDataStream & operator<< ( Q_LONG i )

QDataStream & operator<< ( Q_ULONG i )

QDataStream & operator<< ( float f )

QDataStream & operator<< ( double f )

QDataStream & operator<< ( const char * s )

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

QDataStream & readRawBytes ( char * s, uint len )

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

QDataStream & writeRawBytes ( const char * s, uint len )
 

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 IO device.

Example (write binary data to a stream): 



    QFile file( "file.dat" );


    file.open( IO_WriteOnly );


    QDataStream stream( &file ); // we will serialize the data into the file


    stream << "the answer is";   // serialize a string


    stream << (Q_INT32)42;       // serialize an integer

Example (read binary data from a stream): 



    QFile file( "file.dat" );


    file.open( IO_ReadOnly );


    QDataStream stream( &file );  // read the data serialized from the file


    QString str;


    Q_INT32 a;


    stream >> 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 NUL byte ('&#92;0'), followed by all the characters of the string including the NUL 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 NUL are read.

The initial IODevice 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 IODevice set) atEnd() will return TRUE.

If you want the data to be compatible with an earlier version of Qt use setVersion().

If you want the data to be human-readable, e.g. for debugging, you can set the data stream into printable data mode with setPrintableData(). The data is then written slower, in a bloated but human readable format.

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( IO_WriteOnly );


    QDataStream stream( &file );




    // Write a header with a "magic number" and a version


    stream << (Q_UINT32)0xA0B0C0D0;


    stream << (Q_INT32)123;




    // Write the data


    stream << [lots of interesting data]

Then read it in with: 



    QFile file( "file.xxx" );


    file.open( IO_ReadOnly );


    QDataStream stream( &file );




    // Read and check the header


    Q_UINT32 magic;


    stream >> magic;


    if ( magic != 0xA0B0C0D0 )


        return XXX_BAD_FILE_FORMAT;




    // Read the version


    Q_INT32 version;


    stream >> version;


    if ( version < 100 )


        return XXX_BAD_FILE_TOO_OLD;


    if ( version > 123 )


        return XXX_BAD_FILE_TOO_NEW;


    if ( version <= 110 )


        stream.setVersion(1);




    // Read the data


    stream >> [lots of interesting data];


    if ( version > 120 )


        stream >> [data new in XXX version 1.2];


    stream >> [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 readRawBytes(). Similarly data can be written to the stream using writeRawBytes(). Notice 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 Q_UINT32 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 Q_UINT32 containing the length of the data, followed by the data. Notice that any encoding/decoding of the data (apart from the length Q_UINT32) must be done by you.

See also QTextStream, QVariant, and Input/Output and Networking.  

Member Type Documentation

 

QDataStream::ByteOrder

The byte order used for reading/writing the data.
QDataStream::BigEndian - the default
QDataStream::LittleEndian
 

MEMBER FUNCTION DOCUMENTATION

 

QDataStream::QDataStream ()

Constructs a data stream that has no IO device.

See also setDevice().  

QDataStream::QDataStream ( QIODevice * d )

Constructs a data stream that uses the IO device d.

Warning: If you use QSocket or QSocketDevice as the IO 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, int mode )

Constructs a data stream that operates on a byte array, a, through an internal QBuffer device. The mode is a QIODevice::mode(), usually either IO_ReadOnly or IO_WriteOnly.

Example: 



    static char bindata[] = { 231, 1, 44, ... };


    QByteArray a;


    a.setRawData( bindata, sizeof(bindata) );   // a points to bindata


    QDataStream stream( a, IO_ReadOnly );       // open on a's data


    stream >> [something];                      // read raw bindata


    a.resetRawData( bindata, sizeof(bindata) ); // finished

The QByteArray::setRawData() function is not for the inexperienced.  

QDataStream::~QDataStream () [virtual]

Destroys the data stream.

The destructor will not affect the current IO device, unless it is an internal IO device processing a QByteArray passed in the constructor, in which case the internal IO device is destroyed.  

bool QDataStream::atEnd () const

Returns TRUE if the IO device has reached the end position (end of the stream or file) or if there is no IO device set; otherwise returns FALSE, i.e. if the current position of the IO device is before the end position.

See also QIODevice::atEnd().  

int QDataStream::byteOrder () const

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

See also setByteOrder().  

QIODevice * QDataStream::device () const

Returns the IO device currently set.

See also setDevice() and unsetDevice().  

bool QDataStream::eof () const

This function is obsolete. It is provided to keep old source working. We strongly advise against using it in new code.

Returns TRUE if the IO device has reached the end position (end of stream or file) or if there is no IO device set.

Returns FALSE if the current position of the read/write head of the IO device is somewhere before the end position.

See also QIODevice::atEnd().  

bool QDataStream::isPrintableData () const

Returns TRUE if the printable data flag has been set; otherwise returns FALSE.

See also setPrintableData().  

QDataStream & QDataStream::operator<< ( Q_INT8 i )

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

QDataStream & QDataStream::operator<< ( Q_UINT8 i )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

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

QDataStream & QDataStream::operator<< ( Q_INT16 i )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

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

QDataStream & QDataStream::operator<< ( Q_UINT16 i )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

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

QDataStream & QDataStream::operator<< ( Q_INT32 i )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

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

QDataStream & QDataStream::operator<< ( Q_UINT32 i )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Writes an unsigned integer, i, to the stream as a 32-bit unsigned integer (Q_UINT32). Returns a reference to the stream.  

QDataStream & QDataStream::operator<< ( Q_INT64 i )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

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

QDataStream & QDataStream::operator<< ( Q_UINT64 i )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

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

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

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Writes a signed integer i, of the system's word length, to the stream and returns a reference to the stream.  

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

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Writes an unsigned integer i, of the system's word length, to the stream and returns a reference to the stream.  

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

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Writes a 32-bit floating point number, f, to the stream using the standard IEEE754 format. Returns a reference to the stream.  

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

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Writes a 64-bit floating point number, f, to the stream using the standard IEEE754 format. Returns a reference to the stream.  

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

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Writes the '&#92;0'-terminated string s to the stream and returns a reference to the stream.

The string is serialized using writeBytes().  

QDataStream & QDataStream::operator>> ( Q_INT8 & i )

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

QDataStream & QDataStream::operator>> ( Q_UINT8 & i )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

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

QDataStream & QDataStream::operator>> ( Q_INT16 & i )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

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

QDataStream & QDataStream::operator>> ( Q_UINT16 & i )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

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

QDataStream & QDataStream::operator>> ( Q_INT32 & i )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

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

QDataStream & QDataStream::operator>> ( Q_UINT32 & i )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

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

QDataStream & QDataStream::operator>> ( Q_INT64 & i )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

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

QDataStream & QDataStream::operator>> ( Q_UINT64 & i )

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

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

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

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Reads a signed integer of the system's word length from the stream into i, and returns a reference to the stream.  

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

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Reads an unsigned integer of the system's word length from the stream, into i, and returns a reference to the stream.  

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

This is an overloaded member function, provided for convenience. It behaves essentially like the above function.

Reads a 32-bit floating point number from the stream into f, using the standard IEEE754 format