QDataStream Class Reference
[QtCore module]

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.

 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);

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