com.trolltech.qt.core
Class QDataStream

java.lang.Object
  extended by com.trolltech.qt.internal.QSignalEmitterInternal
      extended by com.trolltech.qt.QSignalEmitter
          extended by com.trolltech.qt.QtJambiObject
              extended by com.trolltech.qt.core.QDataStream
All Implemented Interfaces:
QtJambiInterface

public class QDataStream
extends QtJambiObject

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 = new QFile("file.dat");
        QFile.OpenMode mode = new QFile.OpenMode();
        mode.set(QFile.OpenModeFlag.WriteOnly);
        file.open(mode);
        QDataStream out = new QDataStream(file);   // we will serialize the data into the file
        out.writeString("the answer is");   // serialize a string
        out.writeInt(42);        // serialize an integer
Example (read binary data from a stream):
        QFile file = new QFile("file.dat");
        QFile.OpenMode mode = new QFile.OpenMode();
        mode.set(QFile.OpenModeFlag.ReadOnly);
        file.open(mode);
        QDataStream in = new QDataStream(file);    // read the data serialized from the file
        // extract "the answer is" and 42
        String str = in.readString();
        int a = in.readInt();
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.Version.Qt_4_0.value());
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 = new QFile("file.xxx");
        QFile.OpenMode mode = new QFile.OpenMode();
        mode.set(QFile.OpenModeFlag.WriteOnly);
        file.open(mode);
        QDataStream out = new QDataStream(file);

        // Write a header with a "magic number" and a version
        out.writeInt(0xA0B0C0D0);
        out.writeInt(123);

        out.setVersion(QDataStream.Version.Qt_4_0.value());

        // Write the data
        out.writeBytes(lots_of_interesting_data.toByteArray());
Then read it in with:
        QFile file = new QFile("file.xxx");
        QFile.OpenMode mode = new QFile.OpenMode();
        mode.set(QFile.OpenModeFlag.ReadOnly);
        file.open(mode);
        QDataStream in = new QDataStream(file);

        // Read and check the header
        int magic = in.readInt();
        if (magic != 0xA0B0C0D0)
            return XXX_BAD_FILE_FORMAT;

        // Read the version
        int version = in.readInt();
        if (version < 100)
            return XXX_BAD_FILE_TOO_OLD;
        if (version > 123)
            return XXX_BAD_FILE_TOO_NEW;

        if (version <= 110)
            in.setVersion(QDataStream.Version.Qt_3_1.value());
        else
            in.setVersion(QDataStream.Version.Qt_4_0.value());

        // Read the data
        QByteArray lots_of_interesting_data = new QByteArray();
        QByteArray data_new_in_XXX_version_1_2 = new QByteArray();
        QByteArray other_interesting_data = new QByteArray();

        lots_of_interesting_data.readFrom(in);
        if (version >= 120)
            data_new_in_XXX_version_1_2.readFrom(in);
        other_interesting_data.readFrom(in);
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.


Nested Class Summary
static class QDataStream.Status
          This enum describes the current status of the data stream.
static class QDataStream.Version
          This enum provides symbolic synonyms for the data serialization format version numbers.
 
Nested classes/interfaces inherited from class com.trolltech.qt.QSignalEmitter
QSignalEmitter.AbstractSignal, QSignalEmitter.PrivateSignal0, QSignalEmitter.PrivateSignal1, QSignalEmitter.PrivateSignal2, QSignalEmitter.PrivateSignal3, QSignalEmitter.PrivateSignal4, QSignalEmitter.PrivateSignal5, QSignalEmitter.PrivateSignal6, QSignalEmitter.PrivateSignal7, QSignalEmitter.PrivateSignal8, QSignalEmitter.PrivateSignal9, QSignalEmitter.Signal0, QSignalEmitter.Signal1, QSignalEmitter.Signal2, QSignalEmitter.Signal3, QSignalEmitter.Signal4, QSignalEmitter.Signal5, QSignalEmitter.Signal6, QSignalEmitter.Signal7, QSignalEmitter.Signal8, QSignalEmitter.Signal9
 
Nested classes/interfaces inherited from class com.trolltech.qt.internal.QSignalEmitterInternal
com.trolltech.qt.internal.QSignalEmitterInternal.AbstractSignalInternal
 
Field Summary
 
Fields inherited from class com.trolltech.qt.internal.QSignalEmitterInternal
currentSender
 
Constructor Summary
QDataStream()
          Constructs a data stream that has no I/O device.
QDataStream(QByteArray arg__1)
          Constructs a read-only data stream that operates on byte array a.
QDataStream(QByteArray a, QIODevice.OpenMode mode)
          Constructs a data stream that operates on a byte array, a.
QDataStream(QByteArray a, QIODevice.OpenModeFlag[] mode)
          Constructs a data stream that operates on a byte array, a.
QDataStream(QIODevice arg__1)
          Constructs a data stream that uses the I/O device d.
 
Method Summary
 boolean atEnd()
          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.
 QIODevice device()
          Returns the I/O device currently set.
 boolean readBoolean()
          This function reads a boolean from the stream.
 byte readByte()
          This function read a byte from the stream.
 int readBytes(byte[] buffer)
          Reads data from the stream and puts the data in buffer.
 int readBytes(byte[] buffer, int length)
          Reads length bytes from the stream, and puts them in buffer.
 char readChar()
          Reads a character from the stream.
 double readDouble()
          This function read a double from the stream.
 float readFloat()
          This function reads a float from the stream.
 int readInt()
          This function reads an int from the stream.
 long readLong()
          This function reads a long from the stream.
 short readShort()
          This function reads a short from the stream.
 java.lang.String readString()
          This function reads a string from the stream.
 void resetStatus()
          Resets the status of the data stream.
 void setDevice(QIODevice arg__1)
          void QDataStream.:setDevice(QIODevice *d)
 void setStatus(QDataStream.Status status)
          Sets the status of the data stream to the status given.
 void setVersion(int arg__1)
          Sets the version number of the data serialization format to v.
 int skipRawData(int len)
          Skips len bytes from the device.
 QDataStream.Status status()
          Returns the status of the data stream.
 void unsetDevice()
          Unsets the I/O device.
 int version()
          Returns the version number of the data serialization format.
 QDataStream writeBoolean(boolean i)
          Writes a boolean value, i, to the stream.
 QDataStream writeByte(byte i)
          Writes the byte i to the stream.
 int writeBytes(byte[] buffer)
          Writes the bytes in data to the stream, This function returns the bytes written or -1 if an error occurred.
 int writeBytes(byte[] buffer, int length)
          Writes length bytes from data.
 QDataStream writeDouble(double f)
          This is an overloaded member function, provided for convenience.
 QDataStream writeFloat(float f)
          This is an overloaded member function, provided for convenience.
 QDataStream writeInt(int i)
          Writes a signed byte, i, to the stream and returns a reference to the stream.
 QDataStream writeLong(long i)
          This is an overloaded member function, provided for convenience.
 QDataStream writeShort(short s)
          This function writes the short s/tt> to the stream.
 void writeString(java.lang.String string)
          This function writes the short s/tt> to the stream.
 
Methods inherited from class com.trolltech.qt.QtJambiObject
dispose, disposed, equals, finalize, reassignNativeResources, tr, tr, tr
 
Methods inherited from class com.trolltech.qt.QSignalEmitter
blockSignals, disconnect, disconnect, signalsBlocked, signalSender, thread
 
Methods inherited from class com.trolltech.qt.internal.QSignalEmitterInternal
__qt_signalInitialization
 
Methods inherited from class java.lang.Object
clone, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 
Methods inherited from interface com.trolltech.qt.QtJambiInterface
disableGarbageCollection, nativeId, nativePointer, reenableGarbageCollection, setJavaOwnership
 

Constructor Detail

QDataStream

public QDataStream()
Constructs a data stream that has no I/O device.

See also:
setDevice().


QDataStream

public QDataStream(QIODevice arg__1)
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

public QDataStream(QByteArray arg__1)
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

public 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

public QDataStream(QByteArray a,
                   QIODevice.OpenModeFlag[] 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.

Method Detail

atEnd

public final boolean atEnd()
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().


device

public final QIODevice device()
Returns the I/O device currently set.

See also:
setDevice(), and unsetDevice().


writeBoolean

public final QDataStream writeBoolean(boolean i)
Writes a boolean value, i, to the stream. Returns a reference to the stream.


writeDouble

public final QDataStream writeDouble(double f)
This is an overloaded member function, provided for convenience.

Writes a 64-bit floating point number, f, to the stream using the standard IEEE 754 format. Returns a reference to the stream.


writeFloat

public final QDataStream writeFloat(float f)
This is an overloaded member function, provided for convenience.

Writes a 32-bit floating point number, f, to the stream using the standard IEEE 754 format. Returns a reference to the stream.


writeInt

public final QDataStream writeInt(int i)
Writes a signed byte, i, to the stream and returns a reference to the stream.


writeLong

public final QDataStream writeLong(long i)
This is an overloaded member function, provided for convenience.

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


writeByte

public final QDataStream writeByte(byte i)
Writes the byte i to the stream. i


resetStatus

public final void resetStatus()
Resets the status of the data stream.

See also:
Status , status(), and setStatus().


setDevice

public final void setDevice(QIODevice arg__1)
void QDataStream.:setDevice(QIODevice *d)

Sets the I/O device to d.

See also:
device(), and unsetDevice().


setStatus

public final void setStatus(QDataStream.Status status)
Sets the status of the data stream to the status given.

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


setVersion

public final void setVersion(int arg__1)
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.

Qt Version
QDataStream Version
Qt 4.4 10
Qt 4.3 9
Qt 4.2 8
Qt 4.0, 4.1 7
Qt 3.3 6
Qt 3.1, 3.2 5
Qt 3.0 4
Qt 2.1, 2.2, 2.3 3
Qt 2.0 2
Qt 1.x 1
The Version enum provides symbolic constants for the different versions of Qt. For example:
        QDataStream out = new QDataStream(file);
        out.setVersion(QDataStream.Version.Qt_4_0.value());

See also:
version(), and Version .


skipRawData

public final int 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.

See also:
QIODevice::seek().


status

public final QDataStream.Status status()
Returns the status of the data stream.

See also:
Status , setStatus(), and resetStatus().


unsetDevice

public final void unsetDevice()
Unsets the I/O device. Use setDevice(0) instead.


version

public final int version()
Returns the version number of the data serialization format.

See also:
setVersion(), and Version .


readBoolean

public final boolean readBoolean()
This function reads a boolean from the stream.


readByte

public final byte readByte()
This function read a byte from the stream.


readShort

public final short readShort()
This function reads a short from the stream.


readInt

public final int readInt()
This function reads an int from the stream.


readLong

public final long readLong()
This function reads a long from the stream.


readChar

public final char readChar()
Reads a character from the stream.


readFloat

public final float readFloat()
This function reads a float from the stream.


readDouble

public final double readDouble()
This function read a double from the stream.


writeShort

public final QDataStream writeShort(short s)
This function writes the short s/tt> to the stream. It returns a reference to the stream after it has been read from.


readString

public final java.lang.String readString()
This function reads a string from the stream.


writeString

public final void writeString(java.lang.String string)
This function writes the short s/tt> to the stream. It returns a reference to the stream after it has been read from. string


writeBytes

public final int writeBytes(byte[] buffer)
Writes the bytes in data to the stream, This function returns the bytes written or -1 if an error occurred.


writeBytes

public final int writeBytes(byte[] buffer,
                            int length)
Writes length bytes from data. The function returns the number of bytes written or -1 if an error occurred.


readBytes

public final int readBytes(byte[] buffer)
Reads data from the stream and puts the data in buffer. It returns the number of bytes read or -1 if an error occurred.


readBytes

public final int readBytes(byte[] buffer,
                           int length)
Reads length bytes from the stream, and puts them in buffer. The method returns the number of bytes read or -1 if an error occurred.