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java.lang.Objectcom.trolltech.qt.QSignalEmitter
com.trolltech.qt.QtJambiObject
com.trolltech.qt.core.QDataStream
public class QDataStream
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 << "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.
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.
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.
Nested Class Summary | |
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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 |
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QSignalEmitter.Signal0, QSignalEmitter.Signal1<A>, QSignalEmitter.Signal2<A,B>, QSignalEmitter.Signal3<A,B,C>, QSignalEmitter.Signal4<A,B,C,D>, QSignalEmitter.Signal5<A,B,C,D,E>, QSignalEmitter.Signal6<A,B,C,D,E,F>, QSignalEmitter.Signal7<A,B,C,D,E,F,G>, QSignalEmitter.Signal8<A,B,C,D,E,F,G,H>, QSignalEmitter.Signal9<A,B,C,D,E,F,G,H,I> |
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 arg__1. |
|
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 arg__1. |
Method Summary | |
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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. |
static QDataStream |
fromNativePointer(QNativePointer nativePointer)
This function returns the QDataStream instance pointed to by nativePointer |
boolean |
readBoolean()
This function reads a boolean from the stream. |
byte |
readByte()
This function read a byte from the stream. |
int |
readBytes(byte[] buffer)
This function reads bytes from the stream into buffer. |
int |
readBytes(byte[] buffer,
int length)
This function reads length bytes from the stream into buffer. |
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 arg__1. |
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)
This function writes the bytes in buffer to the stream. |
int |
writeBytes(byte[] buffer,
int length)
This function writes the length bytes of buffer to the stream. |
QDataStream |
writeDouble(double f)
Writes a 64-bit floating point number, f, to the stream using the standard IEEE 754 format. |
QDataStream |
writeFloat(float f)
Writes a 32-bit floating point number, f, to the stream using the standard IEEE 754 format. |
QDataStream |
writeInt(int i)
Writes a signed 32-bit integer, i, to the stream and returns a reference to the stream. |
QDataStream |
writeLong(long i)
Writes a signed 64-bit integer, i, to the stream and returns a reference to the stream. |
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 |
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dispose, disposed, finalize, reassignNativeResources, tr, tr, tr |
Methods inherited from class com.trolltech.qt.QSignalEmitter |
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blockSignals, disconnect, disconnect, signalsBlocked, signalSender, thread |
Methods inherited from class java.lang.Object |
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clone, equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
Methods inherited from interface com.trolltech.qt.QtJambiInterface |
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disableGarbageCollection, nativeId, nativePointer, reenableGarbageCollection, setJavaOwnership |
Constructor Detail |
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public QDataStream(QIODevice arg__1)
Constructs a data stream that uses the I/O device arg__1.
Warning: If you use QSocket or QSocketDevice as the I/O device arg__1 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.
public QDataStream(QByteArray arg__1)
Constructs a read-only data stream that operates on byte array arg__1. 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.
public QDataStream()
Constructs a data stream that has no I/O device.
public QDataStream(QByteArray a, QIODevice.OpenMode mode)
public QDataStream(QByteArray a, QIODevice.OpenModeFlag... mode)
Method Detail |
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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.
public final QIODevice device()
Returns the I/O device currently set.
public final QDataStream writeFloat(float f)
Writes a 32-bit floating point number, f, to the stream using the standard IEEE 754 format. Returns a reference to the stream.
public final QDataStream writeInt(int i)
Writes a signed 32-bit integer, i, to the stream and returns a reference to the stream.
public final QDataStream writeBoolean(boolean i)
Writes a boolean value, i, to the stream. Returns a reference to the stream.
public final QDataStream writeLong(long i)
Writes a signed 64-bit integer, i, to the stream and returns a reference to the stream.
public final QDataStream writeByte(byte i)
public final QDataStream writeDouble(double f)
Writes a 64-bit floating point number, f, to the stream using the standard IEEE 754 format. Returns a reference to the stream.
public final void resetStatus()
Resets the status of the data stream.
public final void setDevice(QIODevice arg__1)
void QDataStream::setDevice(QIODevice *d)
Sets the I/O device to arg__1.
public final void setStatus(QDataStream.Status status)
Sets the status of the data stream to the status given.
public final void setVersion(int arg__1)
Sets the version number of the data serialization format to arg__1.
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 4.2 | 8 |
Qt 4.0 | 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(file); out.setVersion(QDataStream::Qt_4_0);
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.
public final QDataStream.Status status()
Returns the status of the data stream.
public final void unsetDevice()
Unsets the I/O device. This is the same as calling setDevice(0).
public final int version()
Returns the version number of the data serialization format.
public static QDataStream fromNativePointer(QNativePointer nativePointer)
nativePointer
- the QNativePointer of which object should be returned.public final boolean readBoolean()
public final byte readByte()
public final short readShort()
public final int readInt()
public final long readLong()
public final float readFloat()
public final double readDouble()
public final QDataStream writeShort(short s)
public final java.lang.String readString()
public final void writeString(java.lang.String string)
public final int writeBytes(byte[] buffer)
public final int writeBytes(byte[] buffer, int length)
public final int readBytes(byte[] buffer)
public final int readBytes(byte[] buffer, int length)
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