QDataStream 类提供把二进制数据序列化到 QIODevice . 更多...
QDataStream 类提供把二进制数据序列化到 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.
还可以使用数据流去读/写 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.
范例 (把二进制数据写入流)：
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 序列化 Qt 数据类型 .
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 * 使用 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 () 和 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.
The Qt container classes can also be serialized to a QDataStream. These include QList , QLinkedList , QVector , QSet , QHash ，和 QMap 。 stream operators are declared as non-members of the 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 类：
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 相关非成员 章节的 class's documentation page.
用于读/写数据的字节序。
| 常量 | 值 | 描述 |
|---|---|---|
| QDataStream.BigEndian | QSysInfo.BigEndian | Most significant byte first (the default) |
| QDataStream.LittleEndian | QSysInfo.LittleEndian | Least significant byte first |
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.
警告： The floating point precision must be set to the same value on the object that writes and the object that reads the data stream.
| 常量 | 值 | 描述 |
|---|---|---|
| QDataStream.SinglePrecision | 0 | All floating point numbers in the data stream have 32-bit precision. |
| QDataStream.DoublePrecision | 1 | All floating point numbers in the data stream have 64-bit precision. |
另请参阅 setFloatingPointPrecision () and floatingPointPrecision ().
This enum describes the current status of the data stream.
| 常量 | 值 | 描述 |
|---|---|---|
| QDataStream.Ok | 0 | The data stream is operating normally. |
| QDataStream.ReadPastEnd | 1 | The data stream has read past the end of the data in the underlying device. |
| QDataStream.ReadCorruptData | 2 | The data stream has read corrupt data. |
| QDataStream.WriteFailed | 3 | The data stream cannot write to the underlying 设备。 |
This enum provides symbolic synonyms for the data serialization format version numbers.
| 常量 | 值 | 描述 |
|---|---|---|
| QDataStream.Qt_1_0 | 1 | Version 1 (Qt 1.x) |
| QDataStream.Qt_2_0 | 2 | Version 2 (Qt 2.0) |
| QDataStream.Qt_2_1 | 3 | Version 3 (Qt 2.1, 2.2, 2.3) |
| QDataStream.Qt_3_0 | 4 | Version 4 (Qt 3.0) |
| QDataStream.Qt_3_1 | 5 | Version 5 (Qt 3.1, 3.2) |
| QDataStream.Qt_3_3 | 6 | Version 6 (Qt 3.3) |
| QDataStream.Qt_4_0 | 7 | Version 7 (Qt 4.0, Qt 4.1) |
| QDataStream.Qt_4_1 | Qt_4_0 | Version 7 (Qt 4.0, Qt 4.1) |
| QDataStream.Qt_4_2 | 8 | Version 8 (Qt 4.2) |
| QDataStream.Qt_4_3 | 9 | Version 9 (Qt 4.3) |
| QDataStream.Qt_4_4 | 10 | Version 10 (Qt 4.4) |
| QDataStream.Qt_4_5 | 11 | Version 11 (Qt 4.5) |
| QDataStream.Qt_4_6 | 12 | Version 12 (Qt 4.6, Qt 4.7, Qt 4.8) |
| QDataStream.Qt_4_7 | Qt_4_6 | Same as Qt_4_6. |
| QDataStream.Qt_4_8 | Qt_4_7 Qt_4_9 = Qt_4_8 | Same as Qt_4_6. |
另请参阅 setVersion () 和 version ().
Constructs a data stream that has no I/O device.
另请参阅 setDevice ().
Constructs a data stream that uses the I/O device d .
警告： 若使用 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.
另请参阅 setDevice () 和 device ().
Constructs a data stream that operates on a byte array, a 。 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.
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.
另请参阅 QIODevice.atEnd ().
Returns the current byte order setting -- either BigEndian or LittleEndian .
另请参阅 setByteOrder ().
Returns the I/O device currently set, or 0 if no device is currently set.
另请参阅 setDevice ().
Returns the floating point precision of the data stream.
该函数在 Qt 4.6 引入。
另请参阅 FloatingPointPrecision and setFloatingPointPrecision ().
Reads the buffer s from the stream and returns a reference to the stream.
The buffer s is allocated using new . Destroy it 采用 delete[] operator.
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.
另请参阅 readRawData () 和 writeBytes ().
读取最多 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.
另请参阅 readBytes (), QIODevice.read ()，和 writeRawData ().
Resets the status of the data stream.
另请参阅 状态 , status ()，和 setStatus ().
Sets the serialization byte order to bo .
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.
另请参阅 byteOrder ().
void QDataStream.setDevice( QIODevice *d)
Sets the I/O device to d , which can be 0 to unset to current I/O device.
另请参阅 device ().
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.
默认为 DoublePrecision .
警告： This property must be set to the same value on the object that writes and the object that reads the data stream.
该函数在 Qt 4.6 引入。
另请参阅 floatingPointPrecision ().
Sets the status of the data stream to the status 给定。
Subsequent calls to setStatus() are ignored until resetStatus () 被调用。
另请参阅 状态 , status ()，和 resetStatus ().
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 版本化 in the Detailed Description.
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 版本 |
|---|---|
| Qt 4.6 | 12 |
| Qt 4.5 | 11 |
| 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 |
版本 enum provides symbolic constants for the different versions of Qt. For example:
QDataStream out(file); out.setVersion(QDataStream.Qt_4_0);
Skips len bytes from the device. Returns the number of bytes actually skipped, or -1 on error.
这相当于调用 readRawData () on a buffer of length len and ignoring the buffer.
该函数在 Qt 4.1 引入。
另请参阅 QIODevice.seek ().
Returns the status of the data stream.
另请参阅 状态 , setStatus ()，和 resetStatus ().
Returns the version number of the data serialization format.
另请参阅 setVersion () 和 版本 .
Writes the length specifier len and the buffer s to the stream and returns a reference to the stream.
len is serialized as a quint32, followed by len 字节来自 s . Note that the data is not encoded.
另请参阅 writeRawData () 和 readBytes ().
写入 len 字节来自 s to the stream. Returns the number of bytes actually written, or -1 on error. The data is not encoded.
另请参阅 writeBytes (), QIODevice.write ()，和 readRawData ().
This method is only available if the QtGui module is imported.
This method is only available if the QtNetwork module is imported.
This method is only available if the QtScript module is imported.
This method is only available if the QtWebKit module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtNetwork module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtNetwork module is imported.
This method is only available if the QtScript module is imported.
This method is only available if the QtWebKit module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtNetwork module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
This method is only available if the QtGui module is imported.
