File and Datastream Functions¶
QIODeviceclass is the base interface class of all I/O devices in Qt Core .
QIODeviceprovides both a common implementation and an abstract interface for devices that support reading and writing of blocks of data. The device can be a memory buffer, a file, or a datastream.
Some subclasses like
QFilehave been implemented using a memory buffer for intermediate storing of data. This speeds up programs by reducing read/write operations. Buffering makes functions like
putChar()fast, as they can operate on the memory buffer instead of directly on the device itself.QBuffer buffer; char ch; buffer.open(QBuffer::ReadWrite); buffer.write("Qt rocks!"); buffer.seek(0); buffer.getChar(&ch); // ch == 'Q' buffer.getChar(&ch); // ch == 't' buffer.getChar(&ch); // ch == ' ' buffer.getChar(&ch); // ch == 'r'
open()to open the buffer. Then call
putChar()to write to the buffer, and
getChar()to read from it.
size()returns the current size of the buffer, and you can seek to arbitrary positions in the buffer by calling
seek(). When you are done with accessing the buffer, call
QDataStreamclass provides serialization of binary data to a
QIODevice. A data stream is a binary stream of encoded information which is 100% inde- pendent 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.
QTextStreamclass provides a convenient interface for reading and writing text.
QTextStreamcan operate on a
QTextStream‘s streaming operators, you can conveniently read and write words, lines and numbers. It’s also common to use
QTextStreamto read console input and write console output.
There are three general ways to use
QTextStreamwhen reading text files:
Chunk by chunk, by calling
Character by character, by streaming into
QCharor char types. This method is often used for convenient input handling when parsing files, independent of character encoding and end-of-line semantics. To skip white space, call
QByteArraycan be used to store both raw bytes (including
\0) and traditional 8-bit ‘\0’-terminated strings. Using
QByteArrayis much more convenient than using const char *. It always ensures that the data is followed by a ‘\0’ terminator, and uses implicitly shared classes (copy-on-write) to reduce memory usage and avoid needless copying of data.
In addition to
QByteArray, Qt also provides the
QStringclass to store string data. For most purposes,
QStringis the most appropriate class to use. It stores 16-bit Unicode characters. It is, however, a good idea to use
QByteArraywhen you need to store raw binary data, and when memory conservation is critical (for example, with Qt for Embedded Linux).
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