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The QTextStream class provides a convenient interface for reading and writing text. More...
The QTextStream class provides a convenient interface for reading and writing text.
QTextStream can operate on a QIODevice, a QByteArray or a QString. Using QTextStream's streaming operators, you can conveniently read and write words, lines and numbers. For generating text, QTextStream supports formatting options for field padding and alignment, and formatting of numbers. Example:
QFile data("output.txt");
if (data.open(QFile::WriteOnly | QFile::Truncate)) {
QTextStream out(&data);
out << "Result: " << qSetFieldWidth(10) << left << 3.14 << 2.7 << endl;
// writes "Result: 3.14 2.7 \n"
}
It's also common to use QTextStream to read console input and write console output. QTextStream is locale aware, and will automatically decode standard input using the correct codec. Example:
QTextStream stream(stdin); QString line; do { line = stream.readLine(); } while (!line.isNull());
Note that you cannot use QTextStream::atEnd(), which returns true when you have reached the end of the data stream, with stdin.
Besides using QTextStream's constructors, you can also set the device or string QTextStream operates on by calling setDevice or setString(). You can seek to a position by calling seek, and atEnd will return true when there is no data left to be read. If you call flush, QTextStream will empty all data from its write buffer into the device and call flush on the device.
Internally, QTextStream uses a Unicode based buffer, and QTextCodec is used by QTextStream to automatically support different character sets. By default, QTextCodec::codecForLocale() is used for reading and writing, but you can also set the codec by calling setCodec. Automatic Unicode detection is also supported. When this feature is enabled (the default behavior), QTextStream will detect the UTF-16 BOM (Byte Order Mark) and switch to the appropriate UTF-16 codec when reading. QTextStream does not write a BOM by default, but you can enable this by calling setGenerateByteOrderMark(true). When QTextStream operates on a QString directly, the codec is disabled.
There are three general ways to use QTextStream when reading text files:
Since the text stream uses a buffer, you should not read from the stream using the implementation of a superclass. For instance, if you have a QFile and read from it directly using QFile::readLine() instead of using the stream, the text stream's internal position will be out of sync with the file's position.
By default, when reading numbers from a stream of text, QTextStream will automatically detect the number's base representation. For example, if the number starts with "0x", it is assumed to be in hexadecimal form. If it starts with the digits 1-9, it is assumed to be in decimal form, and so on. You can set the integer base, thereby disabling the automatic detection, by calling setIntegerBase. Example:
QTextStream in("0x50 0x20"); int firstNumber, secondNumber; in >> firstNumber; // firstNumber == 80 in >> dec >> secondNumber; // secondNumber == 0 char ch; in >> ch; // ch == 'x'
QTextStream supports many formatting options for generating text. You can set the field width and pad character by calling setFieldWidth and setPadChar. Use setFieldAlignment to set the alignment within each field. For real numbers, call setRealNumberNotation and setRealNumberPrecision to set the notation (SmartNotation, ScientificNotation, FixedNotation) and precision in digits of the generated number. Some extra number formatting options are also available through setNumberFlags.
Like <iostream> in the standard C++ library, QTextStream also defines several global manipulator functions:
In addition, Qt provides three global manipulators that take a parameter: qSetFieldWidth(), qSetPadChar(), and qSetRealNumberPrecision().
See also QDataStream, QIODevice, QFile, QBuffer, QTcpSocket, and Codecs Example.
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