The QIODevice class is the base class of I/O devices.
.PP
An I/O device represents a medium that one can read bytes from and/or write bytes to. The QIODevice class is the abstract superclass of all such devices; classes such as QFile, QBuffer and QSocket inherit QIODevice and implement virtual functions such as write() appropriately.
.PP
Although applications sometimes use QIODevice directly, it is usually better to use QTextStream and QDataStream, which provide stream operations on any QIODevice subclass. QTextStream provides text-oriented stream functionality (for human-readable ASCII files, for example), whereas QDataStream deals with binary data in a totally platform-independent manner.
.PP
The public member functions in QIODevice roughly fall into two groups: the action functions and the state access functions. The most important action functions are:
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open() opens a device for reading and/or writing, depending on the mode argument.
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close() closes the device and tidies up (e.g. flushes buffered data)
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readBlock() reads a block of data from the device.
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writeBlock() writes a block of data to the device.
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readLine() reads a line (of text, usually) from the device.
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flush() ensures that all buffered data are written to the real device.
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There are also some other, less used, action functions:
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getch() reads a single character.
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ungetch() forgets the last call to getch(), if possible.
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putch() writes a single character.
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size() returns the size of the device, if there is one.
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at() returns the current read/write pointer's position, if there is one for this device, or it moves the pointer if given an offset.
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atEnd() indicates whether there is more to read, if this is meaningful for this device.
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reset() moves the read/write pointer to the start of the device, if that is possible for this device.
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The state access are all "get" functions. The QIODevice subclass calls setState() to update the state, and simple access functions tell the user of the device what the device's state is. Here are the settings, and their associated access functions:
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Access type. Some devices are direct access (it is possible to read/write anywhere), whereas others are sequential. QIODevice provides the access functions (isDirectAccess(), isSequentialAccess(), and isCombinedAccess()) to tell users what a given I/O device supports.
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Buffering. Some devices are accessed in raw mode, whereas others are buffered. Buffering usually provides greater efficiency, particularly for small read/write operations. isBuffered() tells the user whether a given device is buffered. (This can often be set by the application in the call to open().)
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Synchronicity. Synchronous devices work immediately (for example, files). When you read from a file, the file delivers its data straight away. Other kinds of device, such as a socket connected to a HTTP server, may not deliver the data until seconds after you ask to read it. isSynchronous() and isAsynchronous() tell the user how this device operates.
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CR/LF translation. For simplicity, applications often like to see just a single CR/LF style, and QIODevice subclasses can provide this. isTranslated() returns TRUE if this object translates CR/LF to just LF. (This can often be set by the application in the call to open().)
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Permissions. Some files cannot be written. For example, isReadable(), isWritable() and isReadWrite() tell the application whether it can read from and write to a given device. (This can often be set by the application in the call to open().)
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Finally, isOpen() returns TRUE if the device is open, i.e. after an open() call.
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QIODevice provides numerous pure virtual functions that you need to implement when subclassing it. Here is a skeleton subclass with all the members you are sure to need and some that you will probably need:
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class MyDevice : public QIODevice
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{
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public:
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MyDevice();
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~MyDevice();
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bool open( int mode );
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void close();
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void flush();
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uint size() const;
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int at() const; // non-pure virtual
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bool at( int ); // non-pure virtual
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bool atEnd() const; // non-pure virtual
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int readBlock( char *data, uint maxlen );
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int writeBlock( const char *data, uint len );
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int readLine( char *data, uint maxlen );
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int getch();
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int putch( int );
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int ungetch( int );
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};
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.fi
.PP
The three non-pure virtual functions need not be reimplemented for sequential devices.
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See also QDataStream, QTextStream, and Input/Output and Networking.
Virtual function that sets the I/O device position to \fIpos\fR. Returns TRUE if the position was successfully set, i.e. \fIpos\fR is within range and the seek was successful; otherwise returns FALSE.
Reads a single byte/character from the I/O device.
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Returns the byte/character read, or -1 if the end of the I/O device has been reached.
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This virtual function must be reimplemented by all subclasses.
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See also putch() and ungetch().
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Reimplemented in QFile and QSocket.
.SH "bool QIODevice::isAsynchronous () const"
Returns TRUE if the device is an asynchronous device; otherwise returns FALSE, i.e. if the device is a synchronous device.
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This mode is currently not in use.
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See also isSynchronous().
.SH "bool QIODevice::isBuffered () const"
Returns TRUE if the I/O device is a buffered device; otherwise returns FALSE, i.e. the device is a raw device.
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See also isRaw().
.SH "bool QIODevice::isCombinedAccess () const"
Returns TRUE if the I/O device is a combined access (both direct and sequential) device; otherwise returns FALSE.
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This access method is currently not in use.
.SH "bool QIODevice::isDirectAccess () const"
Returns TRUE if the I/O device is a direct access device; otherwise returns FALSE, i.e. if the device is a sequential access device.
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See also isSequentialAccess().
.SH "bool QIODevice::isInactive () const"
Returns TRUE if the I/O device state is 0, i.e. the device is not open; otherwise returns FALSE.
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See also isOpen().
.SH "bool QIODevice::isOpen () const"
Returns TRUE if the I/O device has been opened; otherwise returns FALSE.
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See also isInactive().
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Example: network/networkprotocol/nntp.cpp.
.SH "bool QIODevice::isRaw () const"
Returns TRUE if the device is a raw device; otherwise returns FALSE, i.e. if the device is a buffered device.
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See also isBuffered().
.SH "bool QIODevice::isReadWrite () const"
Returns TRUE if the I/O device was opened using IO_ReadWrite mode; otherwise returns FALSE.
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See also isReadable() and isWritable().
.SH "bool QIODevice::isReadable () const"
Returns TRUE if the I/O device was opened using IO_ReadOnly or IO_ReadWrite mode; otherwise returns FALSE.
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See also isWritable() and isReadWrite().
.SH "bool QIODevice::isSequentialAccess () const"
Returns TRUE if the device is a sequential access device; otherwise returns FALSE, i.e. if the device is a direct access device.
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Operations involving size() and at(int) are not valid on sequential devices.
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See also isDirectAccess().
.SH "bool QIODevice::isSynchronous () const"
Returns TRUE if the I/O device is a synchronous device; otherwise returns FALSE, i.e. the device is an asynchronous device.
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See also isAsynchronous().
.SH "bool QIODevice::isTranslated () const"
Returns TRUE if the I/O device translates carriage-return and linefeed characters; otherwise returns FALSE.
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A QFile is translated if it is opened with the IO_Translate mode flag.
.SH "bool QIODevice::isWritable () const"
Returns TRUE if the I/O device was opened using IO_WriteOnly or IO_ReadWrite mode; otherwise returns FALSE.
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See also isReadable() and isReadWrite().
.SH "int QIODevice::mode () const"
Returns bits OR'ed together that specify the current operation mode.
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These are the flags that were given to the open() function.
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The flags are IO_ReadOnly, IO_WriteOnly, IO_ReadWrite, IO_Append, IO_Truncate and IO_Translate.
.SH "bool QIODevice::open ( int mode )\fC [pure virtual]\fR"
Opens the I/O device using the specified \fImode\fR. Returns TRUE if the device was successfully opened; otherwise returns FALSE.
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The mode parameter \fImode\fR must be an OR'ed combination of the following flags. <center>.nf
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l - l. Mode flags Meaning IO_Raw specifies raw (unbuffered) file access. IO_ReadOnly opens a file in read-only mode. IO_WriteOnly opens a file in write-only mode. IO_ReadWrite opens a file in read/write mode. IO_Append sets the file index to the end of the file. IO_Truncate truncates the file. IO_Translate
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</center>
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This virtual function must be reimplemented by all subclasses.
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See also close().
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Example: grapher/grapher.cpp.
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Reimplemented in QFile and QSocket.
.SH "int QIODevice::putch ( int ch )\fC [pure virtual]\fR"
Writes the character \fIch\fR to the I/O device.
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Returns \fIch\fR, or -1 if an error occurred.
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This virtual function must be reimplemented by all subclasses.
Reads a line of text, (or up to \fImaxlen\fR bytes if a newline isn't encountered) plus a terminating '\0' into \fIdata\fR. If there is a newline at the end if the line, it is not stripped.
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Returns the number of bytes read including the terminating '\0', or -1 if an error occurred.
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This virtual function can be reimplemented much more efficiently by the most subclasses.
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See also readBlock() and QTextStream::readLine().
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Reimplemented in QFile.
.SH "bool QIODevice::reset ()"
Sets the device index position to 0.
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See also at().
.SH "void QIODevice::resetStatus ()"
Sets the I/O device status to IO_Ok.
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See also status().
.SH "void QIODevice::setFlags ( int flags )\fC [protected]\fR"
Used by subclasses to set the device flags to the \fIflags\fR specified.
.SH "void QIODevice::setMode ( int mode )\fC [protected]\fR"
Used by subclasses to set the device mode to the \fImode\fR specified.
.SH "void QIODevice::setState ( int state )\fC [protected]\fR"
Used by subclasses to set the device state to the \fIstate\fR specified.
.SH "void QIODevice::setStatus ( int s )\fC [protected]\fR"
Used by subclasses to set the device status (not state) to \fIs\fR.
.SH "void QIODevice::setType ( int type )\fC [protected]\fR"
Used by subclasses to set the device type to the \fItype\fR specified.
Virtual function that returns the size of the I/O device.
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See also at().
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Reimplemented in QFile and QSocket.
.SH "int QIODevice::state () const"
Returns bits OR'ed together that specify the current state.
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The flags are: \fCIO_Open\fR.
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Subclasses may define additional flags.
.SH "int QIODevice::status () const"
Returns the I/O device status.
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The I/O device status returns an error code. If open() returns FALSE or readBlock() or writeBlock() return -1, this function can be called to find out the reason why the operation failed.
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The status codes are: <center>.nf
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l - l. Status code Meaning IO_Ok The operation was successful. IO_ReadError Could not read from the device. IO_WriteError Could not write to the device. IO_FatalError A fatal unrecoverable error occurred. IO_OpenError Could not open the device. IO_ConnectError Could not connect to the device. IO_AbortError The operation was unexpectedly aborted. IO_TimeOutError The operation timed out. IO_UnspecifiedError
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.fi
</center>
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See also resetStatus().
.SH "int QIODevice::ungetch ( int ch )\fC [pure virtual]\fR"
Puts the character \fIch\fR back into the I/O device and decrements the index position if it is not zero.
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This function is normally called to "undo" a getch() operation.
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Returns \fIch\fR, or -1 if an error occurred.
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This virtual function must be reimplemented by all subclasses.
