QIODevice Class Reference

Detailed Description

The QIODevice class is the base class of I/O devices.

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.

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.

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:

• open() opens a device for reading and/or writing, depending on the mode argument.

• close() closes the device and tidies up (e.g. flushes buffered data)

• readBlock() reads a block of data from the device.

• writeBlock() writes a block of data to the device.

• readLine() reads a line (of text, usually) from the device.

• flush() ensures that all buffered data are written to the real device.

There are also some other, less used, action functions:

• getch() reads a single character.

• ungetch() forgets the last call to getch(), if possible.

• putch() writes a single character.

• size() returns the size of the device, if there is one.

• 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.

• atEnd() indicates whether there is more to read, if this is meaningful for this device.

• reset() moves the read/write pointer to the start of the device, if that is possible for this device.

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:

• 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.

• 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().)

• 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.

• 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().)

• 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().)

• Finally, isOpen() returns TRUE if the device is open, i.e. after an open() call.

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:

    class MyDevice : public QIODevice
    {
    public:
        MyDevice();
        ~MyDevice();

        bool open( int mode );
        void close();
        void flush();

        uint size() const;
        int  at() const;        // non-pure virtual
        bool at( int );         // non-pure virtual
        bool atEnd() const;     // non-pure virtual

        int readBlock( char *data, uint maxlen );
        int writeBlock( const char *data, uint len );
        int readLine( char *data, uint maxlen );

        int getch();
        int putch( int );
        int ungetch( int );
    };
    

The three non-pure virtual functions need not be reimplemented for sequential devices.

See also QDataStream, QTextStream and Input/Output and Networking.

Member Type Documentation

QIODevice::Offset

The offset within the device.

Member Function Documentation

QIODevice::QIODevice ()

QIODevice::~QIODevice () [virtual]

Offset QIODevice::at () const [virtual]

This is the position of the data read/write head of the I/O device.

See also size().

Reimplemented in QSocket.

bool QIODevice::at ( Offset pos ) [virtual]

See also size().

Reimplemented in QFile and QSocket.

bool QIODevice::atEnd () const [virtual]

Reimplemented in QFile and QSocket.

void QIODevice::close () [pure virtual]

Closes the I/O device.

This virtual function must be reimplemented by all subclasses.

See also open().

Example: grapher/grapher.cpp.

Reimplemented in QFile and QSocket.

int QIODevice::flags () const

Returns the current I/O device flags setting.

Flags consists of mode flags and state flags.

See also mode() and state().

void QIODevice::flush () [pure virtual]

Flushes an open I/O device.

This virtual function must be reimplemented by all subclasses.

Reimplemented in QFile and QSocket.

int QIODevice::getch () [pure virtual]

Reads a single byte/character from the I/O device.

Returns the byte/character read, or -1 if the end of the I/O device has been reached.

This virtual function must be reimplemented by all subclasses.

See also putch() and ungetch().

Reimplemented in QFile and QSocket.

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.

This mode is currently not in use.

See also isSynchronous().

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.

See also isRaw().

bool QIODevice::isCombinedAccess () const

Returns TRUE if the I/O device is a combined access (both direct and sequential) device; otherwise returns FALSE.

This access method is currently not in use.

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.

See also isSequentialAccess().

bool QIODevice::isInactive () const

Returns TRUE if the I/O device state is 0, i.e. the device is not open; otherwise returns FALSE.

See also isOpen().

bool QIODevice::isOpen () const

Returns TRUE if the I/O device has been opened; otherwise returns FALSE.

See also isInactive().

Example: network/networkprotocol/nntp.cpp.

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.

See also isBuffered().

bool QIODevice::isReadWrite () const

Returns TRUE if the I/O device was opened using IO_ReadWrite mode; otherwise returns FALSE.

See also isReadable() and isWritable().

bool QIODevice::isReadable () const

Returns TRUE if the I/O device was opened using IO_ReadOnly or IO_ReadWrite mode; otherwise returns FALSE.

See also isWritable() and isReadWrite().

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.

Operations involving size() and at(int) are not valid on sequential devices.

See also isDirectAccess().

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.

See also isAsynchronous().

bool QIODevice::isTranslated () const

Returns TRUE if the I/O device translates carriage-return and linefeed characters; otherwise returns FALSE.

A QFile is translated if it is opened with the IO_Translate mode flag.

bool QIODevice::isWritable () const

Returns TRUE if the I/O device was opened using IO_WriteOnly or IO_ReadWrite mode; otherwise returns FALSE.

See also isReadable() and isReadWrite().

int QIODevice::mode () const

Returns bits OR'ed together that specify the current operation mode.

These are the flags that were given to the open() function.

The flags are IO_ReadOnly, IO_WriteOnly, IO_ReadWrite, IO_Append, IO_Truncate and IO_Translate.

bool QIODevice::open ( int mode ) [pure virtual]

Opens the I/O device using the specified mode. Returns TRUE if the device was successfully opened; otherwise returns FALSE.

The mode parameter mode must be an OR'ed combination of the following flags.

This virtual function must be reimplemented by all subclasses.

See also close().

Example: grapher/grapher.cpp.

Reimplemented in QFile and QSocket.

int QIODevice::putch ( int ch ) [pure virtual]

Writes the character ch to the I/O device.

Returns ch, or -1 if an error occurred.

This virtual function must be reimplemented by all subclasses.

See also getch() and ungetch().

Example: grapher/grapher.cpp.

Reimplemented in QFile and QSocket.

QByteArray QIODevice::readAll () [virtual]

Q_LONG QIODevice::readBlock ( char * data, Q_ULONG maxlen ) [pure virtual]

Reads at most maxlen bytes from the I/O device into data and returns the number of bytes actually read.

This function should return -1 if a fatal error occurs.

This virtual function must be reimplemented by all subclasses.

See also writeBlock().

Reimplemented in QFile, QSocket and QSocketDevice.

Q_LONG QIODevice::readLine ( char * data, Q_ULONG maxlen ) [virtual]

Returns the number of bytes read including the terminating '\0', or -1 if an error occurred.

This virtual function can be reimplemented much more efficiently by the most subclasses.

See also readBlock() and QTextStream::readLine().

Reimplemented in QFile.

bool QIODevice::reset ()

Sets the device index position to 0.

See also at().

void QIODevice::resetStatus ()

Sets the I/O device status to IO_Ok.

See also status().

Offset QIODevice::size () const [pure virtual]

Virtual function that returns the size of the I/O device.

See also at().

Reimplemented in QFile and QSocket.

int QIODevice::state () const

Returns bits OR'ed together that specify the current state.

The flags are: IO_Open.

Subclasses may define additional flags.

int QIODevice::status () const

Returns the I/O device status.

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.

The status codes are:

See also resetStatus().

int QIODevice::ungetch ( int ch ) [pure virtual]

Puts the character ch back into the I/O device and decrements the index position if it is not zero.

This function is normally called to "undo" a getch() operation.

Returns ch, or -1 if an error occurred.

This virtual function must be reimplemented by all subclasses.

See also getch() and putch().

Reimplemented in QFile and QSocket.

Q_LONG QIODevice::writeBlock ( const char * data, Q_ULONG len ) [pure virtual]

Writes len bytes from data to the I/O device and returns the number of bytes actually written.

This function should return -1 if a fatal error occurs.

This virtual function must be reimplemented by all subclasses.

See also readBlock().

Reimplemented in QBuffer, QSocket and QSocketDevice.

Q_LONG QIODevice::writeBlock ( const QByteArray & data )

This convenience function is the same as calling writeBlock( data.data(), data.size() ).

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