QIODevice

NAME

SYNOPSIS

#include <qiodevice.h>

Inherited by QBuffer, QFile, QSocket, and QSocketDevice.

Public Members

Protected Members

DESCRIPTION

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

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().

Example: distributor/distributor.ui.h.

Reimplemented in QSocket.  

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

See also size().

Reimplemented in QSocket.  

bool QIODevice::atEnd () const [virtual]

Reimplemented in QFile and QSocket.  

void QIODevice::close () [pure virtual]

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

Flags consists of mode flags and state flags.

See also mode() and state().  

void QIODevice::flush () [pure virtual]

This virtual function must be reimplemented by all subclasses.

Reimplemented in QFile and QSocket.  

int QIODevice::getch () [pure virtual]

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

This mode is currently not in use.

See also isSynchronous().  

bool QIODevice::isBuffered () const

See also isRaw().  

bool QIODevice::isCombinedAccess () const

This access method is currently not in use.  

bool QIODevice::isDirectAccess () const

See also isSequentialAccess().  

bool QIODevice::isInactive () const

See also isOpen().  

bool QIODevice::isOpen () const

See also isInactive().

Example: network/networkprotocol/nntp.cpp.  

bool QIODevice::isRaw () const

See also isBuffered().  

bool QIODevice::isReadWrite () const

See also isReadable() and isWritable().  

bool QIODevice::isReadable () const

See also isWritable() and isReadWrite().  

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