QFile Class Reference
^{[QtCore module]}

Detailed Description

The QFile class provides an interface for reading from and writing to files.

QFile is an I/O device for reading and writing text and binary files and resources. A QFile may be used by itself or, more conveniently, with a QTextStream or QDataStream.

The file name is usually passed in the constructor, but it can be set at any time using setFileName(). You can check for a file's existence using exists(), and remove a file using remove(). (More advanced file system related operations are provided by QFileInfo and QDir.)

The file is opened with open(), closed with close(), and flushed with flush(). Data is usually read and written using QDataStream or QTextStream, but you can also call the QIODevice-inherited functions read(), readLine(), readAll(), write(). QFile also inherits getChar(), putChar(), and ungetChar(), which work one character at a time.

The size of the file is returned by size(). You can get the current file position using pos(), or move to a new file position using seek(). If you've reached the end of the file, atEnd() returns true.

The following example reads a text file line by line:

        QFile file("in.txt");
        if (!file.open(QIODevice::ReadOnly | QIODevice::Text))
            return;

        while (!file.atEnd()) {
            QByteArray line = file.readLine();
            process_line(line);
        }

The QIODevice::Text flag passed to open() tells Qt to convert Windows-style line terminators ("\r\n") into C++-style terminators ("\n"). By default, QFile assumes binary, i.e. it doesn't perform any conversion on the bytes stored in the file.

The next example uses QTextStream to read a text file line by line:

        QFile file("in.txt");
        if (!file.open(QIODevice::ReadOnly | QIODevice::Text))
            return;

        QTextStream in(&file);
        while (!in.atEnd()) {
            QString line = in.readLine();
            process_line(line);
        }

QTextStream takes care of converting the 8-bit data stored on disk into a 16-bit Unicode QString. By default, it assumes that the user system's local 8-bit encoding is used (e.g., ISO 8859-1 for most of Europe; see QTextCodec::codecForLocale() for details). This can be changed using setCodec().

To write text, we can use operator<<(), which is overloaded to take a QTextStream on the left and various data types (including QString) on the right:

        QFile file("out.txt");
        if (!file.open(QIODevice::WriteOnly | QIODevice::Text))
            return;

        QTextStream out(&file);
        out << "The magic number is: " << 49 << "\n";

QDataStream is similar, in that you can use operator<<() to write data and operator>>() to read it back. See the class documentation for details.

When you use QFile, QFileInfo, and QDir to access the file system with Qt, you can use Unicode file names. On Unix, these file names are converted to an 8-bit encoding. If you want to use standard C++ APIs (<cstdio> or <iostream>) or platform-specific APIs to access files instead of QFile, you can use the encodeName() and decodeName() functions to convert between Unicode file names and 8-bit file names.

On Unix, there are some special system files (e.g. in /proc) for which size() will always return 0, yet you may still be able to read more data from such a file; the data is generated in direct response to you calling read(). In this case, however, you cannot use atEnd() to determine if there is more data to read (since atEnd() will return true for a file that claims to have size 0). Instead, you should either call readAll(), or call read() or readLine() repeatedly until no more data can be read. The next example uses QTextStream to read /proc/modules line by line:

        QFile file("/proc/modules");
        if (!file.open(QIODevice::ReadOnly | QIODevice::Text))
            return;

        QTextStream in(&file);
        QString line = in.readLine();
        while (!line.isNull()) {
            process_line(line);
            line = in.readLine();
        }

See also QTextStream, QDataStream, QFileInfo, QDir, and The Qt Resource System.

Member Type Documentation

typedef QFile::DecoderFn

This is a typedef for a pointer to a function with the following signature:

    QString myDecoderFunc(const QByteArray &localFileName);

See also setDecodingFunction().

typedef QFile::EncoderFn

This is a typedef for a pointer to a function with the following signature:

    QByteArray myEncoderFunc(const QString &fileName);

See also setEncodingFunction() and encodeName().

enum QFile::FileError

This enum describes the errors that may be returned by the error() function.

enum QFile::Permission
flags QFile::Permissions

This enum is used by the permission() function to report the permissions and ownership of a file. The values may be OR-ed together to test multiple permissions and ownership values.

Warning: Because of differences in the platforms supported by Qt, the semantics of ReadUser, WriteUser and ExeUser are platform-dependent: On Unix, the rights of the owner of the file are returned and on Windows the rights of the current user are returned. This behavior might change in a future Qt version.

The Permissions type is a typedef for QFlags<Permission>. It stores an OR combination of Permission values.

typedef QFile::PermissionSpec

Use QFile::Permission instead.

Member Function Documentation

QFile::QFile ( const QString & name )

Constructs a new file object to represent the file with the given name.

QFile::QFile ( QObject * parent )

Constructs a new file object with the given parent.

QFile::QFile ( const QString & name, QObject * parent )

Constructs a new file object with the given parent to represent the file with the specified name.

QFile::~QFile ()

Destroys the file object, closing it if necessary.

bool QFile::atEnd () const   [virtual]

Returns true if the end of the file has been reached; otherwise returns false.

For regular empty files on Unix (e.g. those in /proc), this function returns true, since the file system reports that the size of such a file is 0. Therefore, you should not depend on atEnd() when reading data from such a file, but rather call read() until no more data can be read.

Reimplemented from QIODevice.

bool QFile::copy ( const QString & newName )

Copies the file currently specified by fileName() to a file called newName. Returns true if successful; otherwise returns false.

Note that if a file with the name newName already exists, copy() returns false (i.e. QFile will not overwrite it).

The source file is closed before it is copied, and the new file's timestamp will be the time of the copy operation.

See also setFileName().

bool QFile::copy ( const QString & fileName, const QString & newName )   [static]

This is an overloaded member function, provided for convenience.

Copies the file fileName to newName. Returns true if successful; otherwise returns false.

If a file with the name newName already exists, copy() returns false (i.e., QFile will not overwrite it).

See also rename().

QString QFile::decodeName ( const QByteArray & localFileName )   [static]

This does the reverse of QFile::encodeName() using localFileName.

See also setDecodingFunction() and encodeName().

QString QFile::decodeName ( const char * localFileName )   [static]

This is an overloaded member function, provided for convenience.

Returns the Unicode version of the given localFileName. See encodeName() for details.

QByteArray QFile::encodeName ( const QString & fileName )   [static]

By default, this function converts fileName to the local 8-bit encoding determined by the user's locale. This is sufficient for file names that the user chooses. File names hard-coded into the application should only use 7-bit ASCII filename characters.

See also decodeName() and setEncodingFunction().

FileError QFile::error () const

Returns the file error status.

The I/O device status returns an error code. For example, if open() returns false, or a read/write operation returns -1, this function can be called to find out the reason why the operation failed.

See also unsetError().

bool QFile::exists ( const QString & fileName )   [static]

Returns true if the file specified by fileName exists; otherwise returns false.

bool QFile::exists () const

This is an overloaded member function, provided for convenience.

Returns true if the file specified by fileName() exists; otherwise returns false.

See also fileName() and setFileName().

QString QFile::fileName () const

Returns the name set by setFileName().

See also setFileName() and QFileInfo::fileName().

bool QFile::flush ()

Flushes any buffered data to the file.

int QFile::handle () const

Returns the file handle of the file.

This is a small positive integer, suitable for use with C library functions such as fdopen() and fcntl(). On systems that use file descriptors for sockets (i.e. Unix systems, but not Windows) the handle can be used with QSocketNotifier as well.

If the file is not open, or there is an error, handle() returns -1.

See also QSocketNotifier.

bool QFile::isSequential () const   [virtual]

Returns true if the file can only be manipulated sequentially; otherwise returns false.

Most files support random-access, but some special files may not.

Reimplemented from QIODevice.

See also QIODevice::isSequential().

bool QFile::link ( const QString & linkName )

Creates a link named linkName that points to the file currently specified by fileName(). What a link is depends on the underlying filesystem (be it a shortcut on Windows or a symbolic link on Unix). Returns true if successful; otherwise returns false.

See also setFileName().

bool QFile::link ( const QString & fileName, const QString & linkName )   [static]

This is an overloaded member function, provided for convenience.

Creates a link named linkName that points to the file fileName. What a link is depends on the underlying filesystem (be it a shortcut on Windows or a symbolic link on Unix). Returns true if successful; otherwise returns false.

See also link().

bool QFile::open ( OpenMode mode )   [virtual]

Opens the file using OpenMode mode.

The mode must be QIODevice::ReadOnly, QIODevice::WriteOnly, or QIODevice::ReadWrite. It may also have additional flags, such as QIODevice::Text and QIODevice::Unbuffered.

Reimplemented from QIODevice.

See also QIODevice::OpenMode.

bool QFile::open ( FILE * fh, OpenMode mode )

This is an overloaded member function, provided for convenience.

Opens the existing file handle fh in the given mode. Returns true if successful; otherwise returns false.

Example:

    #include <stdio.h>

    void printError(const char* msg)
    {
        QFile file;
        file.open(stderr, QIODevice::WriteOnly);
        file.write(msg, qstrlen(msg));        // write to stderr
        file.close();
    }

When a QFile is opened using this function, close() does not actually close the file, but only flushes it.

Warning: If fh is stdin, stdout, or stderr, you may not be able to seek(). See QIODevice::isSequentialAccess() for more information.

See also close().

bool QFile::open ( int fd, OpenMode mode )

This is an overloaded member function, provided for convenience.

Opens the existing file descripter fd in the given mode. Returns true if successful; otherwise returns false.

When a QFile is opened using this function, close() does not actually close the file.

The QFile that is opened using this function is automatically set to be in raw mode; this means that the file input/output functions are slow. If you run into performance issues, you should try to use one of the other open functions.

Warning: If fd is 0 (stdin), 1 (stdout), or 2 (stderr), you may not be able to seek(). size() is set to LLONG_MAX (in <climits>).

See also close().

Permissions QFile::permissions () const

Returns the complete OR-ed together combination of QFile::Permission for the file.

See also setPermissions() and setFileName().

Permissions QFile::permissions ( const QString & fileName )   [static]

This is an overloaded member function, provided for convenience.

Returns the complete OR-ed together combination of QFile::Permission for fileName.

QString QFile::readLink ( const QString & fileName )   [static]

Returns the absolute path of the file or directory referred to by the symlink (or shortcut on Windows) specified by fileName, or returns an empty string if the fileName does not correspond to a symbolic link.

This name may not represent an existing file; it is only a string. QFile::exists() returns true if the symlink points to an existing file.

QString QFile::readLink () const

This is an overloaded member function, provided for convenience.

Returns the absolute path of the file or directory a symlink (or shortcut on Windows) points to, or a an empty string if the object isn't a symbolic link.

This name may not represent an existing file; it is only a string. QFile::exists() returns true if the symlink points to an existing file.

See also fileName() and setFileName().

bool QFile::remove ()

Removes the file specified by fileName(). Returns true if successful; otherwise returns false.

The file is closed before it is removed.

See also setFileName().

bool QFile::remove ( const QString & fileName )   [static]

This is an overloaded member function, provided for convenience.

Removes the file specified by the fileName given.

Returns true if successful; otherwise returns false.

See also remove().

bool QFile::rename ( const QString & newName )

Renames the file currently specified by fileName() to newName. Returns true if successful; otherwise returns false.

If a file with the name newName already exists, rename() returns false (i.e., QFile will not overwrite it).

The file is closed before it is renamed.

See also setFileName().

bool QFile::rename ( const QString & oldName, const QString & newName )   [static]

This is an overloaded member function, provided for convenience.

Renames the file oldName to newName. Returns true if successful; otherwise returns false.

If a file with the name newName already exists, rename() returns false (i.e., QFile will not overwrite it).

See also rename().

bool QFile::resize ( qint64 sz )

Sets the file size (in bytes) sz. Returns true if the file if the resize succeeds; false otherwise. If sz is larger than the file currently is the new bytes will be set to 0, if sz is smaller the file is simply truncated.

See also size() and setFileName().

bool QFile::resize ( const QString & fileName, qint64 sz )   [static]

This is an overloaded member function, provided for convenience.

Sets fileName to size (in bytes) sz. Returns true if the file if the resize succeeds; false otherwise. If sz is larger than fileName currently is the new bytes will be set to 0, if sz is smaller the file is simply truncated.

See also resize().

void QFile::setDecodingFunction ( DecoderFn function )   [static]

Sets the function for decoding 8-bit file names. The default uses the locale-specific 8-bit encoding.

Warning: This function is not reentrant.

See also setEncodingFunction() and decodeName().

void QFile::setEncodingFunction ( EncoderFn function )   [static]

Sets the function for encoding Unicode file names. The default encodes in the locale-specific 8-bit encoding.

Warning: This function is not reentrant.

See also encodeName() and setDecodingFunction().

void QFile::setFileName ( const QString & name )

Sets the name of the file. The name can have no path, a relative path, or an absolute path.

Do not call this function if the file has already been opened.

If the file name has no path or a relative path, the path used will be the application's current directory path at the time of the open() call.

Example:

    QFile file;
    QDir::setCurrent("/tmp");
    file.setFileName("readme.txt");
    QDir::setCurrent("/home");
    file.open(QIODevice::ReadOnly);      // opens "/home/readme.txt" under Unix

Note that the directory separator "/" works for all operating systems supported by Qt.

See also fileName(), QFileInfo, and QDir.

bool QFile::setPermissions ( Permissions permissions )

Sets the permissions for the file to permissions.

See also permissions() and setFileName().

bool QFile::setPermissions ( const QString & fileName, Permissions permissions )   [static]

This is an overloaded member function, provided for convenience.

Sets the permissions for fileName file to permissions.

qint64 QFile::size () const   [virtual]

Returns the size of the file.

For regular empty files on Unix (e.g. those in /proc), this function returns 0; the contents of such a file are generated on demand in response to you calling read().

Reimplemented from QIODevice.

void QFile::unsetError ()

Sets the file's error to QFile::NoError.

See also error().