Qt Documentation

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QFile Class

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

Header: #include <QFile>
qmake: QT += core
Inherits: QFileDevice
Inherited By:

QTemporaryFile

Note: All functions in this class are reentrant.

Public Types

typedef DecoderFn

Public Functions

QFile()
QFile(const QString &name)
QFile(QObject *parent)
QFile(const QString &name, QObject *parent)
~QFile()
bool copy(const QString &newName)
bool exists() const
bool link(const QString &linkName)
bool open(FILE *fh, OpenMode mode, FileHandleFlags handleFlags = DontCloseHandle)
bool open(int fd, OpenMode mode, FileHandleFlags handleFlags = DontCloseHandle)
bool remove()
bool rename(const QString &newName)
void setFileName(const QString &name)
QString symLinkTarget() const

Reimplemented Public Functions

virtual QString fileName() const
virtual bool open(OpenMode mode)
virtual Permissions permissions() const
virtual bool resize(qint64 sz)
virtual bool setPermissions(Permissions permissions)
virtual qint64 size() const
  • 16 public functions inherited from QFileDevice
  • 43 public functions inherited from QIODevice
  • 31 public functions inherited from QObject

Static Public Members

bool copy(const QString &fileName, const QString &newName)
QString decodeName(const QByteArray &localFileName)
QString decodeName(const char *localFileName)
QByteArray encodeName(const QString &fileName)
bool exists(const QString &fileName)
bool link(const QString &fileName, const QString &linkName)
Permissions permissions(const QString &fileName)
bool remove(const QString &fileName)
bool rename(const QString &oldName, const QString &newName)
bool resize(const QString &fileName, qint64 sz)
bool setPermissions(const QString &fileName, Permissions permissions)
QString symLinkTarget(const QString &fileName)
  • 11 static public members inherited from QObject

Additional Inherited Members

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(). QFile expects the file separator to be '/' regardless of operating system. The use of other separators (e.g., '\') is not supported.

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.

Reading Files Directly

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.

Using Streams to Read Files

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., UTF-8 on most unix based operating systems; see QTextCodec::codecForLocale() for details). This can be changed using QTextStream::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();
    }

Signals

Unlike other QIODevice implementations, such as QTcpSocket, QFile does not emit the aboutToClose(), bytesWritten(), or readyRead() signals. This implementation detail means that QFile is not suitable for reading and writing certain types of files, such as device files on Unix platforms.

Platform Specific Issues

File permissions are handled differently on Unix-like systems and Windows. In a non writable directory on Unix-like systems, files cannot be created. This is not always the case on Windows, where, for instance, the 'My Documents' directory usually is not writable, but it is still possible to create files in it.

Qt's understanding of file permissions is limited, which affects especially the QFile::setPermissions() function. On Windows, Qt will set only the legacy read-only flag, and that only when none of the Write* flags are passed. Qt does not manipulate access control lists (ACLs), which makes this function mostly useless for NTFS volumes. It may still be of use for USB sticks that use VFAT file systems. POSIX ACLs are not manipulated, either.

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().

Member Function Documentation

QFile::QFile()

Constructs a QFile object.

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::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.

See also setFileName().

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

This is an overloaded function.

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().

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

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

See also encodeName().

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

This is an overloaded function.

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

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

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().

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

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

Note: If fileName is a symlink that points to a non-existing file, false is returned.

bool QFile::exists() const

This is an overloaded function.

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

See also fileName() and setFileName().

[virtual] QString QFile::fileName() const

Reimplemented from QFileDevice::fileName().

Returns the name set by setFileName() or to the QFile constructors.

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

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.

This function will not overwrite an already existing entity in the file system; in this case, link() will return false and set error() to return RenameError.

Note: To create a valid link on Windows, linkName must have a .lnk file extension.

See also setFileName().

This is an overloaded function.

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().

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

Reimplemented from QIODevice::open().

Opens the file using OpenMode mode, returning true if successful; otherwise false.

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

Note: In WriteOnly or ReadWrite mode, if the relevant file does not already exist, this function will try to create a new file before opening it.

See also QIODevice::OpenMode and setFileName().

bool QFile::open(FILE *fh, OpenMode mode, FileHandleFlags handleFlags = DontCloseHandle)

This is an overloaded function.

Opens the existing file handle fh in the given mode. handleFlags may be used to specify additional options. 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, behaviour of close() is controlled by the AutoCloseHandle flag. If AutoCloseHandle is specified, and this function succeeds, then calling close() closes the adopted handle. Otherwise, close() does not actually close the file, but only flushes it.

Warning:

  1. If fh does not refer to a regular file, e.g., it is stdin, stdout, or stderr, you may not be able to seek(). size() returns 0 in those cases. See QIODevice::isSequential() for more information.
  2. Since this function opens the file without specifying the file name, you cannot use this QFile with a QFileInfo.

Note for the Windows Platform

fh must be opened in binary mode (i.e., the mode string must contain 'b', as in "rb" or "wb") when accessing files and other random-access devices. Qt will translate the end-of-line characters if you pass QIODevice::Text to mode. Sequential devices, such as stdin and stdout, are unaffected by this limitation.

You need to enable support for console applications in order to use the stdin, stdout and stderr streams at the console. To do this, add the following declaration to your application's project file:

CONFIG += console

See also close().

bool QFile::open(int fd, OpenMode mode, FileHandleFlags handleFlags = DontCloseHandle)

This is an overloaded function.

Opens the existing file descriptor fd in the given mode. handleFlags may be used to specify additional options. Returns true if successful; otherwise returns false.

When a QFile is opened using this function, behaviour of close() is controlled by the AutoCloseHandle flag. If AutoCloseHandle is specified, and this function succeeds, then calling close() closes the adopted handle. Otherwise, close() does not actually close the file, but only flushes it.

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 not a regular file, e.g, it is 0 (stdin), 1 (stdout), or 2 (stderr), you may not be able to seek(). In those cases, size() returns 0. See QIODevice::isSequential() for more information.

Warning: Since this function opens the file without specifying the file name, you cannot use this QFile with a QFileInfo.

See also close().

[virtual] Permissions QFile::permissions() const

Reimplemented from QFileDevice::permissions().

See also setPermissions().

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

This is an overloaded function.

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

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().

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

This is an overloaded function.

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.

If the rename operation fails, Qt will attempt to copy this file's contents to newName, and then remove this file, keeping only newName. If that copy operation fails or this file can't be removed, the destination file newName is removed to restore the old state.

See also setFileName().

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

This is an overloaded function.

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().

[virtual] bool QFile::resize(qint64 sz)

Reimplemented from QFileDevice::resize().

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

This is an overloaded function.

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::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.

[virtual] bool QFile::setPermissions(Permissions permissions)

Reimplemented from QFileDevice::setPermissions().

Sets the permissions for the file to the permissions specified. Returns true if successful, or false if the permissions cannot be modified.

Warning: This function does not manipulate ACLs, which may limit its effectiveness.

See also permissions() and setFileName().

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

This is an overloaded function.

Sets the permissions for fileName file to permissions.

[virtual] qint64 QFile::size() const

Reimplemented from QIODevice::size().

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

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.

This function was introduced in Qt 4.2.

QString QFile::symLinkTarget() const

This is an overloaded function.

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.

This function was introduced in Qt 4.2.

See also fileName() and setFileName().

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