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The QFile class provides an interface for reading from and writing to files. More...
#include <QFile>
Inherits QIODevice.
Inherited by QTemporaryFile.
Note: All the functions in this class are reentrant, except setEncodingFunction() and setDecodingFunction().
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.
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();
}
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.
File permissions are handled differently on Linux/Mac OS X and Windows. In a non writable directory on Linux, 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.
See also QTextStream, QDataStream, QFileInfo, QDir, and The Qt Resource System.
This is a typedef for a pointer to a function with the following signature:
QString myDecoderFunc(const QByteArray &localFileName);
See also setDecodingFunction().
This is a typedef for a pointer to a function with the following signature:
QByteArray myEncoderFunc(const QString &fileName);
See also setEncodingFunction() and encodeName().
This enum describes the errors that may be returned by the error() function.
| Constant | Value | Description |
|---|---|---|
| QFile::NoError | 0 | No error occurred. |
| QFile::ReadError | 1 | An error occurred when reading from the file. |
| QFile::WriteError | 2 | An error occurred when writing to the file. |
| QFile::FatalError | 3 | A fatal error occurred. |
| QFile::ResourceError | 4 | |
| QFile::OpenError | 5 | The file could not be opened. |
| QFile::AbortError | 6 | The operation was aborted. |
| QFile::TimeOutError | 7 | A timeout occurred. |
| QFile::UnspecifiedError | 8 | An unspecified error occurred. |
| QFile::RemoveError | 9 | The file could not be removed. |
| QFile::RenameError | 10 | The file could not be renamed. |
| QFile::PositionError | 11 | The position in the file could not be changed. |
| QFile::ResizeError | 12 | The file could not be resized. |
| QFile::PermissionsError | 13 | The file could not be accessed. |
| QFile::CopyError | 14 | The file could not be copied. |
This enum describes special options that may be used by the map() function.
| Constant | Value | Description |
|---|---|---|
| QFile::NoOptions | 0 | No options. |
This enum was introduced in Qt 4.4.
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.
| Constant | Value | Description |
|---|---|---|
| QFile::ReadOwner | 0x4000 | The file is readable by the owner of the file. |
| QFile::WriteOwner | 0x2000 | The file is writable by the owner of the file. |
| QFile::ExeOwner | 0x1000 | The file is executable by the owner of the file. |
| QFile::ReadUser | 0x0400 | The file is readable by the user. |
| QFile::WriteUser | 0x0200 | The file is writable by the user. |
| QFile::ExeUser | 0x0100 | The file is executable by the user. |
| QFile::ReadGroup | 0x0040 | The file is readable by the group. |
| QFile::WriteGroup | 0x0020 | The file is writable by the group. |
| QFile::ExeGroup | 0x0010 | The file is executable by the group. |
| QFile::ReadOther | 0x0004 | The file is readable by anyone. |
| QFile::WriteOther | 0x0002 | The file is writable by anyone. |
| QFile::ExeOther | 0x0001 | The file is executable by anyone. |
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.
Use QFile::Permission instead.
Constructs a new file object to represent the file with the given name.
Constructs a new file object with the given parent.
Constructs a new file object with the given parent to represent the file with the specified name.
Destroys the file object, closing it if necessary.
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.
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().
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().
This does the reverse of QFile::encodeName() using localFileName.
See also setDecodingFunction() and encodeName().
This is an overloaded member function, provided for convenience.
Returns the Unicode version of the given localFileName. See encodeName() for details.
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().
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().
Returns true if the file specified by fileName exists; otherwise returns false.
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().
Returns the name set by setFileName() or to the QFile constructors.
See also setFileName() and QFileInfo::fileName().
Flushes any buffered data to the file. Returns true if successful; otherwise returns false.
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.
This function is not supported on Windows CE.
See also QSocketNotifier.
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().
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 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().
Maps size bytes of the file into memory starting at offset. A file should be open for a map to succeed but the file does not need to stay open after the memory has been mapped. When the QFile is destroyed or a new file is opened with this object, any maps that have not been unmapped will automatically be unmapped.
Any mapping options can be passed through flags.
Returns a pointer to the memory or 0 if there is an error.
Note: On Windows CE 5.0 the file will be closed before mapping occurs.
This function was introduced in Qt 4.4.
See also unmap() and QAbstractFileEngine::supportsExtension().
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.
Note: Because of limitations in the native API, QFile ignores the Unbuffered flag on Windows.
Reimplemented from QIODevice.
See also QIODevice::OpenMode and setFileName().
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:
Note: For Windows CE you may not be able to call seek() and resize(). Also, size() is set to 0.
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() and qmake Variable Reference.
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>).
Warning: For Windows CE you may not be able to call seek(), setSize(), fileTime(). size() is set to 0.
Warning: Since this function opens the file without specifying the file name, you cannot use this QFile with a QFileInfo.
See also close().
Returns the complete OR-ed together combination of QFile::Permission for the file.
See also setPermissions() and setFileName().
This is an overloaded member function, provided for convenience.
Returns the complete OR-ed together combination of QFile::Permission for fileName.
Removes the file specified by fileName(). Returns true if successful; otherwise returns false.
The file is closed before it is removed.
See also setFileName().
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().
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().
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().
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().
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().
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().
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().
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.
Sets the permissions for the file to the permissions specified. Returns true if successful, or false if the permissions cannot be modified.
See also permissions() and setFileName().
This is an overloaded member function, provided for convenience.
Sets the permissions for fileName file to permissions.
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.
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.
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.
This function was introduced in Qt 4.2.
See also fileName() and setFileName().
Unmaps the memory address.
Returns true if the unmap succeeds; false otherwise.
This function was introduced in Qt 4.4.
See also map() and QAbstractFileEngine::supportsExtension().
Sets the file's error to QFile::NoError.
See also error().
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