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: |
Note: All functions in this class are reentrant.
Public Types
| typedef | DecoderFn |
Public Functions
| QFile(const QString &name, QObject *parent) | |
| QFile(QObject *parent) | |
| QFile(const QString &name) | |
| QFile() | |
| virtual | ~QFile() |
| bool | copy(const QString &newName) |
| bool | exists() const |
| bool | link(const QString &linkName) |
| bool | moveToTrash() |
| 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 override |
| virtual bool | open(OpenMode mode) override |
| virtual Permissions | permissions() const override |
| virtual bool | resize(qint64 sz) override |
| virtual bool | setPermissions(Permissions permissions) override |
| virtual qint64 | size() const override |
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) |
| bool | moveToTrash(const QString &fileName, QString *pathInTrash = nullptr) |
| 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) |
Detailed Description
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.
On Android, some limitations apply when dealing with content URIs:
- Access permissions might be needed by prompting the user through the QFileDialog which implements Android's native file picker.
- Aim to follow the Scoped storage guidelines, such as using app specific directories instead of other public external directories. For more information, also see storage best practices.
- Due to the design of Qt APIs (e.g. QFile), it's not possible to fully integrate the latter APIs with Android's MediaStore APIs.
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(const QString &name, QObject *parent)
Constructs a new file object with the given parent to represent the file with the specified name.
QFile::QFile(QObject *parent)
Constructs a new file object with the given parent.
QFile::QFile(const QString &name)
Constructs a new file object to represent the file with the given name.
QFile::QFile()
Constructs a QFile object.
[virtual] QFile::~QFile()
Destroys the file object, closing it if necessary.
bool QFile::copy(const QString &newName)
Copies the file named fileName() to newName.
This file is closed before it is copied.
If the copied file is a symbolic link (symlink), the file it refers to is copied, not the link itself. With the exception of permissions, which are copied, no other file metadata is copied.
Returns true if successful; otherwise returns false.
Note that if a file with the name newName already exists, copy() returns false. This means QFile will not overwrite it.
Note: On Android, this operation is not yet supported for content scheme URIs.
See also setFileName().
[static] bool QFile::copy(const QString &fileName, const QString &newName)
This is an overloaded function.
Copies the file named fileName to newName.
This file is closed before it is copied.
If the copied file is a symbolic link (symlink), the file it refers to is copied, not the link itself. With the exception of permissions, which are copied, no other file metadata is copied.
Returns true if successful; otherwise returns false.
Note that if a file with the name newName already exists, copy() returns false. This means QFile will not overwrite it.
Note: On Android, this operation is not yet supported for content scheme URIs.
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().
[override virtual] QString QFile::fileName() const
Reimplements: QFileDevice::fileName() const.
Returns the name set by setFileName() or to the QFile constructors.
See also setFileName() and QFileInfo::fileName().
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.
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().
[static] bool QFile::link(const QString &fileName, const QString &linkName)
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().
bool QFile::moveToTrash()
Moves the file specified by fileName() to the trash. Returns true if successful, and sets the fileName() to the path at which the file can be found within the trash; otherwise returns false.
Note: On systems where the system API doesn't report the location of the file in the trash, fileName() will be set to the null string once the file has been moved. On systems that don't have a trash can, this function always returns false.
This function was introduced in Qt 5.15.
[static] bool QFile::moveToTrash(const QString &fileName, QString *pathInTrash = nullptr)
This is an overloaded function.
Moves the file specified by fileName() to the trash. Returns true if successful, and sets pathInTrash (if provided) to the path at which the file can be found within the trash; otherwise returns false.
Note: On systems where the system API doesn't report the path of the file in the trash, pathInTrash will be set to the null string once the file has been moved. On systems that don't have a trash can, this function always returns false.
This function was introduced in Qt 5.15.
[override virtual] bool QFile::open(OpenMode mode)
Reimplements: QIODevice::open(OpenMode mode).
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. On Android, it's expected to have access permission to the parent of the file name, otherwise, it won't be possible to create this non-existing file.
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:
- If fh does not refer to a regular file, e.g., it is
stdin,stdout, orstderr, you may not be able to seek(). size() returns0in those cases. See QIODevice::isSequential() for more information. - 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.
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().
[override virtual] Permissions QFile::permissions() const
Reimplements: QFileDevice::permissions() const.
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().
[override virtual] bool QFile::resize(qint64 sz)
Reimplements: QFileDevice::resize(qint64 sz).
[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 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.
Warning: This function can fail if the file doesn't exist.
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.
[override virtual] bool QFile::setPermissions(Permissions permissions)
Reimplements: QFileDevice::setPermissions(Permissions permissions).
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.
[override virtual] qint64 QFile::size() const
Reimplements: QFileDevice::size() const.
[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().