QSaveFile Class

The QSaveFile class provides an interface for safely writing to files. More...

Header: #include <QSaveFile>
CMake: find_package(Qt6 REQUIRED COMPONENTS Core)
target_link_libraries(mytarget PRIVATE Qt6::Core)
qmake: QT += core
Inherits: QFileDevice

Note: All functions in this class are reentrant.

Public Functions

QSaveFile(const QString &name)
QSaveFile(QObject *parent = nullptr)
QSaveFile(const QString &name, QObject *parent)
virtual ~QSaveFile()
void cancelWriting()
bool commit()
bool directWriteFallback() const
void setDirectWriteFallback(bool enabled)
void setFileName(const QString &name)

Reimplemented Public Functions

virtual QString fileName() const override
virtual bool open(QIODeviceBase::OpenMode mode) override

Reimplemented Protected Functions

virtual qint64 writeData(const char *data, qint64 len) override

Detailed Description

QSaveFile is an I/O device for writing text and binary files, without losing existing data if the writing operation fails.

While writing, the contents will be written to a temporary file, and if no error happened, commit() will move it to the final file. This ensures that no data at the final file is lost in case an error happens while writing, and no partially-written file is ever present at the final location. Always use QSaveFile when saving entire documents to disk.

QSaveFile automatically detects errors while writing, such as the full partition situation, where write() cannot write all the bytes. It will remember that an error happened, and will discard the temporary file in commit().

Much like with QFile, the file is opened with open(). Data is usually read and written using QDataStream or QTextStream, but you can also call the QIODevice-inherited functions read(), readLine(), readAll(), write().

Unlike QFile, calling close() is not allowed. commit() replaces it. If commit() was not called and the QSaveFile instance is destroyed, the temporary file is discarded.

To abort saving due to an application error, call cancelWriting(), so that even a call to commit() later on will not save.

See also QTextStream, QDataStream, QFileInfo, QDir, QFile, and QTemporaryFile.

Member Function Documentation

[explicit] QSaveFile::QSaveFile(const QString &name)

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

[explicit] QSaveFile::QSaveFile(QObject *parent = nullptr)

Constructs a new file object with the given parent.

[explicit] QSaveFile::QSaveFile(const QString &name, QObject *parent)

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

[virtual noexcept] QSaveFile::~QSaveFile()

Destroys the file object, discarding the saved contents unless commit() was called.

void QSaveFile::cancelWriting()

Cancels writing the new file.

If the application changes its mind while saving, it can call cancelWriting(), which sets an error code so that commit() will discard the temporary file.

Alternatively, it can simply make sure not to call commit().

Further write operations are possible after calling this method, but none of it will have any effect, the written file will be discarded.

This method has no effect when direct write fallback is used. This is the case when saving over an existing file in a readonly directory: no temporary file can be created, so the existing file is overwritten no matter what, and cancelWriting() cannot do anything about that, the contents of the existing file will be lost.

See also commit().

bool QSaveFile::commit()

Commits the changes to disk, if all previous writes were successful.

It is mandatory to call this at the end of the saving operation, otherwise the file will be discarded.

If an error happened during writing, deletes the temporary file and returns false. Otherwise, renames it to the final fileName and returns true on success. Finally, closes the device.

See also cancelWriting().

bool QSaveFile::directWriteFallback() const

Returns true if the fallback solution for saving files in read-only directories is enabled.

See also setDirectWriteFallback().

[override virtual] QString QSaveFile::fileName() const

Reimplements: QFileDevice::fileName() const.

Returns the name set by setFileName() or to the QSaveFile constructor.

See also setFileName().

[override virtual] bool QSaveFile::open(QIODeviceBase::OpenMode mode)

Reimplements: QIODevice::open(QIODeviceBase::OpenMode mode).

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

Important: the mode must include QIODevice::WriteOnly. It may also have additional flags, such as QIODevice::Text and QIODevice::Unbuffered.

QIODevice::ReadWrite, QIODevice::Append, QIODevice::NewOnly and QIODevice::ExistingOnly are not supported at the moment.

See also QIODevice::OpenMode and setFileName().

void QSaveFile::setDirectWriteFallback(bool enabled)

Allows writing over the existing file if necessary.

QSaveFile creates a temporary file in the same directory as the final file and atomically renames it. However this is not possible if the directory permissions do not allow creating new files. In order to preserve atomicity guarantees, open() fails when it cannot create the temporary file.

In order to allow users to edit files with write permissions in a directory with restricted permissions, call setDirectWriteFallback() with enabled set to true, and the following calls to open() will fallback to opening the existing file directly and writing into it, without the use of a temporary file. This does not have atomicity guarantees, i.e. an application crash or for instance a power failure could lead to a partially-written file on disk. It also means cancelWriting() has no effect, in such a case.

Typically, to save documents edited by the user, call setDirectWriteFallback(true), and to save application internal files (configuration files, data files, ...), keep the default setting which ensures atomicity.

See also directWriteFallback().

void QSaveFile::setFileName(const QString &name)

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

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

[override virtual protected] qint64 QSaveFile::writeData(const char *data, qint64 len)

Reimplements: QFileDevice::writeData(const char *data, qint64 len).

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