QSaveFile Class

Member Function Documentation

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

Constructs a new file object with the given parent.

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

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

[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 mode flags. Returns true if successful; otherwise returns false.

Important: The flags for mode must include QIODeviceBase::WriteOnly. Other common flags you can use are Text and Unbuffered. Flags not supported at the moment are ReadOnly (and therefore ReadWrite), Append, NewOnly and ExistingOnly; they will generate a runtime warning.

See also setFileName() and QT_USE_NODISCARD_FILE_OPEN.

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).