QTemporaryFile Class

The QTemporaryFile class is an I/O device that operates on temporary files. More...

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

List of all members, including inherited members

Note: All functions in this class are reentrant.

Public Functions

	QTemporaryFile(const QString &templateName, QObject *parent)
	QTemporaryFile(QObject *parent)
	QTemporaryFile(const QString &templateName)
	QTemporaryFile()
virtual	~QTemporaryFile()
bool	autoRemove() const
QString	fileTemplate() const
bool	open()
bool	rename(const QString &newName)
void	setAutoRemove(bool b)
void	setFileTemplate(const QString &name)

Reimplemented Public Functions

virtual QString fileName() const override

Static Public Members

QTemporaryFile *	createNativeFile(QFile &file)
QTemporaryFile *	createNativeFile(const QString &fileName)

Reimplemented Protected Functions

virtual bool open(QIODeviceBase::OpenMode flags) override

Detailed Description

QTemporaryFile is used to create unique temporary files safely. The file itself is created by calling open(). The name of the temporary file is guaranteed to be unique (i.e., you are guaranteed to not overwrite an existing file), and the file will subsequently be removed upon destruction of the QTemporaryFile object. This is an important technique that avoids data corruption for applications that store data in temporary files. The file name is either auto-generated, or created based on a template, which is passed to QTemporaryFile's constructor.

Example:

    // Within a function/method...

    QTemporaryFile file;
    if (file.open()) {
        // file.fileName() returns the unique file name
    }

    // The QTemporaryFile destructor removes the temporary file
    // as it goes out of scope.

Reopening a QTemporaryFile after calling close() is safe. For as long as the QTemporaryFile object itself is not destroyed, the unique temporary file will exist and be kept open internally by QTemporaryFile.

The file name of the temporary file can be found by calling fileName(). Note that this is only defined after the file is first opened; the function returns an empty string before this.

A temporary file will have some static part of the name and some part that is calculated to be unique. The default filename will be determined from QCoreApplication::applicationName() (otherwise qt_temp) and will be placed into the temporary path as returned by QDir::tempPath(). If you specify your own filename, a relative file path will not be placed in the temporary directory by default, but be relative to the current working directory. It is important to specify the correct directory if the rename() function will be called, as QTemporaryFile can only rename files within the same volume / filesystem as the temporary file itself was created on.

Specified filenames can contain the following template XXXXXX (six upper case "X" characters), which will be replaced by the auto-generated portion of the filename. Note that the template is case sensitive. If the template is not present in the filename, QTemporaryFile appends the generated part to the filename given.

Note: On Linux, QTemporaryFile will attempt to create unnamed temporary files. If that succeeds, open() will return true but exists() will be false. If you call fileName() or any function that calls it, QTemporaryFile will give the file a name, so most applications will not see a difference.

See also QDir::tempPath() and QFile.

Member Function Documentation

QTemporaryFile::QTemporaryFile(const QString &templateName, QObject **parent*)

Constructs a QTemporaryFile with a template filename of templateName and the specified parent. Upon opening the temporary file this will be used to create a unique filename.

If the templateName does not contain XXXXXX it will automatically be appended and used as the dynamic portion of the filename.

If templateName is a relative path, the path will be relative to the current working directory. You can use QDir::tempPath() to construct templateName if you want use the system's temporary directory. It is important to specify the correct directory if the rename() function will be called, as QTemporaryFile can only rename files within the same volume / filesystem as the temporary file itself was created on.

See also open() and fileTemplate().

QTemporaryFile::QTemporaryFile(QObject **parent*)

Constructs a QTemporaryFile (with the given parent) using as file template the application name returned by QCoreApplication::applicationName() (otherwise qt_temp) followed by ".XXXXXX". The file is stored in the system's temporary directory, QDir::tempPath().

See also setFileTemplate().

QTemporaryFile::QTemporaryFile(const QString &templateName)

Constructs a QTemporaryFile with a template filename of templateName. Upon opening the temporary file this will be used to create a unique filename.

If the templateName does not contain XXXXXX it will automatically be appended and used as the dynamic portion of the filename.

If templateName is a relative path, the path will be relative to the current working directory. You can use QDir::tempPath() to construct templateName if you want use the system's temporary directory. It is important to specify the correct directory if the rename() function will be called, as QTemporaryFile can only rename files within the same volume / filesystem as the temporary file itself was created on.

See also open() and fileTemplate().

QTemporaryFile::QTemporaryFile()

Constructs a QTemporaryFile using as file template the application name returned by QCoreApplication::applicationName() (otherwise qt_temp) followed by ".XXXXXX". The file is stored in the system's temporary directory, QDir::tempPath().

See also setFileTemplate() and QDir::tempPath().

`[virtual]` QTemporaryFile::~QTemporaryFile()

Destroys the temporary file object, the file is automatically closed if necessary and if in auto remove mode it will automatically delete the file.

See also autoRemove().

bool QTemporaryFile::autoRemove() const

Returns true if the QTemporaryFile is in auto remove mode. Auto-remove mode will automatically delete the filename from disk upon destruction. This makes it very easy to create your QTemporaryFile object on the stack, fill it with data, read from it, and finally on function return it will automatically clean up after itself.

Auto-remove is on by default.

See also setAutoRemove() and remove().

`[static]` QTemporaryFile QTemporaryFile::createNativeFile(QFile &file*)

If file is not already a native file, then a QTemporaryFile is created in QDir::tempPath(), the contents of file is copied into it, and a pointer to the temporary file is returned. Does nothing and returns 0 if file is already a native file.

For example:

  QFile f(":/resources/file.txt");
  QTemporaryFile::createNativeFile(f); // Returns a pointer to a temporary file

  QFile f("/users/qt/file.txt");
  QTemporaryFile::createNativeFile(f); // Returns 0

See also QFileInfo::isNativePath().

`[static]` QTemporaryFile QTemporaryFile::createNativeFile(const QString &fileName*)

This is an overloaded function.

Works on the given fileName rather than an existing QFile object.

`[override virtual]` QString QTemporaryFile::fileName() const

Reimplements: QFile::fileName() const.

Returns the complete unique filename backing the QTemporaryFile object. This string is null before the QTemporaryFile is opened, afterwards it will contain the fileTemplate() plus additional characters to make it unique.

See also fileTemplate().

QString QTemporaryFile::fileTemplate() const

Returns the set file template. The default file template will be called qcoreappname.XXXXXX and be placed in QDir::tempPath().

See also setFileTemplate().

bool QTemporaryFile::open()

A QTemporaryFile will always be opened in QIODevice::ReadWrite mode, this allows easy access to the data in the file. This function will return true upon success and will set the fileName() to the unique filename used.

See also fileName().

`[override virtual protected]` bool QTemporaryFile::open(QIODeviceBase::OpenMode flags)

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

Creates a unique file name for the temporary file, and opens it. You can get the unique name later by calling fileName(). The file is guaranteed to have been created by this function (i.e., it has never existed before).

bool QTemporaryFile::rename(const QString &newName)

Renames the current temporary file to newName and returns true if it succeeded.

This function has an important difference compared to QFile::rename(): it will not perform a copy+delete if the low-level system call to rename the file fails, something that could happen if newName specifies a file in a different volume or filesystem than the temporary file was created on. In other words, QTemporaryFile only supports atomic file renaming.

This functionality is intended to support materializing the destination file with all contents already present, so another process cannot see an incomplete file in the process of being written. The QSaveFile class can be used for a similar purpose too, particularly if the destination file is not temporary.

See also QSaveFile, QSaveFile::commit(), and QFile::rename().

void QTemporaryFile::setAutoRemove(bool b)

Sets the QTemporaryFile into auto-remove mode if b is true.

Auto-remove is on by default.

If you set this property to false, ensure the application provides a way to remove the file once it is no longer needed, including passing the responsibility on to another process. Always use the fileName() function to obtain the name and never try to guess the name that QTemporaryFile has generated.

On some systems, if fileName() is not called before closing the file, the temporary file may be removed regardless of the state of this property. This behavior should not be relied upon, so application code should either call fileName() or leave the auto removal functionality enabled.

See also autoRemove() and remove().

void QTemporaryFile::setFileTemplate(const QString &name)

Sets the static portion of the file name to name. If the file template contains XXXXXX that will automatically be replaced with the unique part of the filename, otherwise a filename will be determined automatically based on the static portion specified.

If name contains a relative file path, the path will be relative to the current working directory. You can use QDir::tempPath() to construct name if you want use the system's temporary directory. It is important to specify the correct directory if the rename() function will be called, as QTemporaryFile can only rename files within the same volume / filesystem as the temporary file itself was created on.