QSaveFile Class
The QSaveFile class provides an interface for safely writing to files. 更多...
| 头文件: | #include <QSaveFile> |
| qmake: | QT += core |
| 开始支持版本: | Qt 5.1 |
| 基类: | QFileDevice |
Note: All functions in this class are reentrant.
公有函数
| QSaveFile(const QString &name) | |
| QSaveFile(QObject *parent = Q_NULLPTR) | |
| QSaveFile(const QString &name, QObject *parent) | |
| ~QSaveFile() | |
| void | cancelWriting() |
| bool | commit() |
| bool | directWriteFallback() const |
| void | setDirectWriteFallback(bool enabled) |
| void | setFileName(const QString &name) |
重新实现的公有函数
- 16 个公有函数继承自 QFileDevice
- 43 个公有函数继承自 QIODevice
- 32 个公有函数继承自 QObject
重新实现的受保护函数
| virtual qint64 | writeData(const char *data, qint64 len) |
- 3 个受保护的函数继承自 QFileDevice
- 5 个受保护的函数继承自 QIODevice
- 9 个受保护的函数继承自 QObject
其他继承的成员
- 1 个属性继承自 QObject
- 1 个公有槽函数继承自 QObject
- 6 个信号继承自 QIODevice
- 2 个信号继承自 QObject
- 11 个静态公有成员继承自 QObject
- 3 个受保护的函数继承自 QFileDevice
- 5 个受保护的函数继承自 QIODevice
- 9 个受保护的函数继承自 QObject
详细描述
The QSaveFile class provides an interface for safely writing to files.
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.
参见 QTextStream, QDataStream, QFileInfo, QDir, QFile, and QTemporaryFile.
成员函数
QSaveFile::QSaveFile(const QString &name)
Constructs a new file object to represent the file with the given name.
QSaveFile::QSaveFile(QObject *parent = Q_NULLPTR)
Constructs a new file object with the given parent.
QSaveFile::QSaveFile(const QString &name, QObject *parent)
Constructs a new file object with the given parent to represent the file with the specified name.
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.
参见 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.
参见 cancelWriting().
bool QSaveFile::directWriteFallback() const
Returns true if the fallback solution for saving files in read-only directories is enabled.
参见 setDirectWriteFallback().
[virtual] QString QSaveFile::fileName() const
Reimplemented from QFileDevice::fileName().
Returns the name set by setFileName() or to the QSaveFile constructor.
参见 setFileName().
[virtual] bool QSaveFile::open(OpenMode mode)
Reimplemented from QIODevice::open().
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 and QIODevice::Append are not supported at the moment.
参见 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.
参见 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.
参见 QFile::setFileName() and fileName().
[virtual protected] qint64 QSaveFile::writeData(const char *data, qint64 len)
Reimplemented from QIODevice::writeData().