1. Depth
First specify the in-depth analysis depth, otherwise you will fall into the ocean of code.
This article only analyzes the Win32 API/Windows Com component level, and does not explore the lower-level code.
This article mainly understands QFile and its specific implementation. Where the parent class data is used, only the key points are discussed.
2. The function of QFile
Read and write file information
3. Inheritance relationship
QFile>>QFileDevice>>QIODevice>>QObject
4. Data variables
QFile itself has no added member variables, and all member variables are inherited from the parent class.
Most Qt classes have a class name + Private class. This class is mainly used to save data and implement class functions.
For example, QObject has QObjectPrivate to implement specific functions. , QFile has QFilePrivate to realize file reading and writing.
This data method is to update the exported header file without touching it, so that the program does not need to be modified when changing the Qt version, and can be directly compiled and passed.
QFile has a member variable d_ptr from QObject, its type is QObjectData pointer, d_ptr points to QObjectPrivate, and QObjectPrivate inherits from QObjectData.
d_ptr in QFile points to QFilePrivate
5. QFilePrivate inheritance relationship
QFilePrivate>>QFileDevicePrivate>>QIODevicePrivate>>QObjectPrivate
6. QFilePrivate data variable
The variable that QFilePrivate has alone is QString fileName, which represents the open file path. All other variables come from the parent class
Next, introduce the main functions of QFile:
7. Call sequence
Firstly, introduce the general call process of functions.
Most of the functions in QFile are first called into QFileDevice, and then the QFSFileEngine engine is created, and then the corresponding functions are implemented by QFSFileEngine.
8. QFile::permissions
a. 函数作用
获取文件的权限和所有权
b. 具体实现函数
依靠Win32 API GetFileAttributesEx来获取文件属性，C语言函数_waccess来获取文件读写权限。
_waccess在Windows中也是依靠GetFileAttributesEx实现
c. 调用顺序
QFile::permissions调用QFileDevice::permissions。
QFileDevice::permissions调用QFilePrivate::engine。
QFilePrivate::engine中如果fileEngine指针为空，则调用QAbstractFileEngine::create创建QFSFileEngine，并将fileEngine指向QFSFileEngine
通过fileEngine调用QFSFileEngine::fileFlags
QFSFileEngine::fileFlags调用QFSFileEnginePrivate::doStat
QFSFileEnginePrivate::doStat调用QFileSystemEngine::fillMetaData
QFileSystemEngine::fillMetaData调用Win32 API GetFileAttributesEx来获取文件属性并填充QFSFileEnginePrivate::metaData、QFSFileEnginePrivate::fileEntry信息
依照QFSFileEnginePrivate::metaData、QFSFileEnginePrivate::fileEntry返回权限标识
9. QFile::setPermissions
a. 函数作用
设置文件的权限和所有权
b. 具体实现函数
C语言函数_wchmod修改文件权限，_wchmod依靠Win32 API SetFileAttributesEx设置文件权限
c. 调用顺序
QFile::setPermissions调用QFileDevice::setPermissions
QFileDevice::setPermissions调用QFilePrivate::engine，QFilePrivate::engine作用同上
通过fileEngine调用QFSFileEngine::setPermissions
QFSFileEngine::setPermissions调用QFileSystemEngine::setPermissions
QFileSystemEngine::setPermissions具体由C语言_wchmod实现
_wchmod具体由Win32 API SetFileAttributesEx实现，C语言函数更好跨平台，因此很多地方优先C语言函数
10. QFile::size
a. 函数作用
获取文件大小
b. 具体实现函数
依靠Win32 API GetFileAttributesEx来获取文件属性
11. QFile::copy(const QString &newName)
a. 函数作用
拷贝文件
b. 具体实现
Win32 API CopyFile
12. QFile::exists
a. 函数作用
检查文件是否存在
b. 具体实现
Win32 API GetFileAttributesEx
13. QFile::rename
a. 函数作用
文件重命名
b. 具体实现
Win32 API MoveFile
14. QFile::link
a. 函数作用
创建链接文件
b. 具体实现
通过Com组件，先找到CLSID_ShellLink，再查询到IID_IPersistFile，最后调用IPersistFile::Save创建链接
为什么不用CreateSymbolicLink实现？
CreateSymbolicLink：最低客户端Windows Vista，最低服务器Windows Server 2008
IShellLink：最低客户端Windows XP，最低服务器Windows Server 2000
15. QFile::setFileName
a. 函数作用
更改QFile代表的文件，并不会打开文件
b. 具体实现
关闭之前打开的文件并更改QFilePrivate中保存的文件名称
16. QFile::moveToTrash
a. 函数作用
移动到回收站
b. 具体实现
大于win7
通过Com组件找到CLSID_FileOperation，最后调用IFileOperation::DeleteItem再执行IFileOperation::PerformOperations
小于等于win7：SHFileOperation
为什么不用DeleteFile实现？
DeleteFile最低客户端XP、最低服务器Windows Server 2003，并且无法递归删除目录中的文件、无法删除空目录
IFileOperation最低客户端Windows Vista，最低服务器Windows Server 2008。能删目录和递归删除
SHFileOperation最低客户端XP最低服务器Windows 2000 Server。能删目录和递归删除在Windows Vista版本时已被IFileOperation替代
17. QFile::symLinkTarget
a. 函数作用
返回符号链接指向的路径
b. 具体实现
通过Com组件，先找到CLSID_ShellLink，再查询到IID_IPersistFile，最后调用IShellLink::GetPath获取链接指向的文件全路径
18. QFile::open
a. 函数作用
打开文件
b. 具体实现
Win32 API CreateFile
19. QFile::readAll
a. 函数作用
读取文件中所有数据
b. 具体实现
Win32 API ReadFile
20. QFile::write
a. 函数作用
拷贝文件
b. 具体实现
write只是将buffer放在QIODevicePrivate::writeBuffer，除非内存大于0x4000才会写入文件中
21. QFile::flush
a. 函数作用
将QIODevicePrivate::writeBuffer中数据写入到文件中
b. 具体实现
Win32 API WriteFile和fflush
22. QFile::remove
a. 函数作用
删除文件
b. 具体实现
Win32 API DeleteFile
23. QFile::close
a. 函数作用
关闭文件
b. 具体实现
Win32 API CloseHandle