I'd like to learn how to use RAII in c++. I think I know what it is, but have no idea how to implement it in my programs. A quick google search did not show any nice tutorials.
Does any one have any nice links to teach me RAII?
There's nothing to it (that is, I don't think you need a full tutorial).
RAII can be shortly explained as "Every resource requiring cleanup should be given to an object's constructor."
In other words:
Pointers should be encapsulated in smart pointer classes (see std::auto_ptr, boost::shared_ptr and boost::scoped_ptr for examples).
Handles requiring cleanup should be encapsulated in classes that automatically free/release the handles upon destruction.
Synchronization should rely on releasing the mutex/synchronization primitive upon scope exit (see boost::mutex::scoped_lock usage for an example).
I don't think you can really have a tutorial on RAII (not anymore than you can have one on design patterns for example). RAII is more of a way of looking at resources than anything else.
For example, at the moment I'm coding using WinAPI and I wrote the following class:
template<typename H, BOOL _stdcall CloseFunction(H)> class checked_handle { public: typedef checked_handle<H,CloseFunction> MyType; typedef typename H HandleType; static const HandleType NoValue; checked_handle(const HandleType value) : _value(value) { } ~checked_handle() { Close(); } HandleType* operator &() { return &_value; } operator HandleType() { return _value; } private: HandleType _value; void Close(const HandleType newValue = NoValue) { CloseFunction(_value); _value = newValue; } }; template<typename H,BOOL _stdcall CloseFunction(H)> const typename checked_handle<H,CloseFunction>::HandleType checked_handle<H,CloseFunction>::NoValue = checked_handle<H,CloseFunction>::HandleType(INVALID_HANDLE_VALUE); typedef checked_handle<HANDLE,::CloseHandle> CheckedHandle; typedef checked_handle<HWINSTA,::CloseWindowStation> WinStationHandle; typedef checked_handle<HDESK,::CloseDesktop> DesktopHandle; typedef checked_handle<HDEVNOTIFY,::UnregisterDeviceNotification> DevNotifyHandle; typedef checked_handle<HWND,::DestroyWindow> WindowHandle; BOOL __stdcall CloseKey(HKEY hKey); typedef checked_handle<HKEY,CloseKey> RegHandle;
This class doesn't include assignment and copy semantics (I removed them to provide a minimal example) so returning by value, will cause the handles to be closed twice.
Here's how it's used:
class declaration:
class Something { public: // ... private: WindowHandle _window; };
This member is allocated but I never call ::CloseWindow(_window._handle)
explicitely (it will be called when instances of Something
go out of scope (as Something::~Something
-> WindowHandle::WindowHandle
-> ::Close(_window._value)
).
The wikipedia explanation isn't bad.
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