RAII tutorial for C++ [closed]

Question

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?

Answer 1

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) ).

Answer 2

The wikipedia explanation isn't bad.