Re: Policy based threading question

From: Aschwin Gopalan (yg-boost-users_at_[hidden])
Date: 2003-05-22 11:29:52

Sorry to reply to myself, but the code below is obviously incorrect
due to some copy-paste braindamage.

Aschwin Gopalan wrote:
> Dear Boosters!
> I am designing a class template with policy controled threading 
> behaviour, like
> 
> template<ThreadingModel=SingleThreaded>
> class Something;
> 
> where there are different ThreadingModels available (see Modern C++ 
> Design by Alexandrescu for a discussion of policies).
> 
> I would like to have Something to inherit ThreadingModel. The idea is 
> that SingleThreaded does not impose any additional penalties. So my Idea
> was to have something like
> 
> class SingleThreaded
> {
> public:
> 	void lock() {}
> 	void release() {}
> }
> 
> class MultiThreaded
> {
> public:
> 	void lock() { lock_mutex(m_mutex) }
> 	void release() { release_mutex(m_mutex) }
> private:
> 	mutex m_mutex;
> }
> and have all functions call lock() and release() accordingly.
> 
> But this is not good, since it does not use Resource Initialization is 
> Aquisition and leads to exeption safety problems and so on. Furthermore, 
> it is not (because of these reasons) possible to model using 
> boost::mutex Žes since they do not expose a lock() release() interface 
> but can only be locked using scoped_locks.
> 
> So another idea is to have something like
> 
> class SingleThreaded
> {
> 	class scoped_lock
> 	{
> 	   // Empty class
> 	}
> }
> 
> class MultiThreaded
> {
> public:
> 	class scoped_lock
> 	{
> 	   scoped_lock() // constructor that locks m_mutex of
>                           // of MultiThreaded
> 	   ~scoped_lock() // unlocks
> 	}
> private:
> 	boost::mutex m_mutex;
> }
> 
> and have the member functions lock the mutex by instantiating
> an embeded scoped_lock object.
> 
> Now there are some questions I have:
> 1.) Does this approach sound sensible, or am I overlooking a commonly
>      used Idiom to achieve this functionality
> 2.) How can I access the m_mutex member of the right object in
>      scoped_lock()? The only way I can imagin is to give the this pointer
>      to the constructor (like scoped_lock  lock(this);). It is somehow
>      not nice, but possible
> 3.) The actual code I am using looks like
> 
> //! Policy class.
> /*! Multithreading, object operations are synchronized via
>   *  a non-recursive lock, obeying strict Resource Acuisition is 
> Initialization
>   *  semantics
>   */
> class LockDefault
> {
> public:
This is to be deleted:
>      LockDefault() :
> 	m_pmutex(new boost::mutex)
> 	{
> 	}
up to here
>      class scoped_lock : public boost::mutex::scoped_lock
>      {
>      public:
> 	scoped_lock(LockDefault &obj) :
> 	    boost::mutex::scoped_lock(obj->m_pmutex)
             boost::mutex::scoped_lock(obj.m_mutex)
> 	    { }
> 	~scoped_lock() { }
>      };
> 
> public:
>      boost::mutex m_mutex;
> 
> };
> 
> This works with gcc and I think it does what it should. It
> fails to compile with VC++6 giving
> 
> error C2512: 'scoped_lock<class boost::recursive_mutex>' : no 
> appropriate default constructor
> 
> which I do not understand, since the default constructor is never
> called.
> Also, I do not like to have the mutex member public, but if I make
> it private and write
> friend class scoped_lock;
> this does not have the desired effect (MSVC still complains about 
> private data member access)
> 
> Somehow, I must be getting something completely wrong, so
> if somebody could point me in the right direction, I would be thankful.
> 
> BTW: I am using boost_1_30_0
> 
> Thanks,
> 
> Aki
> 
> 
> 
> 
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