[boost] Re: Lock unification

From: Eric Niebler (eric_at_[hidden])
Date: 2004-07-16 23:09:35

Peter Dimov wrote:

> Eric Niebler wrote:
> 
>>I continue to believe that a better interface for a unified lock
>>involves descriptively named helper functions:
>>
>>scoped_lock l1( m );                 // lock unconditionally
>>scoped_lock l1 = defer_lock( m );    // don't lock
>>scoped_lock l2 = try_lock( m );      // try lock
>>scoped_lock l3 = timed_lock( m, t ); // timed lock
> 
> 
> Maybe. Do you have a sketch of the specification? (Not the implementation.)

I don't, and I have family in town until Wednesday. (Arg! Lame answer, I 
know.)

I just looked for my post in the ASPN archive, and it seems to have 
vanished into the ether, so I'm reproducing the code here.

   struct lock;
   lock try_lock();

   struct lock_ref
   {
     lock_ref(lock & ref)
       : ref_(ref)
     {
     }
     lock & ref_;
   };

   struct lock
   {
     lock(lock_ref ref)
     {
         // ... move the lock from ref.ref_ ...
     }
     operator lock_ref()
     {
       return lock_ref(*this);
     }
     operator bool() const
     {
       // TODO return true iff we're holding the lock
       return true;
     }

   private:
     friend lock try_lock();
     lock(/*params*/){} // a try-lock c'tor

     // make this type non-copyable, non-assignable
     lock(lock &);
     lock & operator=(lock &);
   };

   inline lock try_lock(/*params*/)
   {
     // call a special try-lock c'tor on lock
     return lock(/*params*/);
   }

   int main()
   {
     if( lock l = try_lock(/*params*/) )
     {}
   }

The idea is to have just one lock class (leaving aside read/write locks) 
and a collection of factory functions like try_lock(), timed_lock(), 
defered_lock() etc. Those functions create a lock object by calling the 
appropriate (private) constructor and returning the new lock object. 
(Some fancy auto_ptr-like move shenanigans are needed to make the lock 
return-able but not copy-able.)

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com