[boost] Re: Moveable locks

From: Eric Niebler (eric_at_[hidden])
Date: 2004-07-16 22:42:58

Christopher Currie wrote:

> 
> As a compromise, we could create lock_transfer<> objects that wouldn't 
> lock a mutex, but would allow initialization syntax in places where the 
> language makes it difficult (this is just a quick sketch, there may be 
> errors):
> 

<code snipped>

This won't work. Try to compile this with Comeau online:

   struct lock_transfer {};

   struct scoped_lock
   {
       scoped_lock( lock_transfer ) {}
   private:
       scoped_lock( scoped_lock const & );
       scoped_lock & operator =( scoped_lock const & );
   };

   int main()
   {
       scoped_lock l = lock_transfer();
       return 0;
   }

You'll get:

   "ComeauTest.c", line 13: error: "scoped_lock::scoped_lock(const
   scoped_lock &)",
           required for copy that was eliminated, is inaccessible
           scoped_lock l = lock_transfer();
                       ^

Non-copyable types can't be initialized this way. The code I posted 
earlier worked around this problem by enabling just enough move 
semantics to allow a lock to be returned from a function, but nothing 
else. The lock is still non-copyable.

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