Re: [Boost-users] Suggestion for boost`s smart pointers

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2006-11-29 13:52:24

On Nov 29, 2006, at 12:50 PM, Andrew Holden wrote:

> I have put together an option based on Andrei Alexandrescu's policy
> based design.  It may take a little time to read, as I modified a copy
> of scoped_ptr.hpp.  What I did was add a second template parameter
> (called deleter) and inherited scoped_ptr from this.

If instead of inheriting from D, you contain a compressed_pair<T*,  
D>, you'll get the same size savings and be able to support function  
pointers as deleters.  With a little more work (in your constructors  
mainly) you can support D& types as a deleter.

D& types are really handy for transferring pointer ownership from one  
object to another when there is a custom deleter of unknown type.  Of  
course since you're working with scoped_ptr, perhaps you're not  
interested in this use case.  But just in case you are:

template<class T, class D>
class A
{
     ...
     void foo(T* p);
private:
     T* p_;
     D  deleter_;
     ...
};

// A::foo establishes ownership of p, but
// must acquire other resources to do so.  A local
// unique_ptr is used as an aid to hold on to p while
// those other resources are acquired.
template<class T, class D>
void
A<T, D>::foo(T* p)
{
     // Establish preliminary ownership without requiring
     // a copy of the deleter D
     std::unique_ptr<T, D&> hold(p, deleter_);  // no throw
     // acquire resources                       // if throws,
     // ...                                     //    deleter_(p)  
executed
     // transfer ownership to A
     p_ = hold.release();                       // no throw
}

-Howard