Re: Implementation of shared_ptr

=?ISO-8859-1?Q?Marcel_M=FCller?= <>
Thu, 29 Jan 2009 10:14:44 +0100

fungus wrote:

This is a question about the implementation of shared_ptr:

Does shared_ptr allocate a little control block to hold the reference


If so, this means double the number of memory allocations
(one for the object, one for the pointer).

If you want to come around this you should have a look at intrusive_ptr.
It looks a bit complicated to implement the free functions
intrusive_ptr_add_ref and intrusive_ptr_release. But there is a very
simple and generic solution: write a simple abstract base class for the
reference count.

Something like that:

class ref_count
{ friend void intrusive_ptr_add_ref(ref_count*);
   friend void intrusive_ptr_release(ref_count*);
   unsigned count;
   ref_count : count(0) {}
   virtual ~ref_count() {}
   bool ref_is_managed() { return ref_count != 0; }
   bool ref_is_unique() { return ref_count == 1; }
   // Only a return value of true is thread-safe.

void intrusive_ptr_add_ref(ref_count* ref)
{ interlocked_increment(&ref->count);

void intrusive_ptr_release(ref_count* ref)
{ interlocked_decrement(&ref->count);
   if (!ref->is_managed())
     delete ref;

Now all you have to do for an intrusive reference count is to inherit
from ref_count. This will add the size of the reference count to your
data object, and the intrusive pointer instances are binary identical to
ordinary C pointers. Of course, they are semantically different.
Whether the virtual table pointer of ref_count takes additional space
depends. If your object is polymorphic anyway and you derive from
ref_count without multiple inheritance, there is usually no additional
space required.

If you do not want ref_count to have to have virtual functions all you
have to change is to move the delete operation in a template function
that is aware of the real type of your object. In this case you can
safely downcast ref_count* to your type and delete the result. This will
allow you to use non-polymorphic objects with intrusive_ptr. Of course,
the destructor of ref_count has to be non-virtual in this case.

Note that the above implementation is thread-safe, as long as you
provide appropriate interlocked increment and decrement functions.


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