Why do C++ smart pointer implementations keep the reference counter on the heap together with the pointee?

Question

Reading Alexandrescu and wikipipidia I see the pointee and the reference counter are stored on the heap. Then there is mention that reference counting is inefficient as counter must be allocated on the heap? Why isn't it stored on the stack?

Answer 1

Because you would lose it as soon as the current instance of the smart pointer goes out of scope.

A smart pointer is used to simulate automatic storage objects that were allocated dynamically. The smart pointers themselves are managed automatically. So when one is destroyed, anything it stores in automatic storage is also destroyed. But you don't want to lose the reference counter. So you store it in dynamic storage.

Answer 2

It can't be stored on the stack because then a copy of the object would also result in a copy of the refcount, which would defeat its purpose.

Answer 3

As others have pointed out, the stack isn't an appropriate place to keep the reference count because the object may outlive the current stack frame (in which case the reference count would go away!)

It's worth noting that some of the inefficiencies associated with putting the reference count on the heap can be overcome by storing it "together" with the object itself. In boost, this can be accomplished by using boost::make_shared (for shared_ptr's) or boost::intrusive_ptr.