Re: Implementation of shared_ptr

Juha Nieminen <nospam@thanks.invalid>
Thu, 29 Jan 2009 17:26:58 GMT
Marcel M?ller wrote:

fungus wrote:

This is a question about the implementation of shared_ptr:

Does shared_ptr allocate a little control block to hold the reference


  In fact, boost::shared_ptr allocates a block which is rather larger
than one reference count (because it has to support, among other things,
a deleter function and weak pointers, and it must be thread-safe, so it
requires a mutex).

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.

  Your ref_count class lacks a proper copy constructor and assignment
operator, which means that it will fail spectacularly if objects
inherited from it are copied/assigned around (if the reference count in
the source and target are different).

  This is an extremely typical mistake with intrusive reference
counting. It's trivial to fix, though.

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