Re: Implementation of shared_ptr

=?ISO-8859-1?Q?Marcel_M=FCller?= <>
Thu, 29 Jan 2009 21:41:18 +0100
Juha Nieminen wrote:

  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).

You are right. The objects that I protect by a class like this have an
application wide primary key. So they are non-copyable anyway, because
if I copy them the key would no longer be unique.

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

class ref_count
{ friend void intrusive_ptr_add_ref(ref_count*);
   friend void intrusive_ptr_release(ref_count*);
   unsigned count;
   ref_count() : count(0) {}
   ref_count(const ref_count&) : count(0) {}
   virtual ~ref_count() {}
   ref_count& operator=(const ref_count&) { return *this; }
   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.

But maybe it makes more sense to derive from boost::non_copyable
instead, because copying reference counted objects is likely to be not
what you intended. A derived class may still implement copy and
assignment semantics explicitly.

But intrusive reference counting is still a extremely lightweight method
for moderate requirements. The runtime overhead is quite small. OK the
interlocked access to the reference counter does not scale that good on
x86/x64 SMP machines, but this is more related to x86/x64 than to the


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