Re: [boost] storing intrusive_ptr in atomic?!

Subject: Re: [boost] storing intrusive_ptr in atomic?!
From: Oliver Kowalke (oliver.kowalke_at_[hidden])
Date: 2013-10-25 08:35:39

2013/10/25 Andrey Semashev <andrey.semashev_at_[hidden]>

> That won't work for several reasons. First, as outlined before, you can't
> store intrusive_ptr in atomic<>. Assuming you store T* instead, then you
> can't
> use nullptr as the first argument because CAS accepts it as an lvalue
> reference. Next, the removal of the pointer is not correct because when CAS
> fails the pointer is just loaded from the atomic, so you release the
> pointer
> and the dangling pointer is left in the array. You have to load e first,
> then
> make a loop with CAS and only release if it succeeds and the loaded
> pointer is
> not NULL.

array< atomic< T * >, 100 > a{ atomic< T* >( 0) };

T * desired = new T();
T * expected = 0;
if ( a[index].compare_exchange_strong( expected, desired) ) {
  // desired added
}

T * expected1 = 0;
T * expected2 = 0;
if ( ! a[index].compare_exchange_strong( expected1, expected2) ) {
  // expected1 is not NULL, expected1 removed == a[index] contains NULL
}

should be OK