Re: [boost] storing intrusive_ptr in atomic?!

Subject: Re: [boost] storing intrusive_ptr in atomic?!
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2013-10-25 03:41:51

On Friday 25 October 2013 09:35:55 Oliver Kowalke wrote:
> Hello,
> 
> do you confirm that it is valid to store a intrusive_ptr inside an atomic?!
> 
> struct X {
>    std::size_t use_count;
>    X() : use_count( 0) {}
>    friend inline void intrusive_ptr_add_ref( X * p) BOOST_NOEXCEPT
>    { ++p->use_count; }
>    friend inline void intrusive_ptr_release( X * p)
>    { if ( 0 == --p->use_count) delete p; }
> };
> 
> X::ptr_t x1( new X()
> );
> 
> X::ptr_t x2( new X()
> );
> 
> boost::atomic< X::ptr_t > av1(
> x1);
> 
> BOOST_ASSERT( av1.compare_exchange_strong( x1, x2) );
> BOOST_ASSERT( av1.load() != x1);
> BOOST_ASSERT( av1.load() == x2);
> 
> A type T used in generic template atomic<T> is required to be trivial
> copy-able, e.g.
> I think it is trivial copy-able (std::memcpy) only the use_count will not
> incremented.
> The memory layout of intrusive_ptr is a raw-pointer so trivial copy-able
> should be correct.
> 
> What do you think?

intrusive_ptr is not trivial-copyable, precisely because it has to operate on 
the counter. Using memcpy for copying it is not correct. For this reason it is 
not compatible with atomic<>. You have to use raw pointers with atomic<>.