[Boost-users] Using Boost.Atomic for thread-safe reference counting with strong exception guarantee

Subject: [Boost-users] Using Boost.Atomic for thread-safe reference counting with strong exception guarantee
From: Narcoleptic Electron (narcoleptic.electron_at_[hidden])
Date: 2010-03-08 16:38:29

I'm trying to create a thread-safe, strong-exception-guaranteed
"intrusive_pte" (intrusive pointee) class that represents a drop-in
reference-counted base class for a pointee of intrusive_ptr.  I'm
using the pending Boost.Atomic library
(http://www.chaoticmind.net/~hcb/projects/boost.atomic/) and am basing
my code on the example given
(http://www.chaoticmind.net/~hcb/projects/boost.atomic/doc/atomic/usage_examples.html#boost_atomic.usage_examples.example_reference_counters),
but not yet having a sound understanding of memory barriers, I am
unsure if my code for the overflow exception is correct or ideal.

The relevant code snippet is below.  Any feedback would be greatly appreciated.

Thanks!

---
mutable atomic< IntegralType > ref_count;
mutable mutex add_ref_mutex;
void add_ref() const {
    mutex::scoped_lock add_ref_lock( add_ref_mutex );
    if ( integer_traits< IntegralType >::const_max == ref_count.load(
memory_order_relaxed ) ) {
        throw_exception( std::overflow_error( "Reference count overflow." ) );
    }
    ref_count.fetch_add( 1, memory_order_relaxed );
}
friend void intrusive_ptr_add_ref ( intrusive_pte const * const p ) {
    BOOST_ASSERT( p && "The argument cannot be null." );
    p->add_ref();
}
friend void intrusive_ptr_release ( intrusive_pte * const p ) {
    BOOST_ASSERT( p && "The argument cannot be null." );
    BOOST_ASSERT( ( 0 < p->ref_count.load( memory_order_acquire ) ) &&
"Reference count underflow." );
    if ( p->ref_count.fetch_sub( 1, memory_order_release ) == 1 ) {
        atomic_thread_fence( memory_order_acquire );
        checked_delete( p );
    }
}