I've replaced shared_ptr<> by intrusive_ptr<> + atomic< std::size_t > in boost.thread's future implementation. atomic< std::size_t > use_count; // initialized with '0' in ctor friend inline void intrusive_ptr_add_ref( future_continuation_base * p) BOOST_NOEXCEPT { ++p->use_count; // should be seq_cst } friend inline void intrusive_ptr_release( future_continuation_base * p) { if ( 0 == --p->use_count) // should be seq_cst delete p; } I get an error if I compile the code with gcc-4.7.2 on x86_64: segmentation fault backtrace: 0x0000000000426a96 in boost::detail::atomic_exchange_and_add (pw=0x436a40, dv=-1) at ../../../boost/smart_ptr/detail/sp_counted_base_gcc_x86.hpp:50 50 ); (gdb) bt #0 0x0000000000426a96 in boost::detail::atomic_exchange_and_add (pw=0x436a40, dv=-1) at ../../../boost/smart_ptr/detail/sp_counted_base_gcc_x86.hpp:50 #1 0x0000000000426ba7 in boost::detail::sp_counted_base::release (this=0x436a38) at ../../../boost/smart_ptr/detail/sp_counted_base_gcc_x86.hpp:143 #2 0x0000000000426c57 in boost::detail::shared_count::~shared_count (this=0x7fffffffc868, __in_chrg=<optimized out>) at ../../../boost/smart_ptr/detail/shared_count.hpp:368 #3 0x0000000000427c3e in boost::shared_ptr<boost::exception_detail::clone_base const>::~shared_ptr (this=0x7fffffffc860, __in_chrg=<optimized out>) at ../../../boost/smart_ptr/shared_ptr.hpp:316 #4 0x000000000042b2cd in boost::shared_ptr<boost::exception_detail::clone_base const>::operator= (this=0x75df00, r=...) at ../../../boost/smart_ptr/shared_ptr.hpp:485 #5 0x0000000000427d6f in boost::exception_ptr::operator= (this=0x75df00, other=...) at ../../../boost/exception/detail/exception_ptr.hpp:73 #6 0x0000000000429a2f in boost::fibers::detail::future_object_base::mark_exceptional_finish_internal (this=0x75def0, e=..., lock=...) at ../../../boost/fiber/future.hpp:278 #7 0x000000000043749a in boost::fibers::detail::task_base<int>::owner_destroyed (this=0x75def0) at ../../../boost/fiber/future.hpp:1241 #8 0x000000000042e768 in boost::fibers::packaged_task<int>::~packaged_task (this=0x7fffffffc9b0, __in_chrg=<optimized out>) at ../../../boost/fiber/future.hpp:1341 called funtions: ~packaged_task() -> task_base::owner_destroyed() -> future_object_base::mark_exceptional_finish_internal() -> exception_ptr::operator=(excepton_ptr const&) -> shared_ptr -> shared_count -> sp_counted_base_gcc_x86 -> boost::detail::atomic_exchange_and_add() I'm wondering why a simple assignement of excpetion_ptr causes a segmentation fault if future's implementation uses intrusive_ptr + atomic + sequential - consistent ops instead of shared_ptr. Does someone have an explanation?