On x86 this is certainly not an issue, but on machines with more relaxed memory model (ia64, sparc or power), in my opinion, this can be an issue... If you take a look at the IA64 and PPC versions of sp_counted_base, you'll see that they do include memory barriers.
In file boost_1_33_1/boost/detail/sp_counted_base_gcc_ia64.hpp : class sp_counted_base { private: //... long use_count_; // #shared long weak_count_; // #weak + (#shared != 0) public: sp_counted_base(): use_count_( 1 ), weak_count_( 1 ) // <--------- ordinal store In file boost_1_33_1/boost/detail/sp_counted_base_gcc_ppc.hpp the same.... In file boost_1_33_1/boost/detail/atomic_count_gcc.hpp : class atomic_count { public: explicit atomic_count(long v) : value_(v) {} // <--------- ordinal store ///... private: //... mutable _Atomic_word value_; }; gcc can compile to Alpha and to IA-64 and to PPC... What I miss?