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copied!<p>Because <code>volatile</code> is not necessary for multithreading, and does nothing beneficial, but potentially destroys a number of optimizations.</p> <p>In order to ensure safe multithreaded access to a variable, the primitive we need is a memory barrier, which provides both the guarantee of <code>volatile</code> and a few others (it prevents memory access reordering across the barrier, which volatile doesn't do)</p> <p>I believe <code>shared_ptr</code> uses atomic operations when possible, which implicitly provide a memory barrier. Otherwise it falls back to a mutex, which also provides a memory barrier.</p> <p>See <a href="https://stackoverflow.com/questions/2484980/why-is-volatile-not-considered-useful-in-multithreaded-c-or-c-programming">Why is volatile not considered useful in multithreaded C or C++ programming?</a> or <a href="http://software.intel.com/en-us/blogs/2007/11/30/volatile-almost-useless-for-multi-threaded-programming/" rel="nofollow noreferrer">http://software.intel.com/en-us/blogs/2007/11/30/volatile-almost-useless-for-multi-threaded-programming/</a> for more details</p> <p><strong>Edit</strong><br> <code>count_type</code> is <em>not</em> a <code>long</code> in the general case. It is <em>convertible</em> to a <code>long</code>. If you look in <code>atomic_count.hpp</code>, the typedef to long is only applied if no threading is available (in which case of course, no synchronization is necessary). Otherwise it uses the implementation defined in <code>boost/smart_ptr/detail/atomic_count_pthreads.hpp</code> or <code>boost/smart_ptr/detail/atomic_count_win32.hpp</code> or one of the other files listed. And those are synchronized wrapper classes that ensures all operations are done atomically.</p>

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