The threadsafe Template Parameter

The threadsafe and copy_on_write parameters on the integer_t template control the copy-on-write behavior of the library. When using copy_on_write, identical integer objects are allowed to share storage, which is more efficient for both CPU power and memory, but which is generally unsafe for multithreaded use.

That's a simplification though. The only problem multithreaded programs have with copy-on-write is that one thread might change an object while another is reading from it. Since the copy-on-write behavior is entirely internal to the class, no other code can tell when it's safe to make changes to an object. This is only a problem when more than one thread can access a specific object. So long as an integer object is only accessed from the thread that it's created in, there's no danger.

If you want the speed of copy-on-write (which can be noticeable if you're dealing with very large numbers), but need to use integer objects from multiple threads when one or more might alter them, you can use the force_thread_safety parameter on the object's copy constructor to make a duplicate of an object with its own unique storage, even if using the copy_on_write option. You'll only need to do that when the integer object is crossing thread boundaries.