[Boost-users] intrusive_ptr vs shared-ptr

Dear Boosters, The documentation of intrusive_ptr says "As a general rule, if it isn't obvious whether intrusive_ptr better fits your needs than shared_ptr, try a shared_ptr-based design first." but does not explain why. If the classes involved are all my own and deriving them all from a base class that implements reference-counting is not a problem, what are some reasons to prefer shared_ptr to intrusive_ptr? Thanks, Ilya