David Abrahams' Guidelines (republished with permission from http://mail.python.org/pipermail/cplusplus-sig/2008-October/013895.html):

 * handle, essentially a smart pointer.  Use when necessary.

   * a handle<> can be NULL, and maintains a reference count on the object it points to

   * handle<> y(null_ok(x)) allows y to become NULL

   * handle<> y(x), where x is not the result of null_ok, never results in a NULL y.  An exception will be thrown if x is NULL

   * handle<> y(borrowed(x)) presumes that *x is borrowed and thus increments its reference count.

   * handle<> y(x), where x is not the result of borrowed, presumes that someone has already incremented the reference count on *x for us.

   * you can combine borrowed and null_ok in any order, so the following are equivalent:

       * handle<> y(borrowed(null_ok(x)))
       * handle<> y(null_ok(borrowed(x)))

 * object, a higher-level notion.  Use wherever possible.

   * an object can't be constructed from a raw PyObject* because there's no information in that type about whether the refcount has been incremented for this additional reference

   * an object can only be constructed from a handle<>.  Other interfaces are not for public consumption and thus not documented.  Use at your own peril.

   * an instance of object always "points to" something (maybe None). If the constructor argument (handle) is NULL, an exception will be thrown.

You should also always give the handle<> a name instead of making it a temporary for the same reasons as cited in Peter Dimov's guideline: [[http://www.boost.org/doc/libs/1_36_0/libs/smart_ptr/shared_ptr.htm#BestPractices|"Smart Pointer Best Practices"]]