David Abrahams' Guidelines (republished with permission from https://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: "Smart Pointer Best Practices"