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: "Smart Pointer Best Practices"