I was just (re-)reading item 46, "Define non-member functions inside templates when type conversions are desired", in Scott Meyers "Effective C++, 3rd edition", and was wondering how and why the comparison operators for intrusive_ptr work when used with NULL, even though they're > not friend functions as described in Scott Meyer's item 46. I believe you're talking about the ancient technique of friend injection used here for example: http://en.wikipedia.org/wiki/Barton%E2%80%93Nackman_trick
The modern good way of doing this Koenig lookup which I think deprecates that trick. http://en.wikipedia.org/wiki/Argument-dependent_name_lookup Just define the operator== in the namespace of the type it works on. It'll find that name in the first lookup stage and then look for a fitting overload. In your code the first one is taken. Though I'm not sure what U is; I'm guessing void. template<class T, class U> inline bool operator==(intrusive_ptr<T> const & a, U * b); template<class T, class U> inline bool operator==(T * a, intrusive_ptr<U> const & b); Chris