----- Mensaje original ----- De: Jeff Garland <jeff@crystalclearsoftware.com> Fecha: Sábado, Mayo 5, 2007 6:10 pm Asunto: Re: [boost] [1.34.0beta] many, many warnings... :( Para: boost@lists.boost.org
Jonathan Franklin wrote:
Interesting. I'm obviously flaunting my ignorance, but I didn't realize that inheriting from a class sans virtual dtor was ever a Good Thing. I'll have to read up on the issues WRT is_polymorphic.
I find inheriting without virtual functions quite useful. The main reason for the virtual destructor rule goes back to Scott Meyers Effective C++. He points out that if you have Base <-- Derived and you call delete Base* the destructor of Derived is not called. In general, it's hard to ensure your derived classes won't need the destructor called and hence the rule. However, if Derived happens to have a trivial/empty destructor then, really there's no reason to call it.
I agree and disagree with your previous exposition :) I think deriving without virtual constructors is fine in those contexts when the user is not expected to delete passed objects, which are by far the most usual. Almost every function of the form void foo([const] T& t) won't certainly try to delete t, so it's fine to pass an use here an object of some class derived from T (like in your super_string case.) IMHO the contexts in which objects are passed around without ownership transfer are overwhelmingly more numerous than those where transfer is done, so I think this "don't derive without virtual dtor" is somewhat overrated. There's the special case when the base is abstract or has some virtual function: here it is more likely that the class was designed with ownership transer via pointers in mind, and the rule would be more sensible. But then again, a great many classes don't have any virtual member function, much more so in the template-dominated C++ of our days. What I don't agree with you in is your claim that "if Derived happens to have a trivial/empty destructor then, really there's no reason to call it." If my memory serves me well, deleting a Derived object through its Base non-virtual dtor is illegal according to the standard, regardless of whether D's dtor is trivial or not. You're merely being lucky with the actual behavior exhibited by compilers. Joaquín M López Muñoz Telefónica, Investigación y Desarrollo