RE: [boost] Finalizing a class

From: Yitzhak Sapir (ysapir_at_[hidden])
Date: 2002-02-13 10:34:25

Try looking here:
http://www.research.att.com/~bs/bs_faq2.html#no-derivation

> -----Original Message-----
> From: George A. Heintzelman [mailto:georgeh_at_[hidden]]
> Sent: Wednesday, February 13, 2002 5:26 PM
> To: boost_at_[hidden]
> Subject: Re: [boost] Finalizing a class
> 
> 
> 
> > >By 'final', I mean a way to mark a class 'final' like in 
> Java to prevent
> > >others from deriving from that class. A simple solution of this is
> > >something like this:
> > 
> > I've wanted 'final' before, but for a reason this doesn't 
> address: optimization.  Within a 'final' class, the compiler
> > can statically bind all virtual member function references, 
> since it's knows that there can be no more-derived class
> > that it doesn't know about.
> > 
> > That, unfortunately, doesn't appear to have a solution 
> without a language extension or very sophisticated whole-program
> > optimization.
> 
> There is a solution if you wrap the virtual call in 
> non-virtual calls. 
> This is a little bit of a pain, plays a few games with 
> implementation-hiding, and it's possibly a little dangerous if used 
> around unskilled maintenance programmers, so beware the dangers of 
> premature optimization. That said, here's how to do it:
> 
> class Base {
>   virtual void doF();
> public:
>   void F() { doF(); }; // Non-virtual call
> };
> 
> class Derived:public base {
>   virtual void doF(); // final override...
> public:
>   void F() { Derived::doF(); } 
> 
> };
> 
> The explicit call to Derived::doF() enables the compiler to optimize 
> F() calls for Derived and, as a bonus, any further derived classes of 
> Derived. The class itself need not be final, though the override must 
> be. Unfortunately, AFAIK, there's no way of preventing 
> further derived 
> classes from overriding doF(), which would lead to some very bizarre 
> behavior. Hence the danger. But in certain kinds of hierarchies with 
> certain kinds of needs, it can be valuable.
> 
> A language extension to mark virtual overrides as final, ie 
> not virtual 
> for any more-derived classes, would IMHO be welcome as a safer way of 
> doing this. Hey! How about using the static keyword for this? :) It's 
> probably the only context where static is illegal right now, 
> we need to 
> do something about that.
> 
> George Heintzelman
> georgeh_at_[hidden]
> 
> 
> 
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