Re: [boost] Finalizing a class

From: Mark Rodgers (mark.rodgers_at_[hidden])
Date: 2002-02-13 15:33:34

From: "Daniel Frey" <daniel.frey_at_[hidden]>

>   template< typename T > class finalize
>   {
>      friend T;
>      finalize() {}
>   };

This has been suggested many times in the past but unfortunately
the standard gets in the way.  See 7.1.5.3p2:

   "If the identifier resolves to a typedef-name or a template
   type-parameter, the elaborated-type-specifier is ill-formed.
   [Note: this implies that, within a class template with a
   template type-parameter T, the declaration

          friend class T;

   is ill-formed. ]"

Otherwise, yes, your finalize template would be a useful addition
to utility.hpp, although I'd prefer a different name maybe "key"
since it had other uses before it was made illegal.  For example, 
you could use it to restrict a function so that only a certain class
can call it:

struct Y;
struct X { 
    void f( key<Y> );
};
struct Y { 
    void g(X x) { 
        x.f( key<Y>() ); 
    }
};

Since only Y can construct a key<Y>, no-one else can call X::f
(unless Y gave them the key).

Mark