Re: [boost] Re: Question about interpretation of ISO C++ standard

From: John Max Skaller (skaller_at_[hidden])
Date: 2001-05-21 21:52:59

David Abrahams wrote:

> > That's roughly my understanding too. But now note that
> > the original trick relied on injection: inserting the declaration
> > of a function first declared as a friend inside a class
> > just before the class definition: I don't believe that happens
> > any more. Instead, Koenig lookup is used. That means that
> > the friend needs to have an argument of the class type
> > to be found outside the class.
> 
> No, just an argument from the same namespace as the class type.

        I assumed that was understood without mentioning it.
 
> > When you start using templates all hell breaks loose.
> > [The lookup rules appear to be imprecise, incomplete,
> > and/or inadequate; but it is hard to tell]
> 
> They're well-defined. No need for FUD here.

        I post the example I posted to the
committee reflector, this example doesn't even
involve recursive templates (there is recursion,
but only between ordinary classes). 

----------- FAILS UNDER G++ 2.95.3 -------------------

template<class T> struct smart_ptr {
  smart_ptr(T *t) { t->incref(); } // INLINE
  smart_ptr(){}
};
class X;
class Y;
struct X { 
  smart_ptr<Y> y; 
  void incref() {}
  X(Y *ya) : y(ya){} 
  X(){}
};

struct Y { 
  smart_ptr<X> x; 
  void incref() {}
  Y(X *xa) : x(xa){} 
  Y(){}
};

//template<class T> 
//smart_ptr<T>::smart_ptr<T>(T *t) { t-> incref(); }

void main()
{
  X x;
}

----------- WORKS UNDER G++ 2.95.3 --------------------

template<class T> struct smart_ptr {
  smart_ptr(T *t); // OUT OF LINE
  smart_ptr(){}
};
class X;
class Y;
struct X { 
  smart_ptr<Y> y; 
  void incref() {}
  X(Y *ya) : y(ya){} 
  X(){}
};

struct Y { 
  smart_ptr<X> x; 
  void incref() {}
  Y(X *xa) : x(xa){} 
  Y(){}
};

template<class T> 
smart_ptr<T>::smart_ptr<T>(T *t) { t-> incref(); }

void main()
{
  X x;
}

-- 
John (Max) Skaller, mailto:skaller_at_[hidden]
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