"John Nagle" <nagle@animats.com> wrote in message
news:JsT2a.302$9G2.40698922@newssvr14.news.prodigy.com...
> Over the years, C++ has acquired more attributes (volatile,
> mutable, explicit) to deal with similar ambiguous situations.
> What's needed here is a way to say "under construction".
> This leads to restrictions similar to those for an
> class whose declaration isn't visible - pointers to
> it can be manipulated, but you can't access its members.

This fits in my idea of private variables. For example,

struct S {int x; private: int y;};

 int main()
{
   S s;

   private S& ref = s;
   ref.x = 3; //Error; cannot access member of private structure 'ref'

   public S& ref2 = private_cast<public S&>(s);
   ref.y = 4; //OK.
}


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   One common construct is a backpointer, where one object
creates another object that points back to the first.

 class B;
 class A {
 private:
  B& owned;
 public:
  A();
 };

 class B {
 private:
  A& owner;
 public:
  B(A& myowner)
  : owner(myowner) {}
 };

A::A()    // constructor implementation
{ owned = *new B(*this); }  // ILLEGAL - passed unconstructed "this"

This is a common, although illegal, idiom.  It should
be regularized.

Over the years, C++ has acquired more attributes (volatile,
mutable, explicit) to deal with similar ambiguous situations.
What's needed here is a way to say "under construction".
This leads to restrictions similar to those for an
class whose declaration isn't visible - pointers to
it can be manipulated, but you can't access its members.

Comments?

    John Nagle
    Animats

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