Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!ucselx!petunia!kestrel.edu!gyro
From: gyro@kestrel.edu (Scott Layson)
Newsgroups: comp.lang.c++
Subject: Re: possible to overload << and pass an object ptr?
Summary: Two-level class structure
Keywords: overload
Message-ID: <1991Mar24.074308.28666@kestrel.edu>
Date: 24 Mar 91 07:43:08 GMT
References: <todd.669759003@premises1.Quotron.COM>
Organization: Kestrel Institute, Palo Alto, CA
Lines: 42

In article <todd.669759003@premises1.Quotron.COM> todd@Quotron.COM (Todd Booth) writes:
>Consider the following struct:
>
>struct S {
>	void operator<<(char p1) { cout << p1; }
>};
>
>S *o2 = new S;
>o2 << 'a'; // error <--- How can I resolve this?
>*o2 << 'a'; // no problem, but ugly
>
>S o1;
>o1 << 'a'; // no problem

I have used a two-level class structure in cases like this:

struct S {
     struct S_impl {
          // ... whatever
     } *impl;
  public:
     void operator<<(char p1) { cout << p1; }
     // constructors etc.
};

This is a pretty good solution, but it starts to break down when you
want to have parallel hierarchies of inner and outer classes, because
`impl' (the sole data member of S which will be inherited by all of
its derived classes) is always of type `S_impl*', while you will want
it to be a pointer to the appropriate derived class of `S_impl'.
Casting `impl' to be the right type is a bit messy but works, until
you start using multiple inheritance with virtual base classes, at
which point you're totally up a tree.

For these reasons I have toyed with the idea of proposing to the
committee a system for declaring "member functions" on the type `C*'
where C is some class, so that your first example could be made to
work without the two-level class structure.  Unfortunately, I have
never managed to come up with a proposal I find adequately elegant.

-- Scott Layson Burson
   Gyro@Reasoning.COM