Path: utzoo!attcan!uunet!cs.utexas.edu!milano!cadillac!puma!vaughan
From: vaughan@puma.cad.mcc.com (Paul Vaughan)
Newsgroups: comp.lang.c++
Subject: Re: problems with [] operator
Message-ID: <9300@cadillac.CAD.MCC.COM>
Date: 29 Jun 90 15:18:04 GMT
Sender: news@cadillac.CAD.MCC.COM
Lines: 75



From: reberhar@ethz-inf (Rolf Georg Eberhardt)

   hi everybody,

   I've encountered a problem concerning the [] operator. I've defined following class:

   typedef class foo *foo_p;
   class foo {
   public:
     foo_p	next, list;		// each object of foo may contain a list of other "foo's"
     foo_p operator [] (ndx);
   };

   foo_p foo::operator [] (ndx) {
     foo_p tmp = list;

   //  ... run thru list starting at list and return the ndx'th element

     return(tmp);
   };

   I get following error msg's : 

   void foo::do_something(foo_p b) {
     list[i]->next = b; 		// error : non pointer -> next	
     b->next = a[i];		// error : bad assignment type: foo_p = foo
     ...
   };


list is a foo*, list[i] is a foo--not the result of applying the
operator [] to a foo, just the usual array [] on a foo*.  Try
(*list)[i] if you wanted to invoke foo::operator[].

   In all C++ books I've looked into the []-operator must return a reference. Why - on the other 
   hand - does following code compile and run correctly ?

   typedef class bb *bb_p;

   class bb {
   public:
     bb_p next;
     int  key;
     // etc...
   };

   class cc {
   public:
     bb_p list;
     bb_p operator[] (int ndx);		// same code as above
   };


   void main() {
     cc c;
     // do some initializing ..

     c[2]->key = 4;
     cout << c[2]->key;
   };

The operator[] can return whatever you want--no restriction to
references.  That's just a common usage.  Here c is a cc (not a cc*, like above),
thus c[2] invokes cc::operator[] which returns a bb*.  The bb* (being
a pointer) can be deferenced using -> to get the key field, so the
code compiles fine.
-- 
 Paul Vaughan, MCC CAD Program | ARPA: vaughan@mcc.com | Phone: [512] 338-3639
 Box 200195, Austin, TX 78720  | UUCP: ...!cs.utexas.edu!milano!cadillac!vaughan


 Paul Vaughan, MCC CAD Program | ARPA: vaughan@mcc.com | Phone: [512] 338-3639
 Box 200195, Austin, TX 78720  | UUCP: ...!cs.utexas.edu!milano!cadillac!vaughan