Path: utzoo!attcan!uunet!mcsun!cernvax!chx400!ethz!ethz-inf!reberhar
From: reberhar@ethz-inf (Rolf Georg Eberhardt)
Newsgroups: comp.lang.c++
Subject: problems with [] operator
Message-ID: <27893@ethz-inf.UUCP>
Date: 28 Jun 90 15:07:57 GMT
Sender: reberhar@inf.ethz.ch
Organization: Informatik ETH Zurich
Lines: 61


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
  ...
};

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;
};


thanks in advance for your advice

-- rolf eberhardt
-- ETH Zuerich, Switzerland
-- e-mail : reberhar@inf.ethz.ch