Path: utzoo!attcan!uunet!samsung!umich!sharkey!amara!des
From: des@amara.uucp (Dave Steinhoff)
Newsgroups: comp.lang.c++
Subject: Reference pointer function arguments baroquen?
Message-ID: <DES.90May24232259@perch.amara.uucp>
Date: 25 May 90 03:22:59 GMT
Sender: news@adi.COM
Distribution: na
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Lines: 65


I ran into this case in a library someone else developed with a C++1.2
version of cfront.  I am using SUNC++2.0.  The authors apparently got
a different result than I am getting, because their software works.  I
apologize if this has been discussed before.

In the program listed below, the compiler warns me:

 "conversion of lvalue needed: temporary used to initialize reference"

on the call 'f(i2);' It does indeed make a local copy of the pointer,
which I don't understand.  I would have expected it to be handled as a
reference parameter with the actual argument's address passed through,
even though it is a pointer to a derived class.

When I change the call to 'f((Int*)i2);' as shown in the comment
below, the warning message goes away, but the same behavior occurs.

Can anyone explain this, or is the subclass-pointer-reference
mechanism baroquen...

------------------------------

#include <iostream.h>
class Int {};
class Int2 : public Int {};

void f (Int *& i)
 { i = new Int;}
void f2 (Int2 *& i)
 { i = new Int2;}
void f3 (Int **i)
 { *i = new Int;}

main ()
 {Int2* i2 = new Int2;
  Int * i1 = new Int;

  cout << (long) i2 << " " << (long) i1 << "\n";
  f (i2); f(i1);                  // f(i2) -> f((Int*)i2;
  cout << (long) i2 << " " << (long) i1 << "\n";
  f3 ((Int**)&i2);
  cout << (long) i2 << "\n";
  f2(i2);
  cout << (long) i2 << "\n";
  }

/* ------------------------------
Output with f(i2) and f((Int*)i2) is identical:
139832 139840
139832 140884
140892
140900
------------------------------ */

---------------------------------------------
Cheers,
Dave
--
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Dave Steinhoff                              Applied Dynamics, Int'l
des@amara.UUCP                              3800 Stone School Rd.
des@adi.com                                 Ann Arbor, Mi 48108
...uunet!amara!des                          (313)973-1300                 
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