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From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.lang.c++
Subject: Re: When polymorphic, when not?
Message-ID: <10225@alice.UUCP>
Date: 8 Dec 89 03:39:03 GMT
References: <22395@brunix.UUCP>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 68

In article <22395@brunix.UUCP>, sdm@cs.brown.edu (Scott Meyers) writes:

      1	#include <stream.h> 
      2	
      3	struct B { virtual void foo() { cout << "B::foo\n"; }};
      4	struct D: public B { void foo() { cout << "D::foo\n"; }};
      5	
      6	void val(B p) { p.foo(); }
      7	void ref(B &p) { p.foo(); }
      8	
      9	main() {
     10	  {
     11	    D d;
     12	    B b;
     13	    
     14	    b = d;
		// b is still a B

     15	    D &dr = d;
     16	    B &br = d;
     17	    
     18	    val(b);       // calls B::foo
     19	    val(br);      // calls D::foo [g++ calls B::foo]
		// g++ is correct here.  This is a bug in cfront 2.0,
		// which will be fixed in 2.1

     20	    ref(b);       // calls B::foo [g++ calls D::foo]
		// cfront is correct here.  b is a B.

     21	    ref(br);      // calls D::foo
		// correct.  br refers to a D.
     22	    
     23	    val(d);       // calls D::foo [g++ calls B::foo]
     24	    val(dr);      // calls D::foo [g++ calls B::foo]
		// these two lines exhibit the same cfront bug as above.

     25	    ref(d);       // calls D::foo 
     26	    ref(dr);      // calls D::foo
		// correct.

     27	  }
     28	
     29	  // I don't know what results g++ gives for the following section
     30	  {
     31	    D d;
     32	    B b = d;     // g++ won't compile this line ("unexpected argument
     33	                 // to constructor `B'")
		// apparently a bug in g++.  B always has a copy
		// constructor B::const(B&) and you can bind the
		// argument of that constructor to a D.
     34	
     35	    D &dr = d;
     36	    B &br = d;
     37	    
     38	    val(b);       // calls D::foo
     39	    val(br);      // calls D::foo
     40	    ref(b);       // calls D::foo
     41	    ref(br);      // calls D::foo
     42	    
     43	    val(d);       // calls D::foo
     44	    val(dr);      // calls D::foo
     45	    ref(d);       // calls D::foo 
     46	    ref(dr);      // calls D::foo
     47	  }
     48	}
-- 
				--Andrew Koenig
				  ark@europa.att.com