Path: utzoo!attcan!uunet!husc6!cmcl2!rutgers!att!ulysses!andante!alice!ark
From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.lang.c++
Subject: Re: Type Compatibility
Message-ID: <8810@alice.UUCP>
Date: 21 Jan 89 05:22:53 GMT
References: <5485@mit-vax.LCS.MIT.EDU>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 53

In article <5485@mit-vax.LCS.MIT.EDU>, waldemar@mit-vax.LCS.MIT.EDU (Waldemar Horwat) writes:

> Given the declarations below, which of the following seven statements
> (numbered 1, 2, 4, 5, 6, 7, and 8) are acceptable in a "portable" C++ program?

> struct A {int x;};
> struct B:A {int y;};

> extern void f1(void *);
> extern void g1(A *);
> extern void i1(A &);

> extern void f2(void **);
> extern void g2(A **);
> extern void h2(void *&);
> extern void i2(A *&);

> B b,*pb,**ppb;

Here are my comments, along with the messages that appear
when I run it on the (still experimental) cfront on my machine.

> void test()
>   {
>   f1(pb);   //1
		OK.  f1 takes a void* argument so you can give it
		a pointer to anything at all.
>   g1(pb);   //2
		OK.  g1 takes an A*, you've given it a B*, and
		B is (publicly) derived from A.
>   i1(b);    //4
		OK.  References are just like pointers.  i1 takes
		an A&, you've given it a B&.
>   f2(ppb);  //5   bad argument 1 type for f2(): B ** (void ** expected)
		No.  You can convert anything to void* but you can't
		convert anything to void**.  f2 takes a void** and ppb
		is a B**.
>   g2(ppb);  //6   bad argument 1 type for g2(): B ** (A ** expected)
		No.  Again, you can't treat a pointer to an B* as if
		if it were a pointer to an A*.
>   h2(pb);   //7   warning: coercion of lvalue needed: temporary used
			to initialize reference
		Yes (!).  h2 wants a reference to a void* and you've
		given it a B*.  So it converts the B* to a void*
		(no problem there) and binds the reference to that.
>   i2(pb);   //8   warning: coercion of lvalue needed: temporary used
			to initialize reference
		Yes -- same reason.  You can convert pb to an A*
		and then bind a reference to it.
>   }
-- 
				--Andrew Koenig
				  ark@europa.att.com