Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!rochester!cornell!uw-beaver!uw-june!pardo
From: pardo@june.cs.washington.edu (David Keppel)
Newsgroups: comp.std.c
Subject: Re: const in latest draft
Message-ID: <8408@june.cs.washington.edu>
Date: 1 Jun 89 01:22:43 GMT
References: <16259@gryphon.COM>
Reply-To: pardo@cs.washington.edu (David Keppel)
Organization: U of Washington, Computer Science, Seattle
Lines: 82

In article <16259@gryphon.COM> sarima@gryphon.COM (Stan Friesen) writes:
>[Discussion of `const' qualifier]

I posted a related question about this a while back.  I kept the
e-mail that I exchanged, in the hope that it would make sense
eventually.  I've sat on this for a while and I'm still not sure that
I understand.  Here goes again:

I declare a function parameter such as is used by qsort.  Unlike
qsort, I don't in general require that the function have no side
effects, but do want to be able to pass functions that don't have side
effects.

My understanding of the `const' qualifier is that, when applied to
the object type of a pointer function parameter, the function
guarantees that it will not change the storage being pointed to.

    int blah (void const *zork);

Says that `blah' will not change whatever `zork' points to.


I declare some functions.

    typedef int (*functype) (void *object);
    typedef int (*c_functype) (void const *const_object);

    void toplevel (functype);
    functype f1;
    c_functype f2;

Calling the parameter functions directly works as expected.

    const void *object = POINTER_TO_VOID;
    const void *c_object = POINTER_TO_CONST_VOID;

    (*f1) (object);	/* OK */
    (*f2) (object);	/* OK */
    (*f1) (c_object);	/* ERROR */
    (*f2) (c_object);	/* OK */

Basically, any place that `f1' appears in the code, I can replace it
with a call to `f2'.  Calling the parameter functions indirectly does
not do what I expect.

	void
    toplevel (functype f)
    {
	(*f)(object);
    }

    :
    :
	toplevel (f1);	/* OK */
	toplevel (f2);	/* ERROR */

I expect that when the second call will be OK, because I'm passing in
a function that guarantees that it won't change its parameters.  That
function is being used to replace a function that might change its
parameters, so the replacement won't do anything that the original
couldn't do.  Surely there's nothing wrong with that!?  Still, I get a
`argument passing between incompatible pointer types' type clash
warning message from my friendly local dpANS-conformant compiler.

According to a clip of the May '88 draft that somebody mailed me
(Section 3.5.4.2):

    For two function types to be compatible, both shall specify
    compatible return types.  Moreover, the parameter type lists, if
    both are present, shall agree in the number of parameters [...];
    corresponding parameters shall have compatible types [...] For
    each parameter declared with qualified type, its type for these
    comparisons is the unqualified [Emph. mine] version of it's
    declared type.

So what's my compiler doing?  Is this a bug or a feature?  Insight is
appreciated.

	;-D on  ( Promotion rules?  *Surfing* rules! )  Pardo
-- 
		    pardo@cs.washington.edu
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