Re: scope and/or parameters (beginner)

[Hyman Rosen <hymie@prolifics.com>]

Robert Dewar wrote:
>   Hyman Rosen <hymie@prolifics.com> wrote:
> > Correct. A C programmer would, of course, simply declare
> > the parameter as being 'const', then insert the test
> > exactly as you did in Ada.
> 
> The "of course" here is mighty forced :-)
> 
> First, I have never *ever* seen C programmers routinely
> write prototypes with const in them, as in
> 
>    int max (const int a, const int b) {
> 
> so to claim that it is standard practice, as implied by
> your "of course" is really stretching things.

It's not standard practice to do this because generally no one cares
if the parameter is modified. No C programmer thinks of such a parameter
as being an alias of some other object. If it was important to preserve
the value of the parameter throughout the function, then making it const
is the natural solution.

> Second, please note that in C, if you *do* declare things
> this way, the effect of assigning to the const object is
> "undefined" according to the ANSI C standard, not illegal
> but undefined!

Huh? I think you have a serious misunderstanding. An attempt to assign to
a const object is illegal, and any C compiler will prevent you from doing
this. What you are thinking of is an attempt to modify an object that is
declared const through an alias that is not. Naturally, it may be impossible
to detect such an attempt, so the standard makes the effect undefined. Here's
an example:

	void f(const int a)
	{
		int *p;

		a = 1;	/* Illegal assignment to constant */
		p = &a;	/* Illegal - const discarded */
		p = (int *)&b; /* Legal because of explicit cast */
		*p = 1; /* Undefined result, but compiler need not notice. */
	}
 
> perhaps the abbreviation of constant to const is best
> interpreted as meaning that it is sort of constant, but
> not very :-) :-)

C compilers routinely place const global data into readonly segments.
I assure you, they are quite constant!