Do we really need const?

[Janice Caron]

On 9/18/07, Bill Baxter <dnewsgroup at billbaxter.com> wrote:
> Janice Caron wrote:
> > About passing structs by reference: It's one of those things, like
> > register or inline...
> >
> > Once upon a time, programmers used the keyword "register" because they
> > thought they could do a better job than the compiler at figuring how
> > to use its registers.
> >
> > Once upon a time, programmers used the keyword "inline" because they
> > thought they could do a better job than the compiler at figuring out
> > what to inline and what not.
> >
> > Now people explicitly choose between f(s) and f(ref s) (where s is a
> > struct and the function does not modify it) because they think they
> > can do a better job than the compiler at figuring out how to pass
> > parameters to functions. I say give control back to the compiler. Lets
> > let "ref" mean "the function may modify the parameter", and the
> > absense of ref mean "the function may not modify the parameter", but
> > leave it to the compiler to decide what is the most efficient way to
> > pass the data to the function.
>
> This thought has occurred to me before to.  I think issue becomes that
> the function signature may no longer enough for a compiler to tell what
> kind of code it needs to generate to call the function.

Actually, I think it's possible my proposal may not have been
understood. Under my suggestion, if the caller passes a struct...

f(s)

...and the callee declares the function as...

void f(S s)

...then all information is known. Both ends must surely know s.sizeof
at compile time? And since that's the /only/ thing the compiler needs
to know to make the decision.


> So for instance
> "pass by ref unless something in the function modifies the value" is not
> going to work.

Agreed. But that's not what I suggested. You'd probably have to head
back to see the earlier post to see the suggestion in full, because
it's too long to repost, but basically it all works (...or at least, I
think so...) providing you have parameters passed as const by default,
which is an argument in favor of needing const.

(If this discussion gets too complicated, we can take it to another thread)