Re: Reference to void

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++.moderated

Date:

26 Oct 2006 13:32:41 -0400

Message-ID:

<4qc6s1Fma1baU1@individual.net>

* James Kanze:

Lucian Radu Teodorescu wrote:

Salt_Peter wrote:

Is the following code valid?

int i = 5;
void* pv = &i;
void& rv = *pv; // Error here on VC 2005

If not, why isn't it valid?

Its not valid for the same reasons that the following is invalid:
void v = 5;
That is to say: a reference is an object and a valid object.
The same can't be said of a pointer, or any pointer.

Let's make the following assumption: every type is implicitly derived
from void. Many C++ programmers can accept that.

Name one. It's manifestly false; it is, in fact, illegal to
derive from void (or from any incomplete type).

Note that Lucian (as I read that article, especially in light of the
rest of it) was describing a future extension, not the current language.

When discussing a future extension of the language it's hardly an
argument against that extension that it doesn't yet exist. ;-)

However, since 'void' has at least two different meanings (sometimes it
behaves like a type, sometimes like a syntactic placeholder) it's IMO a
poor choice for an 'any' type.

Void, as a type, is an incomplete type, which can never be
completed. And in no case can you derive from an incomplete
type. The rules concerning convertion of T* to void* do NOT
obey the rules of derived to base (which only works if the types
are complete), and of course, work even in cases where
inheritance would be illegal, e.g. int* to void*.

Let's think about what the extension would be used for.

There's not much of a technical problem, for

   void& rv = *pv;

would simply be translated, internally, to

   typeof(pv)& rv = pv;

(except with C++0x mumbo-jumbo-name for typeof) and

   void const& r = SomeExpression();

would be translated, internally, to

   typeof(SomeExpression) const& r = SomeExpression;

but in both cases with a restriction on what you could do with the
reference, namely the restriction implied by 'void'.

But what's that restriction good for?

Not very much, I think.

And then why not use the keyword 'auto' instead, to get the compiler's
deduced type instead of the restrictive 'void'?

Huh, nice idea, but where can I have seen that before?

Aha! <url: http://std.dkuug.dk/JTC1/SC22/WG21/docs/papers/2003/n1478.pdf>.

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