Re: Reference to void

"James Kanze" <>
29 Oct 2006 11:03:40 -0500
Gennaro Prota wrote:

On 28 Oct 2006 11:36:22 -0400, James Kanze wrote:

It's hypothetical code. Of course, in this case, I don't see
much point in the void&; what does it buy us over simply:

    Lock const v( mx ) ;

If instead of Lock, we used a function, and its return value,
perhaps. But even then, we presumably know the return type, so
there's no point in not naming it. (In fact, naming it means
that if I happen to mistype the name, and hit the name of
another function, which returns a different type, I get an error
message; declaring it as void is less safe.)

Interestingly, the example is, IMHO, an argument for anonymous
variables. Supposing that Lock and mx mean what I think they
do, he's had to invent a name which will never be used again in
the function.

Hmm... how would that be different from

  Lock const( mx );


That's currently legal syntax; it creates a temporary, whose
lifetime ends at the end of the full expression. An anonymous
variable would behave exactly like any other variable, including
with respect to lifetime, but it wouldn't have a name.

The most obvious way of doing this is to use a reserved name,
say _. You'd write the declaration with the name, but the
compiler wouldn't enter that name in its symbol table, you
couldn't use the name to refer to the variable later, but you
could define additional variables with the same name in the same
scope, e.g. if you needed two locks:

    Lock const _(mx1) ;
    Lock const _(mx2) ;

The implementation could be something along the lines of the
implementation of anonymous namespaces -- each time the compiler
sees the name, it generates a new, unique, unnamable name in its

FWIW, and to keep your anti-obfuscation radar fit ;-), my SHA-1
implementation has the following:

  const sha1_loop /* loop */ ( state, block );

Opinions/suggestions/insults? (I know that I could just use a
static function, but I'm not convinced that that is an

If you want to call a function, call a function. I'm not sure
what this buys you -- you get two successive function calls in
one statement (construtor and destructor), but I don't see any
real advantage in this. Of course, if you are within a
template, this structure would make shar1_loop a type parameter,
rather than a non-type parameter of type void (*)(...). I
suppose that there are cases where that would be an advantage,
but even then, I think a classical traits class, with e.g. a
static member function loop, would be more readable, so you
would write:

    sha1::loop( state, block ) ;

The const is also obfuscation, since there is really no moment
that the object is const. (A const object doesn't become const
until the constructor has finished, and looses its const-ness as
soon as the destructor is invoked. Since all that is ever done
with your object is to call the constructor, then the
destructor, the object is effectively non-const.)

James Kanze Gabi Software email:
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