Why is it impossible to have a reference-to-void?

Question

Why is it impossible to have a reference to void? The only thing I found in the C++ Standard is this line, at 8.3.2.1

A declarator th

Answer 1

OK, one thing is bugging me about this. The idea of a void*, as mentioned above, is that you still have a valid variable containing an address, but the type is being ignored. This seems allowable since we can still work with the address data - the type is somewhat superfluous (or less important) in this context. Dereferencing it is bad, because to try and access a member doesn't make sense e.g. p.mem. We don't know what class to refer to, and thus the memory to jump to, the vtable pointers to follow.

However, it'd then seem to make sense that p on its own would be OK since it'd only refer to the object, but none of its data. No class information is needed to do so, just the address. I understand there's absolutely no use for this, but it's important in defining when things break down. Allowing this notion, a C++ reference (constantly dereferenced but not accessing anything) e.g. void& ref = static_cast< &void >(obj) also makes sense, and thus would allow void references. I'm not saying anyone should take it up with those in charge, but from a "making sense" point of view, it'd seem correct, no?

As Luc Touraille pointed out above (at least, this is my interpretation), it could be implemented, but the issue is a semantic one. The reasonable explanation I could come to was that since an object variable is a "tag" for a sequence of memory, the type is of important semantic value. Thus, the pointer, being thought of as a variable with an address value, treats the type as somewhat superfluous - not key to defining it.

Would anyone agree with that?

Answer 2

The following is not a defense of the notion of void references. I offer it as an anecdote from the wild. Ask yourself if it doesn't smell funny.

My company was one of the first using C++ commercially, and initially compiled using Cfront. Early developers were still learning the language, and generally using every trick in the book (operators everywhere!). Here is a trick they thought was cool:

void Foo::something(int action, ostream &os = *(ostream *)0)
{
   ostream *os_p = &os;
   if (&os == (ostream *)0) {
      os_p = &cerr;
   }
   // continue with method
}

So here you have, not a void reference, but rather a typed reference with a potentially void binding! A moment's thought should probably suggest better alternatives to this particular idiom...

Answer 3

If you did have a reference to void, what would you do with it? It wouldn't be a number, or a character, or a pointer, or anything like that. Your hypothetical generic function couldn't perform any operation on it, except taking its address (and not its size).

"void" has two uses: to disclaim any knowledge of type (as in void *), and to specify nothing as opposed to something (void function return). In neither case is it possible to say anything about a void something except that it may have an address.

If you can't think of a way something can be useful, and I can't, that is at least evidence that something is useless, and that may well be at least part of the rationale here.

Answer 4

Technically speaking, all that is guaranteed is that a reference to an object is an alias for it. That under the hood reference argument passing is done with pointers is an implementation detail. This can be confusing because of references reusing the & operator which is also address-of, but keep in mind that the operator actually has different meanings in different contexts (in a variable or parameter declaration it denotes a reference type, otherwise it's address-of, except when it's bitwise-and). Because it's technically just an alias for an object, a reference is 'always dereferenced' as Worrier explained.

Answer 5

If they were, they would be semantically non-differentiated from pointers, and would amount to syntactic sugar. A reference says, "I refer to something that is of this type." Allowing void or null reference would weaken that difference from pointers.

Granted, it's still possible for a reference to refer to an object that doesn't exist anymore, but that is an exception.

Answer 6

A reference is a reference to an instance of something. An instance of something can't be of type void. Any instance of something must have a specific type (and possibly base types).