Passing empty "tag" structs

[David Brown]

In C++ programming, it is sometimes helpful to have empty structs acting 
as tags.  An example is "struct nothrow_t {}".

When parameters of these types - such as "nothrow", are passed to 
functions the compiler passes them as a value 0.  Since the type cannot 
hold any kind of value, surely it could be passed without any value at 
all being placed in the corresponding parameter register or stack slot? 
Or do the rules of C++ require a value here?

Going one step further, if a function takes empty tag parameters such as:

	foo(tag1_t, tag2_t, int x);

the first two parameters take up valuable register (or stack) slots for 
no useful information.  Could this function be mangled to be, in effect:

	foo__tag1_t__tag2_t(int x);

That would let you use tags for a variety of safe programming techniques 
with absolutely zero overhead.

mvh.,

David