Walter's second axiom

[Janice Caron]

On 12/8/07, Stewart Gordon <smjg_1998 at yahoo.com> wrote:
> S or MyInt should be a union rather than a struct.  This is because it
> doesn't make sense to add more member variables to the derived type

The axiom says struct.

Also, I don't necessarily agree that adding member variables would be
a problem. The axiom says it should be /possible/ to create a wrapped
type. It doesn't say it should /not/ be possible to create something
with more data. (Implicit casting should fail for any narrowing
conversion though).


> (it
> would increase the size, thereby violating the 'is a' principle of
> inheritance).

Adding a union member is also capable of increasing the size.

Moreover, there is no such principle restricting inheritance. Those of
us who come from a C++ background are quite au fait with stuff like

// C++
    struct A { int x; };
    struct B : public A { int y; };

where B inherits from A and simultaneously increases its size.


> However, it does make sense to add more views of the data,
> which is a common use of unions. I briefly mentioned it here:
> http://www.digitalmars.com/d/archives/digitalmars/D/42035.html#N42048

I think either a struct /or/ a union should be allowed, depending on
the programmer's preference.