Re: Sibling Polymorphism

"Alf P. Steinbach" <>
Sat, 8 Dec 2007 11:30:25 CST
* Greg D:

I've come across a pattern (I'd call it an antipattern) in the course
of my job, recently, and I was wondering:

  > In simplified form:
  > // ---------- begin code
  > class B;
  > class A
  > {
  > public:
  > virtual B* getB() = 0;
  > };
  > class B
  > {
  > public:
  > virtual A* getA() = 0;
  > };
  > class C : public A, public B
  > {
  > public:
  > A* getA() { return (A*)this; }
  > B* getB() { return (B*)this; }
  > };
  > int main()
  > {
  > C* c = new C();
  > A* a = c->getA();
  > B* b = a->getB();
  > a = b->getA();
  > delete c;
  > return 0;
  > }
  > // ---------- end code

A: Who's seen or done this before?

One instance was someone posting in this group about a way to convert
matrices between row-major and column-major form in constant time.

B: Is there even a theoretically good reason for something like this?

See above. The two interfaces could provide two different
interpretations of the same data. It would need a slight rearrangement
to support n interpretations without requiring O(n^2) getter functions.

If there isn't already a name for this, I think I'd call it "Sideways

IMHO, this stinks to high-heaven,

Depends on what it's for. A "goto" generally has some odour, but in
some cases it might be what a good doctor would order. But as a general
way to design classes, yes I'd agree.

but I thought I'd see what the
community experience is--- maybe there's a good reason for this that
I've just never considered. (Note that this software is under active
development and not yet in production-- backward compatibility isn't a
valid excuse, and the same people own A and B.)


Regarding a comment made else-thread: dynamic_cast isn't really an
option because a dynamic_cast might fail.

At least with getA() and getB() the intent that these interfaces are
both present is (partially) expressed in the code, not as comment or
just an assumption.

Cheers, & hth.,

- Alf

A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

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