Monday, June 11, 2007

Interfaces and Diamonds

I have a set of abstract interfaces that go something like this:

class IPoint {
virtual void SetLocation(const Point2& location)=0;
class IPoint_Heading : public virtual IPoint {
virtual void SetHeading(double heading)=0;

I derive my point-heading from the point because it makes the interface more useful for clients. Becasue ALL point-headings MUST BE points, clients can simply set the location of a point-heading and use it like a point without having to do dynamic casting and failure-checking. So that seemed good.

Why is it virtual? Well, here's why:

class WED_Point : public virtual IPoint {
virtual void SetLocation(const Point2&) p;

class WED_Point_Heading : public WED_Point {
virtual void SetHeading(double heading);

Basically I inherit the implementation of my point into my point-with-heading to avoid recoding points. If IPoint isn't a virtual base class, then the implementation of SetLocation doesn't apply to WED_Point_Heading.

Now before I'll go on, let's examine that fact in more detail. It is a surprising detail. You might think: "well, WED_Point_Heading derives from WED_Point whichi implements SetLocation, what else would I need?" That's what I thought until I tried it, and then when GCC rejected it read the C++ spec.

The problem is simple: given a derived class with two bases (B1 and B2), a virtual function in B1 cannot provide an override for B2. (I don't see any C++ implementation reason why the language couldn't work that way, e.g. it is possible to create this structure in memory, but my guess is that the ensuing chaos of mixing in two unrelated bases and having one change the guts of the other would be worse than what we have now, by a lot.)

The reason that the use of IPoint as a virtual base solves this problem is becasue WED_Point_Heading only has one IPoint. The compiler merges the IPoint sub-object and thus WED_Point's overrides apply everywhere, because they're not being copied from one base to another, they're being applied to one virtual base and there is only one virtual base.

Now I tend to say that if you have an inheritence diamond you've probably already screwed up, and this case is no exception. The real problem here is the use of inheritence of implementation to get code reuse. The real way to unravel this is:
  1. Define two non-virtual, non-polymorphic implementation classes (point-guts, heading-guts) that provide the true run-time implementations of the "SetLocation" code and "SetHeading" code.
  2. Derive WED_Point_Heading only from IPoint_Heading.
  3. Use those same guts in WED_Point and WED_Point_Heading. Because the common implementation has been factored out, the only "code duplication" is the calling of the common implementation. I would say this is not even code duplication, but code disambiguation (a careful description of how we want to map our interface hierarchy onto a set of reusable implementation pieces.

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