Wednesday, May 03, 2006


Austin is undertaking a major refactoring of X-Plane right now. First a little C++: we have a lot of code that's hard-coded to work only on the user's aircraft, which is item 0 of an array. Austin wanted to easily detect every case where we were hard-coding 0 as an index and edit the code to be generic for any aircraft in our array of aircraft. This is a typical case of a refactoring where due to the pure quantity of code and lack of a single overarching abstraction we run the risk of missing cases and leaving in bugs. In the case of our flight model, doing half of our work on one plane and half on another would lead to a catagory of bugs that's almost impossible to detect from using the product (e.g. systems mysteriously failing only under certain intermittent conditoins.) I'm not normally a big fan of C++ gymnastics but here's what we came up with:

  1. We invented a named enumeration for our index into our aircraft array. Because we have finite aircraft, making an enum for each aircraft was trivial.
  2. We overloaded operators ++ and — so we could use the enumeration in for-loops. (C++ doesn't naturally provide math operators for named enumerations.)
  3. We moved our aircraft from a real C array to a vector subclass called "safety_vector". Safety vector is a vector with the regular [] operator declared private and a new [] operator declared that takes a named enumeration rather than an integer (okay, std::size_t) as its parameter.

What does this add up to? Code where this is okay:

for (PLANE_INDEX p = plane0; p < plane8; ++p) analyse_engines(p);

But this provides a compile error:


Perfect! One other comment on this operation: this kind of wide-scale refactoring of the X-Plane code happens all the time. And having watched this happen for a few versions (and worked on other products with conventional life-cycles) I would have to say that continual refactoring is not a source of bugs. (What is a source of bugs I think is code interdependency, which makes regression hard by causing bugs in areas that we wouldn't expect to be affected by feature work.) The benefit of the continual refactoring is that the code doesn't have any cruft. When we go to put new features in we have a clean workspace, which I think speeds implementation.

No comments:

Post a Comment