bearophile wrote:
> Jens Mueller:
>
> > I have a rectangle and one of its methods moves the upper left
> > of the rectangle. I have two invariants when moving a rectangle: The
> > width and the height do not change. I could do something like the
> > following:
> >
> > void move(...) {
> > int currentWidth = width;
> > int currentHeight = height;
> > // moving the rectangle here
> > assert(currentWidth == width);
> > assert(currentHeight == height);
> > }
>
> Probably you need one basic feature of DesignByContract that is missing still
> in D2, the "old" that allows at the end of a method to know the originals. It
> was discussed two or more times:
>
> http://www.digitalmars.com/d/archives/digitalmars/D/why_no_old_operator_in_function_postconditions_as_in_Eiffel_54654.html
>
> http://www.digitalmars.com/d/archives/digitalmars/D/Communicating_between_in_and_out_contracts_98252.html
>
> Once the feature is implemented you may solve your problem like this (minus
> syntax changes, but other solutions are possible):
Oh. I should have done better research.
I don't like the "old" approach.
> void move(...)
> in {
> // ...
> } out {
> assert(width == old.width);
> assert(height == old.height);
> } body {
> // moving the rectangle here
> }
>
>
> The problem of implementing the old was solved in C#4, it is named
> PrestateValues(OldValue), see page 8 here:
> http://research.microsoft.com/en-us/projects/contracts/userdoc.pdf
> So probably this problem may be solved in D2 too.
>
> To solve this problem currently you may need to use ghost fields in your
> struct/class that memorize the older values... ghost fields wrapped in
> version(unittest) {...} or some version(debug). This is a bad solution.
Don't get your point here. You neither like the ghost fields (old) nor
the debug {} approach as Andrei suggested? I mean after all the problem
is not that important that one should bother too much. Maybe I'm too
pragmatic. I'll try using it and see how it feels.
Jens
