Out contracts: how to refer to objects' start state

[Andrei Alexandrescu]

On 5/25/13 8:38 PM, Peter Williams wrote:
> For some class methods, to express comprehensive out{} contracts it is
> necessary to be able to refer to the state of the class object before
> the operation as well as after it e.g. if the method adds something to a
> container you need to be able to specify that nothing was accidentally
> deleted from the container during the method. I've scoured the language
> specification and Andrei's book for advice on how to do this without any
> luck.
>
> Can it be done and, if so, how?
>
> Thanks
> Peter

Unfortunately we don't have a solution to that. A while ago I proposed 
that the "in" and "out" contracts share the same scope. That would allow 
us to do:

class A {
   void fun()
   in { auto oldLen = this.length; }
   out { assert(this.length == oldLen + 1); }
   body { ... }
}

That was technically difficult to do back then, and fell by the wayside. 
Today it would break too much code to introduce even if feasible.


Andrei