Out contracts: how to refer to objects' start state

[Peter Williams]

On 26/05/13 10:43, Andrei Alexandrescu wrote:
> 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.

That looks like a good solution to me.

> Today it would break too much code to introduce even if feasible.

How would it break code?  I've tried to imagine how but have failed.

Peter