should postconditions be evaluated even if Exception is thrown?

[Andrei Alexandrescu]

Walter Bright wrote:
> Brad Roberts wrote:
>> Walter Bright wrote:
>>> Andrei Alexandrescu wrote:
>>>> An exception (not an Error) is an expected and documented outcome of a
>>>> function. After having listened to those endless Boeing stories,
>>>> please listen to this one :o). Contract Programming covers the
>>>> correctness of a program, and exceptions are correct behavior. By your
>>>> very Boeing stories that I stoically endured, it seems like the
>>>> logical conclusion is that postconditions must be evaluated upon
>>>> exceptional return.
>>> Consider a constructor. It's postcondition is the class invariant is
>>> satisfied. If it throws, the object is not successfully constructed and
>>> the invariant does not hold.
>>
>> If the constructor fails, the object never existed.  Nothing to 
>> validate is valid.
> 
> Right. And I can't see how you can validate the output of a function 
> that failed. Let's say your function sorts an array, and the post 
> condition is the array is sorted. So, what would the postcondition be if 
> it failed?
> 
>   out (result)
>   {
>     assert(failed || isSorted(result));
>   }
> 
> ? What's the point?

Very, very interesting example that actually makes my point very nicely.

If the function doesn't throw, the postcondition is that the array is 
sorted.

If the function does throw, the postcondition is that the array has not 
lost any element. So at least you know that information wasn't lost.

I can't believe this is working so well in my argument :o).


Andrei