should postconditions be evaluated even if Exception is thrown?

[Andrei Alexandrescu]

Pelle Månsson wrote:
> Andrei Alexandrescu wrote:
>> If a function throws a class inheriting Error but not Exception (i.e. 
>> an unrecoverable error), then the postcondition doesn't need to be 
>> satisfied.
>>
>> I just realized that postconditions, however, must be satisfied if the 
>> function throws an Exception-derived object. There is no more return 
>> value, but the function must leave everything in a consistent state. 
>> For example, a function reading text from a file may have the 
>> postcondition that it closes the file, even though it may throw a 
>> malformed file exception.
>>
>> This may sound crazy, but if you just follow the facts that 
>> distinguish regular error handling from program correctness, you must 
>> live with the consequences. And the consequence is - a function's 
>> postcondition must be designed to take into account exceptional paths. 
>> Only in case of unrecoverable errors is the function relieved of its 
>> duty.
>>
>>
>> Andrei
> Isn't the post-condition mainly to assert the correctness of the return 
> value? Or at least partially? The output cannot be correct if an 
> exception is thrown, so any assertion in the post condition concerning 
> the output would fail by definition, right?
> 
> I would say the invariant() is the correct part to run.

As others have mentioned, you may have different postconditions 
depending on whether an exception was thrown or not.

I think a major failure of exceptions as a language mechanism is that 
they gave the illusion that functions need not worry about what happens 
when an exception traverses them, and only need to focus on the success 
case.


Andrei