Invariant and pre/post-conditions order

[Davidson Corry]

On 1/19/2012 4:08 PM, Jonathan M Davis wrote:
> On Thursday, January 19, 2012 17:29:28 bearophile wrote:
>> If I am mistaken please I'd like to know why the current design is better
>> (or maybe why it's just equally good). Thank you :-)
>
> Honestly, I don't think that the order is all that critical, since all of the
> same assertions are run in either case. But conceptually, the invariant is for
> verifying the state of the object, whereas the post-condition is for verifying
> the state of the return value. And the return value doesn't really matter if
> the object itself just got fried by the function call. Not to mention, if your
> tests of the return value in the post-condition rely on the state of the
> object itself being correct, then your tests in the post-condition aren't
> necessarily going to do what you expect if the invariant would have failed.
>
> I have no idea what Walter's reasoning is though.
>
> - Jonathan M Davis

I certainly can't speak for Walter, either, but to my mind, the 
precondition/invariant/body/invariant/postcondition order makes sense, 
because the object's internal monologue goes like

precondition:  "Does this guy even have any business calling me?"
                (does he meet my method's preconditions?"
invariant:     "Am I in any shape to do the job for him?"
                (do I meet my class invariant, am I a "real" X object?)
body:          Yo.
invariant:     "Am I still in good shape?"
                (did I break something, am I still a valid X?)
postcondition: "Did I do my job, or what?"

Kinda like checking the customer's order to see if we carry those 
widgets (precondition), checking inventory (invariant), getting the 
widget (body), checking inventory again to make sure the books are still 
balanced (invariant), and finally looking to see that what's in my hand 
really is the widget the customer asked for (postcondition), before 
handing the widget over the counter to the customer. That seems like the 
correct order to do things, to me.

Just my opinion.

-- Dai