Correctness bug in TDPL

[Timon Gehr]

Andrei Alexandrescu wrote:
>On 5/13/11 1:01 PM, Timon Gehr wrote:
>>> On 5/13/11 3:25 AM, Timon Gehr wrote:
>>>> On p368 the CheckedInt struct does not check for overflow in the unary minus
>>>> operator.
>>>
>>> Unary minus never overflows. That being said, there is the oddity that
>>> -x is x when x == int.min. Even in that case there is no loss of
>>> information.
>>>
>>> Andrei
>>
>> This behavior is caused by _overflow_ when the error condition that is checked in
>> ++ is overflow:
>>
>> auto x=CheckedInt(int.min);
>> x=-x; //passes
>>
>> x=~x;
>> x++;//throws
>
> Not sure I understand the point here. I do agree that this may be
> confusing and also that it's reasonable to check against int.min in
> unary minus.

In two's complement, -x is the same as ~x+1. The implementation in TDPL says one
is correct and the other is wrong on int.min.

My understanding is that something that claims to be a CheckedInt has exactly two
options on every operation:

1. Behave like a pure whole number, without any strange corner cases.
2. Throw an exception.

struct CheckedInt in TDPL does not guarantee that.
This means it basically gives no reasonable guarantee. You may disagree on this
but in my eyes it is a serious bug.

>
>> Also, the statement that there is no loss of information is just wrong:
>>
>> scanf("%d %d %d",&n_,&m_);
>> auto n=CheckedInt!int(n_),m=CheckedInt!int(m_);
>> enforce(n>0&&  m<0, "provide meaningful input!");
>> foreach(i;0..n) m=-m;
>> writeln(n," is"~(m<0?"odd":"even")); //disaster strikes!
>
> Depends on how one defines "information". I meant it simply as state
> available to the program.

Okay. But I think this is not really applicable to CheckedInt. By this definition
the fact that int.max+1==int.min is just another oddity (when inspected nearer it
is even just the same oddity), not overflow that loses information. TDPL checks
for that though.

>
>
> Andrei

But it is of course up to you if you consider this a worthy addition to some
future version of TDPL.

Timon


OT: I got one of those without an author on it. =) How many are there of those?
BTW, it is one of the best reads I've had!