Re: [boost] [unnullable_ptr] Proposal for a new library

From: Angel Tsankov (fn42551_at_[hidden])
Date: 2008-07-23 05:40:56

vicente.botet wrote:
>
>>
>> Mathias Gaunard wrote:
>>> Do you still provide pointer arithmetic and implicit upcasting?
>>>
[...]
>>
>> As for pointer arithmetic, I've not yet needed it to work with
>> unnullable_ptr's, so I've not considered implementing it.
>> Nevertheless, it turns out that pointer arithmetic is partially
>> supported (via the convertion
>> operator from unnullable_ptr to C pointer); this is to say that
>> subtracting
>> unnullable_ptr's works, but subtracting int's from (or adding int's
>> to) unnullable_ptr's doesn't yet. In fact, subtracting an int from
>> (as well as adding an int to) an unnullable_ptr might result in a
>> null pointer so the result need to be checked. However, the current
>> implementation of unnullable_ptr is exactly as effective as a
>> built-in C pointer (or at least
>> allows instances of unnulable_ptr to be that effective) and I would
>> like to
>> keep it that way. So, subracting and adding int's will probably not
>> be supported, unless a zero-overhead way is devised to guarantee
>> unnullability
>> of the result from these operations.
>
> What is the expected calue of c? It is not 0?
> T t;
> UnnullablePtr<T> a(t);
> UnnullablePtr<T> b(t);
> UnnullablePtr<T> c(t);
>
> c=a-b;
>
This does not compile. More precisely,
int i = a-b; // i = 0;
c = i; // Error: int cannot be converted to UnnullablePtr<T> (neither 
implicitly, nor explicitly)

Regards,
Angel Tsankov