Andrew Haley wrote:
> Evan Jones wrote:
>> Rupert Wood wrote:
>>> I don't know how to fight aliasing rules but as another approach you
>>> could template the function instead, i.e.
>>>
>>> template<typename T> T* exchange(T** ptr, T* next);
>>>
>>> exchange<int>(&a, &v2);
>>>
>>> since you're using C++ here.
>> I initially dismissed this solution since the implementation of exchange
>> is either a typical swap using a temporary, a compare-and-swap sequence
>> or a mutex, depending on various compilation options. I wanted to avoid
>> including the implementation in the header to avoid a bunch of #ifdefs.
>> However, your email made me try to avoid the warning with a template
>> wrapper:
>>
>>
>> template <typename T>
>> T* exchange_template(T** ptr, T* next) {
>> return reinterpret_cast<T*>(
>> exchange(reinterpret_cast<void**>(ptr), next));
>> }
>>
>> ...
>>
>> exchange_template(&a, &v2);
>>
>>
>> I don't understand why, but this compiles without warnings. It also
>> avoids needing to look up the aliasing rules, and places the dangerous
>> cast in a single location, so this seems like a better solution.
>>
>> I would still like to know if my original solution is "correct" or not,
>> since it would be needed for C.
>
> We have no way to know, since you didn't provide us with the source of
> exchange(), and that's where the aliasing violation, if any, would
> occur.
>
> However, the answer is almost certainly no. I don't know why you're
> trying to do something so simple in such a difficult way. If you really
> want to do this in standard portable C, the easiest way is a macro:
>
> #define SWAP(TYPE, A, B) do { TYPE tmp = a; b = b; b = tmp; } while (0)
Err,
#define SWAP(TYPE, A, B) do { TYPE tmp = a; a = b; b = tmp; } while (0)
>
> You can't portably cast a pointer to an int* to a pointer to a void*
> and then dereference the resulting pointers since the things they point
> to might not even be the same size. The union hack will work on gcc,
> though.
>
> Andrew.
>
>
