Re: strict aliasing: how to swap pointers

[Evan Jones <evanj at MIT dot EDU>]

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.

Evan Jones

--
Evan Jones
http://evanjones.ca/