is portable aliasing possible in C++?

[Andrew Haley]

On 11/09/14 00:03, haynberg@yahoo.com wrote:
>>> So it would seem the answer is no, portable aliasing is not possible
>>> in C++.  Though I wonder if it should be (with a new language
>>> feature).
>>
>> In practice, a union is exactly that, and I've not heard of a compiler
>> which doesn't do the right thing.  Given that it's explicitly legal in
>> C11, I can't see any reason which C++ shouldn't simply adopt the same
>> language.
> 
> I agree.  But in the union case, you're copying bytes.  If you don't perform 
> 
> a copy, for example:
> 
> struct msg {
>   // ...
> };
> char *get_bytes();
> msg *p = reinterpret_cast<msg*>(get_bytes());

Why are you doing this?

> if (p->i)
>   // ...
> 
> Then there can't be a portable way to do this, because there's hardware that 
> doesn't permit unaligned reads (e.g. where you'd get a SIGBUS).

But a union will always be correctly aligned for all members.

Andrew.