Re: [boost] [config] RFC PR 82

Subject: Re: [boost] [config] RFC PR 82
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2015-12-02 07:11:36

On 2015-12-01 18:32, Agustín K-ballo Bergé wrote:
> On 12/1/2015 11:59 AM, Andrey Semashev wrote:
>>
>> It's not just syntax. If I'm not mistaken, the union-based type punning
>> has advantage over memcpy - it allows the code to be constexpr.
>
> No, it doesn't. There's no undefined behavior allowed in a constant
> expression. Furthermore, the rules for `union` member access are
> stronger for constant expressions, there's no common initial sequence
> rule nor anything like it since constant expressions do not work under
> the memory object model. Even compilers with implementation-specific
> behavior would reject it.

I see. I tried this locally with gcc and it succeeded, but apparently I 
used bitwise_cast in a context that allowed runtime execution, although 
the compiled code was still using a constant. In a purely constant 
expression it indeed fails to compile. Good to know.

> After all, it would have to be an implementation
> specific choice, if we don't know whether `memcpy` will be optimized
> then we can't assume that the undefined behavior will do the `memcpy`
> you expect.

That's true, although I can't remember any particular compiler 
implementing a different behavior wrt unions.