Re: [boost] [optional] Strict Aliasing Warnings on Trunk

Subject: Re: [boost] [optional] Strict Aliasing Warnings on Trunk
From: Emil Dotchevski (emildotchevski_at_[hidden])
Date: 2009-12-17 20:16:23

On Thu, Dec 17, 2009 at 12:19 PM, Dean Michael Berris
<mikhailberis_at_[hidden]> wrote:
> On Fri, Dec 18, 2009 at 3:10 AM, Emil Dotchevski
> <emildotchevski_at_[hidden]> wrote:
>> On Thu, Dec 17, 2009 at 10:07 AM, Giovanni Piero Deretta
>> <gpderetta_at_[hidden]> wrote:
>>> ... but the dynamic type of the internal buffer of boost::optional has
>>> been changed via placement new, so (modulo compiler bugs) it should be
>>> safe: at any time the buffer has a specific type (when initialized) or
>>> no type at all (it is just a bunch of memory). Boost optional only
>>> dereferences the buffer when it has been initialized, and always with
>>> the same type used for placement new. Of course this is hard
>>> (impossible?) to prove statically and gcc warns even if it should be
>>> perfectly safe .
>>
>> We had a discussion about warnings some time ago, I think this is an
>> excellent example of a warning that warns about something important
>> yet it is completely useless.
>
> I don't understand why it's "completely useless" when in cases where
> they are done wrong it can cause a runtime crash.

Because the information the compiler gives you in this case is "you
have used reinterpret_cast, your program *might* violate the C++
standard" but it can't know if you're violating it or not. So what it
really says is "warning, you're using reinterpret_cast" which is
useless because you know that you are using reinterpret_cast.

>> To someone who doesn't know the
>> semantics of reinterpret_cast, perhaps the warning has some value, but
>> for the rest of us what it really says is "Warning! You have used
>> reinterpret_cast!"
>>
>
> Sure, but isn't reinterpret_cast considered bad because of the fact
> that there is no standard implementation of it?

I'm not sure what you mean. There is a standard implementation of
reinterpret_cast.

Emil Dotchevski
Reverge Studios, Inc.
http://www.revergestudios.com/reblog/index.php?n=ReCode