Re: [boost] [Endian] Performance

Subject: Re: [boost] [Endian] Performance
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2011-09-13 15:29:18

John Filo wrote:
>
> Apologies for getting a bit off track re: this review, but the
> one thing I don't understand from 5.2.10 of the standard:
>
> -7- A pointer to an object can be explicitly converted to a
> pointer to an object of different type.  Except that
> converting an rvalue of type ``pointer to T1'' to the type
> ``pointer to T2'' (where T1 and T2 are object types and where
> the alignment requirements of T2 are no stricter than those
> of T1) and back to its original type yields the original
> pointer value, the result of such a pointer conversion is
> unspecified.
>
> The last sentence seems to contradict itself.  It first says
> that the following assertion will never trigger as long as
> T2's alignment requirements are not stricter than T1's.
>
>      void f(T1 *arg)
>      {
>          T2 *p2 = reinterpret_cast<T2*>(arg)
>          T1 *p1 = reinterpret_cast<T1*>(p2);
>          assert(p1 == arg);
>      }

Right.

> But then it goes on to say that the "conversion is
> unspecified".

No.  It says, "Except that converting ... 'pointer to T1' to ... 'pointer to T2' ... and back ... yields the original pointer value, the result ... is unspecified."

What you did in f() is fine.  Using p2 in any other way is unspecified behavior.  That means what you get is up to the compiler vendor/version.  It may be documented, but probably isn't.  Thus, it is in no way portable.

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer                     using std::disclaimer;
Dev Tools & Components
Susquehanna International Group, LLP  http://www.sig.com

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