On 11/12/08, James Dennett <[EMAIL PROTECTED]> wrote:
> On Wed, Nov 12, 2008 at 11:29 AM, Joe Buck <[EMAIL PROTECTED]> wrote:
> > On Tue, Nov 11, 2008 at 08:43:40PM -0800, James Dennett wrote:
> >> (There are secondary uses of unions for type punning. Most such uses
> >> are not valid portable C++, but g++ supports them because they're so
> >> common in real code.)
> >
> > On the contrary: the uses of unions for type-punning, while not portable,
> > are valid C++, and while not portable, the behavior is
> > implementation-defined (because it exposes the bitwise implementation of C
> > and C++ types).
>
>
> Interesting that you call them "valid C++" when the standard leaves
> them as undefined behavior (not just implementation-defined). As I
> implied, there are some permitted uses of type punning in unions, but
> they're quite restricted.
>
> Let's be concrete:
>
> union {
> int i;
> double d;
> } u;
>
> u.i = 1;
> u.d = 2.0;
>
> printf("%d\n", u.i);
>
> AIUI, the printf() statement above has undefined behavior, and might
> quite possibly print 1, if the compiler uses the knowledge that reads
> from u.i cannot be affected (in code with defined behavior) by writes
> to u.d. Indeed, the write to u.d above might reasonably be eliminated
> by the optimizer if u.d is never read subsequently.
>
> Placing most types inside a union doesn't make it any more valid to
> alias them than if reinterpret_cast<int*>(&d) were used on a double d.
Now would be a good time to mention Google's bit_cast<>.
>
>
> > Without this feature, it would not be possible to
> > implement a function like ldexp or frexp in C or C++, where we want
> > to consider the same storage either as a double or as a sequence of
> > bytes.
>
>
> It's possible to do that without using unions; pointers to character
> types are explicitly permitted to alias any storage.
>
>
> -- James
>
--
Lawrence Crowl
