Re: casting (void *) to (class *)
On Apr 16, 1:16 pm, Pete Becker <p...@versatilecoding.com> wrote:
James Kanze wrote:
On Apr 16, 10:07 am, Maxim Yegorushkin <maxim.yegorush...@gmail.com>
wrote:
On Apr 15, 4:14 pm, Pete Becker <p...@versatilecoding.com> wrote:
Maxim Yegorushkin wrote:
Wow! Why can't it be as simple as:
std::size_t x = v - static_cast<T*>(0);
Because pointer arithmetic is only defined for pointers to
objects in the same array (or one past the end of the
array).
True.
Actually, the original code casts a pointer to an integer,
whereas what I posted is doing a different thing (count of
objects).
Which, of course, fails at compile time if the integral type
is not large enough to hold it. Arguably, on most machines,
the cast to ptrdiff_t should fail (if pointer values are
considered "positive"), since by definition, it won't be
large enough. On a segmented architecture, the conversion
to size_t might not compile either, since size_t might not
provide enough room for the segment. And any number of
architectures, different bit patterns may address the same
memory---comparison of pointers must take this into account,
but the conversion to an integral type may preserve the bit
pattern, and comparison of the resulting integers may take
the bits into consideration.
Which is why C++0x, following the lead of C99, offers
uintptr_t, which is large enough to support round trip
conversions of void*'s. It's found in <stdint.h> or, if you're
sufficiently compulsive, <cstdint>.
Is it guaranteed to exist? Practically, of course, now that
there is long long (required to be 64 bits, at least), I doubt
that there will be any implementations where it doesn't. But in
the past, I've worked on machines where pointers were 48 bits,
and the largest integral type only 32.
And of course, that's only the tip of the iceberg with regards
to the problems in the original code.
Note that I also use something similar (actually a lot
simpler---just a cast to size_t), in specific cases. But my
original question to Alf was for a portable solution; I know how
to calculate a hash code for a pointer on every platform I've
ever worked on, but the soluton does vary depending on the
platform. (For example, it involves "normalizing" the pointer
on an 8086, something that has no meaning on a Sparc.)
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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