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From: JOAQUIN LOPEZ MU?Z (joaquin_at_[hidden])
Date: 2005-03-11 18:29:06

----- Mensaje original -----
De: Alberto Barbati <abarbati_at_[hidden]>
Fecha: Viernes, Marzo 11, 2005 9:01 pm
Asunto: [boost] Re: [review] hash functions

> Hi,
> I have a doubt about the proposed implementation of hash_value()
> for
> pointers, which is:
> template <class T> inline std::size_t hash_value(T* v)
> {
> return static_cast<std::size_t>(v - (T*)0);
> }
> this code calls for undefined behaviour according to §5.7/6, each
> time v
> is a non-null pointer.
> IMHO, the only reasonable way to map generic pointer values to
> integer
> values is through reinterpret_cast<>. The mapping would be
> implementation-defined, but it would be no worse than the intent
> of the
> proposed implementation (if it worked).


> To be picky, in order to accomodate with implementations that may
> have
> sizeof(size_t) < sizeof(T*), we should use two casts:
> template <class T> inline std::size_t hash_value(T* v)
> {
> return static_cast<std::size_t>(
> reinterpret_cast<boost::uintmax_t>(v));
> }
> (the right thing would be to use a C9X-ish uintptr_t type, but
> boost/cstdint.hpp does not provide such a type).
> However, as the double cast may result in suboptimal code if
> size_t is
> large enough, a boost::enable_if trick might be used to provide an
> optimized version with just a single use of reinterpret_cast. The
> result
> would be something similar to:

Please, no SFINAE! This would break not-so-conforming
compilers. Let's stick to compilerwise macro-based branches.
After all, for the vast majority of platforms (AFAIK)

> Unfortunately, there's one more thing... even if p == q,
> reinterpret_cast<uintmax_t>(p) is not guaranteed to be equal to
> reinterpret_cast<uintmax_t>(q) on implementations that have
> multiple
> internal representations for the same address. I don't think much
> can be
> done (in a portable way) for such platforms.

Another reason for resorting to macro-based alternatives,
and not to SFINAE.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo

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