JOAQUIN LOPEZ MU?Z wrote:
De: Alberto Barbati <abarbati@iaanus.com>
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 ยง5.7/6, each time v is a non-null pointer.
Ah... And even if it works, it isn't very good when sizeof(T) is greater than the alignment of T.
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).
Agreed.
This hash value will have problems with alignment. Although, that won't matter with our containers because we use a prime number of buckets (unless your platform has 53-byte alignment...). But if this hash function is to be general purpose it's worth thinking about.
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) sizeof(size_t)==sizeof(T*)
It would be possible to implement Alberto's suggestion without SFINAE. Daniel