Alberto Barbati <abarbati@iaanus.com> writes:
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 ยง5.7/6, each time v is a non-null pointer.
Furthermore, you can't subtract addresses that aren't in the same array. 0 isn't in the same array as anything.
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.
Once you're into reinterpret_cast you've left the land of strict portability anyway. -- Dave Abrahams Boost Consulting www.boost-consulting.com