[boost] Re: [review] hash functions

From: David Abrahams (dave_at_[hidden])
Date: 2005-03-12 22:52:45

JOAQUIN LOPEZ MU?Z <joaquin_at_[hidden]> writes:

> ----- Mensaje original -----
> De: David Abrahams <dave_at_[hidden]>
> Fecha: Domingo, Marzo 13, 2005 0:52 am
> Asunto: [boost] Re: [review] hash functions
>
>> Alberto Barbati <abarbati_at_[hidden]> writes:
>> 
>> > Ah! And there's one more case in which we are helpless, although 
>> I'm 
>> > curious to see an implementation that exploits such latitude: 
>> the 
>> > wording used in the standard does not guarantee that the mapping 
>> from 
>> > pointers to integers always produce the same value over time. In 
>> other 
>> > words:
>> >
>> >   char* p = new char;
>> >   size_t i1 = reinterpret_cast<size_t>(p);
>> >   size_t i2 = reinterpret_cast<size_t>(p);
>> >   assert(i1 == i2); // ???
>> >   char* q1 = reinterpret_cast<char*>(i1);
>> >   char* q2 = reinterpret_cast<char*>(i2);
>> >   assert(p == q1 && q1 == q2);
>> >
>> > The standard guarantees that the second assert passes, but says 
>> nothing 
>> > about the first assert.
>> 
>> There are other cases too.  reinterpret_cast could map all 
>> pointers to
>> the same integer.
>
> If you allow me to be a little pedantic, this is not the
> case. reinterpret_cast<> guarantees back conversion, so it can't
> possibly map every pointer into the same integer.

  4 A pointer can be explicitly converted to any integral type large
  enough to hold it. The mapping function is implementation-defined
  [Note: it is intended to be unsurprising to those who know the
  addressing structure of the underlying machine. ]

  5 A value of integral type or enumeration type can be explicitly
  converted to a pointer. 64) A pointer converted to an integer of
  sufficient size (if any such exists on the implementation) and back
  to the same pointer type will have its original value; mappings
  between pointers and integers are otherwise implementation-defined.

There's no guarantee that the integer type will have sufficient size.

> As for the rest of your argument, I agree 100%.

Now you agree with all of it ;-)
(I hope)

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com