[boost] Re: [review] hash functions

From: David Abrahams (dave_at_[hidden])
Date: 2005-03-12 18:52:39

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.  I'm saying what Peter's saying.  This is
non-portable territory; don't lose sleep over it; practically speaking
it will work most places; everywhere else we'll fix it.

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