Gabriel Dos Reis wrote: >Joe Buck <[EMAIL PROTECTED]> writes: > >| On Tue, Jul 05, 2005 at 08:05:39PM +0200, Gabriel Dos Reis wrote: >| > It is definitely a good thing to use the full bits of value >| > representation if we ever want to make all "interesting" bits part of >| > the hash value. For reasonable or sane representations it suffices to >| > get your hand on the object representation, e.g.: >| > >| > const int objsize = sizeof (double); >| > typedef unsigned char objrep_t[objsize]; >| > double x = ....; >| > objrep_t& p = reintepret_cast<objrep_t&>(x); >| > // ... >| > >| > and let frexp and friends only for less obvious value representation. >| >| I disagree; on an ILP32 machine, we pull out only 32 bits for the hash >| value, and if you aren't careful, your approach will wind up using the >| least significant bits of the mantissa. This will cause all values that >| are exactly representable as floats to collide. > >I'm not sure we're talking about the same thing. With the >representations I'm talking about, value repsentation == object representation. > ... still, the hash functions for float, double and long double that we need for TR1 must return a size_t (6.3.3). How do you get a size_t from an objrep_t? (fulfilling the various requirements briefly reminded by Joe in his first message)
Paolo.
