Re: [Boost-users] [multi_index] problem with hashed index

21 Mar 2007

      ----- Mensaje original -----
De: "JOAQUIN LOPEZ MU?Z" 
Fecha: Miércoles, Marzo 21, 2007 8:33 pm
Asunto: Re: [Boost-users] [multi_index] problem with hashed index
Para: boost-users@lists.boost.org
...
Hi Boris,
----- Mensaje original -----
De: Boris 
Fecha: Miércoles, Marzo 21, 2007 8:19 pm
Asunto: [Boost-users] [multi_index] problem with hashed index
Para: boost-users@lists.boost.org
...
I've been using Boost Multiindex quite a lot but don't 
understand 
why the code below inserts two items. The container has a unique
hash index and the two items return the same hash value. Is this
is a bug in the library or in my code?
[...]
  indexed_by > >
Here's the problem: your key is shared_ptr<foo>, i.e.,
you compare *the pointers* rather than the pointed to
foo objects. What you want is this instead:
indexed_by >
Hi again, I answered too fast, the answer above is more or
less correct but there's more to it: you have overloaded
hash_value for shared_ptr<foo>s so as to use the pointed
to foo objects:

  std::size_t hash_value(const shared_ptr<foo> &f)
  {
  std::size_t seed = 0;
  boost::hash_combine(seed, f->i);
  boost::hash_combine(seed, f->j);
  return seed;
  }

So far so good; but the hashed index depends not only on the
hash functor, but also on an equality predicate, which is
by default (in this case)

  std::equal_to,

which compares pointers, not foo objects. Hence the problem.
Rather than overloading std::equal_to,
which breaks the natural equality semantics of shared_ptr,
my advice is that you change the key of the hashed index
to:

  indexed_by >

as suggested in my previous post, and write the overload
of hash_value for foo, not for shared_ptr<foo>:

std::size_t hash_value(const foo &f)
{
std::size_t seed = 0;
boost::hash_combine(seed, f.i);
boost::hash_combine(seed, f.j);
return seed;
}

Does this work? Please report back,

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo