[boost] [functional] [shared_ptr] boost::hash<shared_ptr<T>> Returns Only Two Values

Subject: [boost] [functional] [shared_ptr] boost::hash<shared_ptr<T>> Returns Only Two Values
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2011-02-18 14:40:03

The following is a program that demonstrates that a null shared_ptr<int> produces a hash value of zero and that (ostensibly) all non-null instances produce a hash value of one.  I expected that there'd be a shared_ptr specialization of hash_value to make it work right, but I don't find one.  Shouldn't there be one?

#include <iostream>
#include <map>
#include <vector>
#include <boost/shared_ptr.hpp>
#include <boost/functional/hash.hpp>

int
main()
{
   typedef boost::shared_ptr<int> ptr_type;
   typedef std::map<std::size_t, int> hashes_type;
   std::vector<ptr_type> v(25);
   hashes_type hashes;
   boost::hash<ptr_type> hasher;
   ptr_type null_ptr;
   ++hashes[hasher(null_ptr)];
   for (int i(0); i < 25; ++i)
   {
      v[i] = ptr_type(new int(i));
      std::size_t const hash(hasher(v[i]));
      ++hashes[hash];
   }
   for (hashes_type::iterator it(hashes.begin()), end(hashes.end());
        it != end; ++it)
   {
      std::cout << it->first << ": " << it->second << '\n';
   }
}

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer, Core Software      using std::disclaimer;
Susquehanna International Group, LLP  http://www.sig.com

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