[Boost-users] Unwanted implicit conversions to bool

Subject: [Boost-users] Unwanted implicit conversions to bool
From: Dominik Steenken (dominik_at_[hidden])
Date: 2011-05-12 06:58:07

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Hello everyone,

  i am having a bit of a problem using boost/hash in combination with
boost/shared_ptr. Essentially, what i want when i hash a shared_ptr
object is to hash the object it points to. So i wrote a template
function to that effect, i.e.

    inline std::size_t hash_value(boost::shared_ptr<const IHashable> obj)
    {
        return obj->hash_value();
    }

  where IHashable is an interface forcing the existence of the
hash_value() function. Now i am getting compiler errors because
apparently there is an implicit conversion to bool from
boost::shared_ptr<T> and obviously there is a hash function for bool.
Thus, something like

  boost::shared_ptr<IHashable> p(new something());
  boost::hasher<something> hasher;
  std::size_t hash = hasher(p);

is now ambiguous. I already found the BOOST_HASH_NO_IMPLICIT_CASTS
switch, but it does not seem to change anything. I still get compiler
errors like this one:

  In file included from /usr/include/boost/functional/hash/hash.hpp:
494:0,
                 from /usr/include/boost/functional/hash.hpp:6,
                 from types/../common.h:14,
                 from types/variables.h:11,
                 from types/abstractformula.h:11,
                 from types/unaryoperations.h:11,
                 from types/unaryoperations.cpp:1:
  types/unaryoperations.cpp:49:37:   instantiated from here
  /usr/include/boost/functional/hash/extensions.hpp:176:34: error:
call of overloaded ?hash_value(const
boost::shared_ptr<terms::CAbstractFormula>&)? is ambiguous
  /usr/include/boost/functional/hash/hash.hpp:120:24: note: candidates
are: size_t boost::hash_value(bool)
  types/../common.h:86:24: note:                 size_t
terms::hash_value(boost::shared_ptr<const terms::IHashable>)

Any idea on what i could to to prevent this would be greatly
appreciated.

cheers,
  Dominik
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