C++ boost shared_ptr as a hash_map key

Question

I'm making a neural network and wanted to use a hash_map to keep weight references for output neurons for each neuron:

class Neuron; //forward declaration was there (sorry I forgot to show it earlier)
typedef double WEIGHT;
typedef stdext::hash_map<boost::shared_ptr<Neuron>,WEIGHT> NeuronWeightMap;
class Neuron
{
private:
    NeuronWeightMap m_outputs;
    //...
public:

    Neuron();
    ~Neuron();
    //...
    WEIGHT GetWeight(const boost::shared_ptr<Neuron>& neuron) const
    {
        NeuronWeightMap::const_iterator itr = m_outputs.find(neuron);
        if( itr != m_outputs.end() )
        {
            return itr->second;
        }
        return 0.0f;
    }
};

I realize that I can't use the boost::shared_ptr as the key of a stdext::hash_map, so what would be another suggestion? Are there any work-arounds or is the only option to use a different key or maybe switch to an std::map? Thanks!

Here is the errors:

1>c:\program files (x86)\microsoft visual studio 8\vc\include\xhash(61) : error C2440: 'type cast' : cannot convert from 'const boost::shared_ptr<T>' to 'size_t'
1>        with
1>        [
1>            T=Neuron
1>        ]
1>        No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
1>        c:\program files (x86)\microsoft visual studio 8\vc\include\xhash(99) : see reference to function template instantiation 'size_t stdext::hash_value<_Kty>(const _Kty &)' being compiled
1>        with
1>        [
1>            _Kty=boost::shared_ptr<Neuron>
1>        ]
1>        c:\program files (x86)\microsoft visual studio 8\vc\include\xhash(98) : while compiling class template member function 'size_t stdext::hash_compare<_Kty,_Pr>::operator ()(const _Kty &) const'
1>        with
1>        [
1>            _Kty=boost::shared_ptr<Neuron>,
1>            _Pr=std::less<boost::shared_ptr<Neuron>>
1>        ]
1>        c:\program files (x86)\microsoft visual studio 8\vc\include\hash_map(80) : see reference to class template instantiation 'stdext::hash_compare<_Kty,_Pr>' being compiled
1>        with
1>        [
1>            _Kty=boost::shared_ptr<Neuron>,
1>            _Pr=std::less<boost::shared_ptr<Neuron>>
1>        ]
1>        c:\program files (x86)\microsoft visual studio 8\vc\include\xhash(119) : see reference to class template instantiation 'stdext::_Hmap_traits<_Kty,_Ty,_Tr,_Alloc,_Mfl>' being compiled
1>        with
1>        [
1>            _Kty=boost::shared_ptr<Neuron>,
1>            _Ty=common_ns::WEIGHT,
1>            _Tr=stdext::hash_compare<boost::shared_ptr<Neuron>,std::less<boost::shared_ptr<Neuron>>>,
1>            _Alloc=std::allocator<std::pair<const boost::shared_ptr<Neuron>,common_ns::WEIGHT>>,
1>            _Mfl=false
1>        ]
1>        c:\program files (x86)\microsoft visual studio 8\vc\include\hash_map(90) : see reference to class template instantiation 'stdext::_Hash<_Traits>' being compiled
1>        with
1>        [
1>            _Traits=stdext::_Hmap_traits<boost::shared_ptr<Neuron>,common_ns::WEIGHT,stdext::hash_compare<boost::shared_ptr<Neuron>,std::less<boost::shared_ptr<Neuron>>>,std::allocator<std::pair<const boost::shared_ptr<Neuron>,common_ns::WEIGHT>>,false>
1>        ]
1>        FILE_PATH_REMOVED\neuralnet.h(21) : see reference to class template instantiation 'stdext::hash_map<_Kty,_Ty>' being compiled
1>        with
1>        [
1>            _Kty=boost::shared_ptr<Neuron>,
1>            _Ty=common_ns::WEIGHT
1>        ]
1>NeuralNet.cpp
1>c:\program files (x86)\microsoft visual studio 8\vc\include\xhash(61) : error C2440: 'type cast' : cannot convert from 'const boost::shared_ptr<T>' to 'size_t'
1>        with
1>        [
1>            T=Neuron
1>        ]
1>        No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
1>        c:\program files (x86)\microsoft visual studio 8\vc\include\xhash(99) : see reference to function template instantiation 'size_t stdext::hash_value<_Kty>(const _Kty &)' being compiled
1>        with
1>        [
1>            _Kty=boost::shared_ptr<Neuron>
1>        ]
1>        c:\program files (x86)\microsoft visual studio 8\vc\include\xhash(98) : while compiling class template member function 'size_t stdext::hash_compare<_Kty,_Pr>::operator ()(const _Kty &) const'
1>        with
1>        [
1>            _Kty=boost::shared_ptr<Neuron>,
1>            _Pr=std::less<boost::shared_ptr<Neuron>>
1>        ]
1>        c:\program files (x86)\microsoft visual studio 8\vc\include\hash_map(80) : see reference to class template instantiation 'stdext::hash_compare<_Kty,_Pr>' being compiled
1>        with
1>        [
1>            _Kty=boost::shared_ptr<Neuron>,
1>            _Pr=std::less<boost::shared_ptr<Neuron>>
1>        ]
1>        c:\program files (x86)\microsoft visual studio 8\vc\include\xhash(119) : see reference to class template instantiation 'stdext::_Hmap_traits<_Kty,_Ty,_Tr,_Alloc,_Mfl>' being compiled
1>        with
1>        [
1>            _Kty=boost::shared_ptr<Neuron>,
1>            _Ty=common_ns::WEIGHT,
1>            _Tr=stdext::hash_compare<boost::shared_ptr<Neuron>,std::less<boost::shared_ptr<Neuron>>>,
1>            _Alloc=std::allocator<std::pair<const boost::shared_ptr<Neuron>,common_ns::WEIGHT>>,
1>            _Mfl=false
1>        ]
1>        c:\program files (x86)\microsoft visual studio 8\vc\include\hash_map(90) : see reference to class template instantiation 'stdext::_Hash<_Traits>' being compiled
1>        with
1>        [
1>            _Traits=stdext::_Hmap_traits<boost::shared_ptr<Neuron>,common_ns::WEIGHT,stdext::hash_compare<boost::shared_ptr<Neuron>,std::less<boost::shared_ptr<Neuron>>>,std::allocator<std::pair<const boost::shared_ptr<Neuron>,common_ns::WEIGHT>>,false>
1>        ]
1>        FILE_PATH_REMOVED\neuralnet.h(21) : see reference to class template instantiation 'stdext::hash_map<_Kty,_Ty>' being compiled
1>        with
1>        [
1>            _Kty=boost::shared_ptr<Neuron>,
1>            _Ty=common_ns::WEIGHT
1>        ]
1>Generating Code...

Answer 1

You probably have Neuron declared in your .h file.

So the below could work:

struct hasher {
    size_t operator()(const boost::shared_ptr<Neuron>& n) { return (size_t)n.get(); }
};

typedef stdext::hash_map<boost::shared_ptr<Neuron>,WEIGHT,hasher> NeuronWeightMap;

The problem probably is that boost:shared_ptr has no default hashing function (what one would look like?), so you just need to provide one.

Answer 2

You could also provide a template specialization for the hash:

#include <functional>
#include <boost/shared_ptr.hpp>

template<class T>
class std::tr1::hash<boost::shared_ptr<T>> {
public:
    size_t operator()(const boost::shared_ptr<T>& key) const {
        return (size_t)key.get();
    }
};

Works perfectly with unordered_set:

class Foo;
typedef boost::shared_ptr<Foo> FooPtr;
typedef std::tr1::unordered_set<FooPtr> FooSet;