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;

Answer 3

A "normal" Neuron* should work as key for the hash_map:

typedef stdext::hash_map<Neuron*, WEIGHT> NeuronWeightMap;

If you want to lookup shared_ptr-Neurons you can use shared_ptrs get() method to access the raw pointer:

NeuronWeightMap weights;
boost::shared_ptr<Neuron> n;

weights.find(n.get());

Answer 4

Since you already use Boost, why not use boost::unordered_map for the hash table? It compiles out of the box with shared_ptr for the key, though you better check how to do it properly (Boost.Unordered)

Answer 5

From the code above what is the compiler error?

Have you tried forward declaring the class Neuron?

class Neuron; // class Neuron must be forward declared to be used

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


class Neuron
{
private:
    NeuronWeightMap m_outputs;
    //...
public:

    Neuron();
    ~Neuron();
    //...
};

a standard map should also be fine in this case, what is your specific reason to use hash_map?

Answer 6

I find by adding a hash_value() function I can use a shared_ptr as a key in a stdext::hash_map. Below I compare using boost::unordered_map with std::hash_map

#include <boost/smart_ptr.hpp>
#include <boost/unordered_map.hpp>
#include <hash_map>
#include <assert.h>

struct MyItem
{
    int i;
};
typedef boost::shared_ptr<const MyItem> MyItem_csptr;

struct MyExtra
{
    double d;
};

//Boost's unordered_map already knows how to hash shared_ptr
typedef boost::unordered_map<MyItem_csptr,MyExtra> MapMyItemToExtra;


//But for stdext::hash_map, we need to write this:
template<class T>
size_t hash_value(const boost::shared_ptr<T>& aSptr)
{
    return reinterpret_cast<size_t>( aSptr.get() );
};
typedef stdext::hash_map<MyItem_csptr,MyExtra> MapMyItemToExtra2;


template<class MAP>
void TryMapMyItemToExtra_T()
{
    MAP tMap;

    MyItem_csptr tItem1(new MyItem);
    MyItem_csptr tItem2(new MyItem);
    tMap[tItem1].d=1.0;
    tMap[tItem1].d=1.1;
    tMap[tItem2].d=2.0;

    assert( tMap[tItem1].d == 1.1 );
    assert( tMap[tItem2].d == 2.0 );
}

void TryMapMyItemToExtra()
{
    TryMapMyItemToExtra_T<MapMyItemToExtra>();
    TryMapMyItemToExtra_T<MapMyItemToExtra2>();
}

Answer 7

Issue with hash function arises for stdext::hash_set as well. Solution with declaring hash_value() is the best since you can declare it just once in main headers file.