hashmap of pointers to functions

{ Note: multi-posted to [alt.comp.lang.learn.c-c++]. -mod }

I'm creating a hash_map with the values being pointers to functions,
and g++ is giving me a compiler error on my assignment. How I've
tried to implement this is to have a base class that defines a virtual
function and a typedef that corresponds to the template of that
function. Then when I assign values in the hash_map, I just point it
to the function in the extended class I want to add in. Here's what
I've got:

BaseSteamClass:

#include <iostream>
#include <ext/hash_map>

using __gnu_cxx::hash_map;

namespace streamclasses {
    class BaseStreamClass {
        public:
            BaseStreamClass();
            virtual ~BaseStreamClass() = 0;
            virtual void toOStream(unsigned, std::ostream *ostream) = 0;
            typedef void (*OStreamFunction)(unsigned, std::ostream*);
        private:
            __gnu_cxx::hash_map<int, OStreamFunction> StreamConversionMap;
    };
}

Log Class:

namespace streamclasses {
    class Log:public streamclasses::BaseStreamClass {
            Log();
            virtual ~Log();
            void toOStream(unsigned, std::ostream *ostream);
    };
}

And here's what I'm calling:

        StreamConversionMap[0] = &streamclasses::Log::toOstream;

The compiler error is

cannot convert ?void (streamclasses::Log::*)(unsigned int,
std::ostream*)? to ?void (*)(unsigned int, std::ostream*)? in
initialization

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