Compile error in

Can someone please explain these weird compile errors I get for this code?

1>C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xrefwrap(283): error : expression preceding parentheses of apparent call must have (pointer-to-) function type
1>C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\functional(1148): error : no instance of overloaded function "std::_Pmd_wrap<_Pmd_t, _Rx, _Farg0>::operator() [with _Pmd_t=int (inputLoader::*)(), _Rx=int (), _Farg0=inputLoader]" matches the argument list
1>C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xrefwrap(283): error : return value type does not match the function type

It obviously has something to do with the way I am using that function<T()> template but other than that I can't see it.

#include <functional>
using std::function;
using std::bind;

template <typename T>
class memoizedProperty {
    bool _calculated = false;
    T* _valuePtr = NULL;
    function<T()> _definition = NULL;

public:
    operator T() const {
        if (_calculated) return *_valuePtr;
        if (!_definition) return T(0);
        *_valuePtr = _definition();
        _calculated = true;
        return *_valuePtr;
    }

    memoizedProperty(T* valuePtr, function<T()> definition) {
        _valuePtr = valuePtr;
        _definition = definition;
    }
};

class inputLoader {
    inforceRecord* _record;
    policyInput* _input;

    int _WBFlag() {
        return _record->PdLB_WBFlag;
    }
public:
    memoizedProperty<int> WBFlag;

    inputLoader(inforceRecord* record, policyInput* input) :
        _record(record), _input(input),
        WBFlag(&input->WBFlag, bind(&inputLoader::_WBFlag, this))
    { }
};