template<class T>
  void FooClass<T>::foo(boost::function<double (int)> f)
{...}

class MyFun {
    public: double operator()(int a) {do something...;}
}

template<class T>
  template <class FunPtr>
    void FooClass<T>::foo(const FunPtr& f)
{...}

MyFun f;
FooClass<double> fooclass;
fooclass.foo(f);

boost::function<double (int)> bMyFun(myFunInstance);
fooClassInstance.foo(bMyFun);

fooclass.foo(boost::function<double (int)>(f));