C++11 Call virtual member function

I`m trying to implement something like this using C++11.

class Proto{
public:    
    virtual void fu();
};

class Impl: public Proto{
public:
    void fu();
};

void Impl::fu(){
    LOG_INFO("im fu");
}

class Inv{
public:
    void useFu(void (Proto::*)());
};

void Inv::useFu(void (Proto::*fu)()){
    //fu();
};

void main(){    
    Impl impl;
    Inv inv;

    //inv.useFu(impl.fu); 
}

useFu(void (Proto::*)()) must be declared in this way because, fu() uses some specific to Proto functionality's

I have two places were things going wrong. First is fu() call itself and second how to pass fu as parameter inv.useFu(impl.fu).