c++ 泛型特化继承

#include <stdio.h>
#include <iostream>

using namespace std;
class Empty{
};
template<typename R, typename T1, typename T2, typename T3>
class Parent{
public:
    virtual ~Parent(){}
    virtual R operator()(T1,T2,T3)=0;
};
template<typename R, typename T1, typename T2>
class Parent<R,T1,T2,Empty>{
public:
    virtual ~Parent(){}
    virtual R operator()(T1,T2)=0;
};
template<typename R, typename T1>
class Parent<R,T1,Empty,Empty>{
public:
    virtual ~Parent(){}
    virtual R operator()(T1)=0;
};
template<typename R, typename T1, typename T2, typename T3>
class Child: public Parent<R,T1,T2,T3>{
public:
    Child(){
    }
    ~Child(){
    }
    R operator()(T1 a,T2 b,T3 c){
        std::cout<<a<<" "<<b<<" "<<c<<std::endl;
    }
    R operator()(T1 a,T2 b){
        std::cout<<a<<" "<<b<<std::endl;
    }
    R operator()(T1 a){
        std::cout<<a<<std::endl;
    }
};
template<typename R, typename T1, typename T2, typename T3>
void test(Parent<R,T1,T2,T3> *p,T1 a,T2 b,T3 c){
    return (*p)(a,b,c);
}
template<typename R, typename T1, typename T2>
void test(Parent<R,T1,T2,Empty> *p,T1 a,T2 b){
    (*p)(a,b);
}
int main(){
    Child<void,int,int,Empty> child;
    test<void,int,int>(&child,1,2);
    Child<void,int,int,int> child1;
    test<void,int,int,int>(&child1,1,2,3);
}