Pure virtual functions with templated inheritance

Question

I am attempting to implement polymorphism with a templated derived class. See below:

//templated base class
template <class num> class A{
protected:
    num x;
    num y;

public:
    A( num x0 = 0, num y0 = 0) {
       x = x0;
       y = y0;
    }
    virtual void print_self() = 0;

};

//derived class
template < class num > class B: public A < num > {
   public:
      B( num x1 = 0, num y1 = 0):shape < num > ( x1 , y1 ) { }
      void print_self(){
          std::cout << "x is " << x << " and y is " << y << endl;
      }
};

The base class has pure virtual function print_self(). When trying to define the function in the derived class, I received the following error:

'x' was not declared in this scope

The same for y. So somehow the derived class doesn't have access to the variables x and y even though it is listed as protected.

Is there some other way to define print_self(), or is this simply not possible? If it is not possible, can you suggest another approach?

You can also use `A::x` and `A::y` or `typedef A base` and then `base::x`, etc. — DeiDei, Feb 05 '17 at 22:26
@Raindrop7 Nope, `this` is in fact needed here. Using Visual C++ by any chance? I think they allow it, but it's not standard. — DeiDei, Feb 05 '17 at 22:29
@DeiDei: Yes I am using `MSVC` and it doesn't complain. So is it really a must to use `this` in this case? What is the OP using (compiler)? — Raindrop7, Feb 05 '17 at 22:31
@Raindrop7 Yes, either use `this->` or the possibilities I mentioned above, or the answer below. Either way, the names inside a base class are not implicitly visible in a derived class. — DeiDei, Feb 05 '17 at 22:32

score 2 · Accepted Answer · answered Feb 05 '17 at 22:23

Since you are using templated inheritance, the existence of x and y are dependent of the template argument. In other words the base class part of this pointer become a dependent name. You must use the this pointer explicitly or use using.

template<typename num>
struct B : A<num> {
    using A<num>::x;
    using A<num>::y;

    void print_self(){
        std::cout << "x is " << x << " and y is " << y << endl;
    }
};

Or even:

template<typename num>
struct B : A<num> {
    void print_self() {
        std::cout << "x is " << this->x << " and y is " << this->y << endl;
    }
};

Pure virtual functions with templated inheritance

1 Answers1