多个 inheritance 模板的成员函数歧义

Question

I am public deriving two instances of class template 'Area', one int and another char into an separate class 'Rectangle'.

template<class T>
class Area {
  public:
    T a;
    T getArea() { return a; }
    void setArea(T t) { a = t; }
};  

class Rectangle : public Area<int>, public Area<char> {
};  

int main() {
  Rectangle a;
  a.setArea(1);
  std::cout << a.getArea() << std::endl;
  Rectangle b;
  b.setArea('c');
  std::cout << b.getArea() << std::endl;
}   

And I see ambiguity with setArea and getArea. Why is that so? I thought after public Area, public Area there would be two definitions of setArea. First, void setArea(int) and another void setArea(char). Please correct me if I am wrong. And If I am correct, why the ambiguity?

Answer 1

If you bring the name setArea from both the base classes into the derived class with using statements:

class Rectangle : public Area<int>, public Area<char> {
  using Area<int>::setArea;
  using Area<char>::setArea;
};  

the compiler will be able to call the right setArea .

This will not work for getArea as the 2 functions differ only in their return type. You will have to distinguish between them at the call site:

std::cout << a.Area<int>::getArea() << std::endl;