Impossible to access protected function in the initialization of the derived class's constructor

Question

I'm trying to implement a class Union which inherits directly from the class Shape (Union is a shape composed of multiple shapes).

The (protected) constructor of Shape takes a Point as input (representing the center of the shape). To construct a Union object, the only input is a list of shapes (const vector<const Shape>). To implement the constructor of Union, I wanted to use an initialization list, as detailed herebelow

class Union : Shape
{
public:
     Union(const std::vector<const Shape> shapes): 
     Shape(shapes[0].get_center()), shapes(shapes) {};
     ...
private:
     const std::vector<const Shape> shapes;

}

with get_center() a protected virtual function of class Shape.

class Shape
{
protected:
     Shape (const Point& center) : center(center) {};
     virtual const Point& get_center() const =0;
     ...
private:
     const Point center;
}

However, when I call get_center() in the initialization list of Union constructor, there is an error saying that "get_center() is a protected member of Shape".

Can someone explain me why I cannot call get_center() from the subclass Union (which should have inherited the function) ?

Thank you!

P.S.: If I set the function get_center() to public, there is no error anymore.

@NeilButterworth Yes. Anyways, when I write `class Union: public Shape {...}`, the problem is still there. — PDC, Aug 23 '19 at 17:10
@Jarod42 But `Union` is a derived class of `Shape`. Shouldn't I have access to `get_center()` function ? — PDC, Aug 23 '19 at 17:14
[This](https://stackoverflow.com/questions/16785069/why-cant-a-derived-class-call-protected-member-function-in-this-code) might help — gimme_danger, Aug 23 '19 at 17:27
Why does `Union` derive from `Shape` when it is, in fact, a collection of `Shape`s? — Paul Sanders, Aug 23 '19 at 17:33
@PaulSanders Because different shapes put together form a new shape. — PDC, Aug 23 '19 at 17:34

score 2 · Accepted Answer · answered Aug 23 '19 at 17:13

Issue can be reduced to that

struct Base
{
protected:
    int i = 0;
};

struct Derived : Base
{
    static void foo(Base& b)
    {
        b.i = 42; // Error
    }

    static void bar(Derived& d)
    {
        d.i = 42; // Ok
    }
};

Demo

You can only access protected members through the derived class.

Impossible to access protected function in the initialization of the derived class's constructor

1 Answers1