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#include <iostream>
using namespace std;
 
struct Point
{
	Point(int x, int y);
};
 
class Shape  {	// deals with color and style, and holds sequence of lines
protected:
	Shape() { }
	Shape(initializer_list<Point> lst);  // add() the Points to this Shape
 
	void add(Point p);
};
 
struct Open_polyline : Shape {   // open sequence of lines
    using Shape::Shape;          // use Shape’s constructors (§A.16)
    void add(Point p) { Shape::add(p); }
};
 
Open_polyline opl = {
        Point{100,100}, Point{150,200}, Point{250,250}, Point{300,200}
    };
 
int main() {
	// your code goes here
	return 0;
}

prog.cpp:24:5: error: ‘Shape::Shape(std::initializer_list<Point>)’ is protected within this context
     };
     ^
prog.cpp:12:2: note: declared protected here
  Shape(initializer_list<Point> lst);  // add() the Points to this Shape
  ^~~~~

Standard output is empty