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#include <iostream>
#include <vector>
using namespace std;
 
class Shape {
    protected:
        Shape() {} // Having a protected constructor only allows you to
                   // call it from the same or derived class
    public:
        virtual void func()=0;
};
 
class Circle : public Shape {
    public:
        Circle() {  }
        void func() { cout << "Hello from a Circle object" << endl; }
};
 
class Square : public Shape {
    public:
        Square() {  }
        void func() { cout << "Hello from a Square object" << endl; }
};
 
int main() {
 
    std::vector<Shape*> shapes;
    Circle c;
    Square s;
    shapes.push_back(&c); // Store the address of the object
    shapes.push_back(&s); // Store the address of the object
 
    // A call through a pointer to a virtul polymorphic class
    // will make sure to call the appropriate function
    Shape ea;
    shapes[0]->func(); // Hello from a circle object
    shapes[1]->func(); // Hello from a square object
 
 
}

prog.cpp: In function ‘int main()’:
prog.cpp:7:9: error: ‘Shape::Shape()’ is protected
         Shape() {} // Having a protected constructor only allows you to
         ^
prog.cpp:35:11: error: within this context
     Shape ea;
           ^
prog.cpp:35:11: error: cannot declare variable ‘ea’ to be of abstract type ‘Shape’
prog.cpp:5:7: note:   because the following virtual functions are pure within ‘Shape’:
 class Shape {
       ^
prog.cpp:10:22: note: 	virtual void Shape::func()
         virtual void func()=0;
                      ^

Standard output is empty