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#include <iostream>
 
using Color = int;
struct Shape
{
  double area = 0;
};
 
class Geometry
{
public:
  // everybody can alter the color of the geometry, so 
  // they get a mutable reference
  Color& color() { return m_color; };
 
  // not everybody can alter the shape of the Geometry, so
  // they get a constant reference to the shape.
  Shape const& shape() const { return m_shape; };
 
protected:
  // derived classes can alter the shape, so they get 
  // access to the mutable reference to alter the shape.
  Shape & shape() { return m_shape; };
 
private:
  Shape m_shape;
  Color m_color;
};
 
 
void someFunction(Geometry & g)
{
  std::cout << g.shape().area << std::endl;
}
 
int main ()
{
  Geometry g1;
  someFunction(g1);
  return 0;
}

prog.cpp: In function ‘void someFunction(Geometry&)’:
prog.cpp:23:11: error: ‘Shape& Geometry::shape()’ is protected
   Shape & shape() { return m_shape; };
           ^
prog.cpp:33:24: error: within this context
   std::cout << g.shape().area << std::endl;
                        ^

Standard output is empty