Which of the following is not a type of constructor?

#include<iostream.h> 
class CuriousTab
{
    int x, y; 
    public:
    CuriousTab(int xx = 10, int yy = 20 )
    {
        x = xx; 
        y = yy;
    }
    void Display()
    {
        cout<< x << " " << y << endl;
    } 
    ~CuriousTab()
    { } 
};
int main()
{
    CuriousTab objCuriousTab; 
    objCuriousTab.Display(); 
    return 0;
}

1. Once a reference variable has been defined to refer to a particular variable it can refer to any other variable.
2. A reference is not a constant pointer.

class Birds {};
class Peacock : protected Birds {};