Why need to initiallise to '0' of the members of constructor of all the classes while writing a code for Virtual function in C++

#include< iostream >
using namespace std;
class Shape {
protected:
    int width, height;
    int mem;
public:
    Shape( int a, int b)
    {
        width = a;
        height = b;
    }
    virtual int area()
    {
        cout << "Parent class area :" <<endl;
        return 0;
    }
    virtual int dis()
    {
        mem=0;
    }
};
class Rectangle: public Shape{
public:
    Rectangle(int a, int b)
    {
        Shape(a, b);
    }
    int area ()
    {
        cout << "Rectangle class area :" <<endl;
        return (width * height);
    }
    int dis()
    {
        cout<<"mem is : "<<mem<<endl;
    }
};
class Triangle: public Shape{
public:
    Triangle(int a, int b)
    {
        Shape(a, b);
    }
    int area ()
    {
        cout << "Triangle class area :" <<endl;
        return (width * height / 2);
    }
};
// Main function for the program
int main( )
{
    Shape *shape;
    Rectangle rec(10,7);
    Triangle  tri(10,5);
    // store the address of Rectangle
    shape = &rec;
    // call rectangle area.
    shape->area();
    shape->dis();
    // store the address of Triangle
    shape = &tri;
    // call triangle area.
    shape->area();

    return 0;
}