Difference between instantiating a class object with Constructor parameter and * operator without parameters C++

Question

I came across this tutorial link when studying OOPS concepts in C++. http://www.tutorialspoint.com/cplusplus/cpp_polymorphism.htm

In the base class Shape, it has constructor with two parameters to set its protected variables and in main() I thought the only way to create an object for class Shape is by doing something like Shape shape(2,4) to match the constructor in Shape class.

Can someone say how the instantiation Shape *shape without any parameters work and whats the difference between creating object by Shape shape(2,4) and Shape *shape.

#include <iostream>

using namespace std;

class Shape{
protected:
    int w,h;

public:

    Shape(int a, int b){
        w=a;
        h=b;
    }
    int area(){
        cout << "Shape class area" << endl;
        return 0;
    }
};

class Rect:public Shape{

public:

    Rect(int a, int b):Shape(a,b) {}
    int area(){
        cout <<"Rect class area " << endl;
        return w*h;
    }
};

class Tri:public Shape{

public:

    Tri(int a, int b):Shape(a,b) {}
    int area(){
        cout << "Tri class area" << endl;
        return (w*h)/2;
    }
};

int main(){

    Shape *shape;
    Rect r(4,5);
    Tri t(4,5);
    shape=&r;
    shape->area();
    shape=&t;
    shape->area();
    return 0;
}

Answer 1

Statement

Shape *shape;

does not create any instance of the class Shape. It declares a variable that is a pointer of type Shape * and that is not initialized that is it has indetermined value provided that the declaration declares a local variable.

As for the constructor then the only constructor of the class with two parameters is also the default constructor of the class because each parameter has a default argument.

Shape( int a=0, int b=0)
           ^^^      ^^^  
{
   //...
}

Thus you can write for example

Shape sh;

and data members of the created object will be initialized by these default arguments. This declaration is equivalent to

Shape sh( 0, 0 );