Shape class with Overloading

#include <iostream>
#include <cstdlib>
#include <cmath>

using namespace std;

class Shape{
    public:
        double area();
};

class Square:public Shape{
    double side;
    public:
        Square(double = 0.0);
        double area();
};

class Circle:public Shape{
    double radius;
    public:
        Circle(double = 0.0);
        double area();
};

class Rectangle:public Shape{
    double length, breadth;
    public:
        Rectangle(double = 0.0, double = 0.0);
        double area();
};

inline Square::Square(double side){
    Square::side = side;
}

inline Circle::Circle(double radius){
    Circle::radius = radius;
}

inline Rectangle::Rectangle(double length, double breadth){
    Rectangle::length = length;
    Rectangle::breadth = breadth;
}

inline double Circle::area(){
    return M_PI * radius * radius;
}

inline double Rectangle::area(){
    return length * breadth;
}

inline double Square::area(){
    return side * side;
}

int main(int argc, char** argv) {
    system("cls");
    cout << "For square" << endl;
    cout << "Enter a side: ";
    double side;
    cin >> side;
    cout << "For circle" << endl;
    cout << "Enter a radius: ";
    double radius;
    cin >> radius;
    cout << "For rectangle" << endl;
    cout << "Enter a length: ";
    double length;
    cin >> length;
    cout << "Enter a breadth: ";
    double breadth;
    cin >> breadth;
    Circle c(radius);
    Rectangle r(length, breadth);
    Square s(side);
    cout << "Area of circle = " << c.area() << endl;
    cout << "Area of square = " << s.area() << endl;
    cout << "Area of rectangle = " << r.area() << endl;
    cin.ignore();
    cin.get();
    return 0;
}