C++ program to define member function outside of the class

//C++ program to demonstrate Class and Object
//Class Rectangle with length and breadth as private members
//and appropriate member functions to read data, calculate and display area
#include <iostream>
using namespace std;
class Rectangle
{
    private:
        float l,b,a;
    public:
        void ReadData();
        void CalculateArea();
        void DisplayArea();
};
void Rectangle::ReadData()
{
    cout<<"Enter length and breadth: ";
    cin>>l>>b;
}
void Rectangle::CalculateArea()
{
    a=l*b;
}
void Rectangle::DisplayArea()
{
    cout<<"The area is "<<a;
}
int main()
{
    Rectangle r1;
    r1.ReadData();
    r1.CalculateArea();
    r1.DisplayArea();
    return 0;
}