Question

I am currently working on Chapter 7 in the book “Starting Out With C++ Early Objects” by the Pearson printing company.

I am unable to understand the function of the variable ‘r’ in this class declaration:

class Circle
{  private:
       double radius;

   public:
       void setRadius(double r)
       {  radius = r; }

  double getArea()
      {  return 3.14 * pow(radius, 2); }
};

Why can’t I just write the ‘radius’ variable like this?

class Circle
{  private:
       double radius;

  double getArea()
      {  return 3.14 * pow(radius, 2); }
};

I don’t understand the function of the

   public:
       void setRadius(double r)
       {  radius = r; }

Statement.

Answer 1

The radius is private. Without that function, you would not be able to set the radius from outside of the class. In order for the class to be useful, you would most likely want to be able to create objects of the type Circle and set their radius. Thus, you need some type of function in order to set that radius.

The easiest and most reasonable way to solve this is to supply a public member function inside the class Circle itself.

This can most easily be done using a setter, such as what you have shown. This allows you to set, and later change, the radius.

void SetRadius(float r)
{
    radius = r;
} 

You could also supply an extra argument to the constructor to ensure that a Circle always has its radius initialized with a user-supplied value, or at least set a default value in the declaration of radius (in C++11).