Classes

Classes, like structures, provide a way for C++ programmers to aggregate data together in a way that makes sense in the context of a specific program. By convention, programmers use structures when member variables are independent of each other, and use classes when member variables are related by an "invariant".

Invariants

An "invariant" is a rule that limits the values of member variables.

For example, in a Date class, an invariant would specify that the member variable day cannot be less than 0. Another invariant would specify that the value of day cannot exceed 28, 29, 30, or 31, depending on the month and year. Yet another invariant would limit the value of month to the range of 1 to 12.

Date Class

Let's define a Date class:

// Use the keyword “class” to define a Date class:
class Date {
  int day{1};
  int month{1};
  int year{0};
};

So far, this class definition provides no invariants. The data members can vary independently of each other.

There is one subtle but important change that takes place when we change struct Date to class Date. By default, all members of a struct default to public, whereas all members of a class default to private. Since we have not specified access for the members of class Date, all of the members are private. In fact, we are not able to assign value to them at all!

Date Accessors And Mutators

As the first step to adding the appropriate invariants, let's specify that the member variable day is private. In order to access this member, we'll provide accessor and mutatot functions. Then we can add the appropriate invariants to the mutators.

class Date {
 public:
  int Day() { return day_; }
  void Day(int d) { day_ = d; }

 private:
  int day_{1};
  int month_{1};
  int year_{0};
};

Date Invariants

Now we can add the invariants within the mutators.

class Date {
 public:
  int Day() { return day; }
  void Day(int d) {
    if (d >= 1 && d <= 31) day_ = d;
  }

 private:
  int day_{1};
  int month_{1};
  int year_{0};
};

Now we have a set of invariants for the the class members!

As a general rule, member data subject to an invariant should be specified private, in order to enforce the invariant before updating the member's value.