14.2. What is a class?

In most object-oriented programming languages, a class is a user-defined type that includes a set of functions. As we have seen, structures in C++ meet the general definition of a class.

But there is another feature in C++ that also meets this definition; confusingly, it is called a class. in C++, a class is just a structure whose instance variables are private by default. For example, I could have written the playing_card definition:

class playing_card
{
  int m_suit, m_rank;

public:
  playing_card ();
  playing_card (int s, int r);

  int rank () const { return m_rank; }
  int suit () const { return m_suit; }
  void rank (int r) { m_rank = r; }
  void suit (int s) { m_suit = s; }
};

I replaced the word struct with the word class and removed the private: label. This result of the two definitions is exactly the same.

In fact, anything that can be written as a struct can also be written as a class, just by adding or removing labels. There is no real reason to choose one over the other, except that as a stylistic choice, most C++ programmers use class.

Also, it is common to refer to all user-defined types in C++ as “classes,” regardless of whether they are defined as a struct or a class.

By default, the data members of a class are private.

True

Correct! We can omit the private: label because the data members are private by default

False

Incorrect! Try again.

Check answer

How can we change deck, which is currently a struct, into a class?

struct deck {
private:
  vector<card> cards;

public:
  deck ();
  deck (int n);

  void print () const;
  void swap_cards (int index1, int index2);
  int find_lowest_card (int index);
  void shuffle_deck ();
  void sort_deck ();
  deck subdeck (int low, int high) const;
  deck merge_sort () const;
  deck merge_sort (deck deck) const;
};

Remove the private: label.

Incorrect! deck is still a struct.

Change struct to class and remove the public: label.

Incorrect! We don't want to make the constructors and all member functions private.

Remove the public: label.

Incorrect! We don't want to make the constructors and all member functions private.

Change struct to class.

Correct! deck is now a class and it's okay that we kept the private: label.

Check answer

Private data members can be accessed within the class.

True

Correct! However, they cannot be accessed outside of the class.

False

Incorrect! We can access private data members as long as we are in the class.

Check answer