Section 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 Card definition:

class Card
{
private:
  int suit, rank;

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

  int getRank() const { return rank; }
  int getSuit() const { return suit; }
  int setRank(int r) { rank = r; }
  int setSuit(int s) { suit = s; }
};

I replaced the word struct with the word class - the result of the two definitions is exactly the same. If I wanted to, I could remove the private: label. In a class, any non-labeled members default to private. Members default to public in a struct, which is why we didn’t have to say public: in any of our structs.

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.

Checkpoint 14.2.1.

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.

Checkpoint 14.2.2.

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 swapCards(int index1, int index2);
  int findLowestCard(int index);
  void shuffleDeck();
  void sortDeck();
  Deck subdeck(int low, int high) const;
  Deck mergeSort() const;
  Deck mergeSort(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.

Checkpoint 14.2.3.

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.

You have attempted of activities on this page.

Prev Top Next