An empty class is pretty useless. Classes are designed to bring together related information and functionality — time to add stuff inside!

Components of a class are called class members. Just like you can get a string’s length using .length(), you can access class members using the dot operator (object.class_member).

There are two types of class members:

  • Attributes, also known as member data, consist of information about an instance of the class.
  • Methods, also known as member functions, are functions that you can use with an instance of the class. We use a . before method names to distinguish them from regular functions.

We encapsulate — or enclose for simpler user access — attributes and methods in a class like this:

class City { // attribute int population; // we'll explain 'public' later public: // method void add_resident() { population++; } };

Unless we have a mostly empty class, it’s common to split function declarations from definitions. We declare methods inside the class (in a header), then define the methods outside the class (in a .cpp file of the same name).

How can we define methods outside a class? We can do this using ClassName:: before the method name to indicate its class like this:

int City::get_population() { return population; }

Unlike with regular functions, we need to include the header file in the .cpp file where we define the methods.

Note: We must declare a method inside the class if we want to define it outside.



Add a Song class to song.hpp. Inside the class, add an std::string attribute called title.


Below title, add public: on a new line. We’ll explain this part later.

Add two method declarations for Song underneath:

  • .add_title(), a void method which accepts a new_title for a song. (Make sure this parameter has a different name than title.)
  • .get_title(), which has no parameters and will return as std::string.

Move over to song.cpp. Add definitions for each method:

  • Song::add_title() should set title to new_title.
  • Song::get_title() should return title.
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