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Classes & Objects

Classes & Objects

Classes

• Model real-world entities

• Blueprint from which objects are created

• A user-defined data-type

• Has attributes (data; instance variables)

• Has methods (functions)

• Can hide data and methods that are only used internally by the class (By using access modifiers)

• The goal of a class is to provide a well-defined public interface that the user of the class can easily use to solve their problem.

• Examples:

  • Account

  • Employee

  • Image

  • std::vector

  • std::string

Objects

• Created from a class

• Represents a specific instance of a class

• Can create many objects out of a class

• Each has its own identity

• Each can use the defined class methods

• Examples:

  • Jack's account is an instance of an Account.

  • Sunny's account is an instance of an Account.

  Each has its own balance, can make deposits, withdrawals, etc.

Examples

• Player class and objects

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  // Class declaration
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  class Player
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  {
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      // Attributes (or instance variables)
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      std::string name;
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      int health;
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      int xp;
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      // Methods
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      void talk(std::string text_to_say);
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      bool is_dead();
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  };
  

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  // Object instantiataion
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  Player jack;
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  Player hero;
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  Player *enemy = new Player();   // Allocated dynamically on the heap
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  delete enemy;
  

• Account class and objects

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  // Class declaration
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  class Account
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  {
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      // Attributes (or instance variables) 
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      std::string name;
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      double balance;
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      // Method
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      bool withdraw(double amount);
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      bool deposit(double amount);
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  };
  

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  Account jack_account;
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  Account sunny_account;
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  Account *yena_account = new Account();  // Allocated dynamically on the heap
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  delete yena_account;
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  Account accounts[] {jack_account, sunny_account};
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  std::vector<Account> accounts1 {jack_account};
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  accounts1.push_back(sunny_account);
  

  Once declared, a class can be used just like any other C++ primitive data types.

Accessing Class Members

• You can access class attributes and class methods.

• Some class members will not be accessible (i.e., hidden members)

• Need an object to access instance variables

• If you have an object, you can access the class members by using the dot operator (.).

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  Account jack_account;
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  jack_account.balance;
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  jack_account.deposit(1000.00);
  

• If you have a pointer to an object, you can access the class members by using the arrow operator or member of pointer operator (->).

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  Account jack_account = new Account();
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  // 1. Dereference the poiner, then use the dot operator
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  (*jack_account).balance;
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  (*jack_account).deposit(1000.00);
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  // 2. Use the arrow operator (member of pointer operator)
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  jack_account->balance;
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  jack_account->deposit(1000.00);
  

   

Class Member Access Modifiers (public, private, protected)

• public - Accessible everywhere

• private - Accessible only by other members of the same class or by friends of the class

• protected - Used with inheritance

Examples

• Player class

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  class Player
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  {
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  private:
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      std::string name;
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      int health;
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      int xp;
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  public:
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      void talk(std::string text_to_say);
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      bool is_dead();
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  };
  

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  Player jack;
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  jack.name = "Jack";     // Compiler error
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  jack.health = 1000;     // Compiler error
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  jack.talk("Hello!");    // OK
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  Player *enemy = new Player();
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  enemy->xp = 100;        // Compiler error
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  enemy->talk("Got you!");// OK
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  delete enemy;
  

  If you try to directly access a private class member from outside of the class, you will get a compiler error.

• Account class

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  class Account
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  {
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  private:
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      std::string name;
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      double balance;
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  public:
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      bool withdraw(double amount);
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      bool deposit(double amount);
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  };
  

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  Account jack_account;
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  jack_ccount.balance = 1000.00;          // Compiler error
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  jack_account.deposit(1000.00);          // OK (Accessing a private member through another member of the class)
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  jack_account.name = "Jack's Account";   // Compiler error
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  Account *sunny_account = new Account();
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  sunny_account->balance = 10000.0;       // Compiler error
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  sunny_account->withdraw(10000.0);       // OK
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  delete sunny_account;
  

  This is very powerful, because, for example, if we have an object whose balance is 20 million dollars and we know that that must have been an error, then the only place where that error could have happened in this example is in the deposit method. There's no way that any other part of the program could have changed that value since the value is private.

  This makes testing and debugging code much easier.

Implementing Member Methods

• Very similar to how we implemented functions

• Member methods have access to member attributes (Don't need to pass them as arguments as we do with regular functions!)

• Can be implemented inside the class declaration

  • If you do this, the method becomes implicitly inline.

  • While this is okay for small and relatively simple methods, it is a good practice to implement larger, more complex methods outside of the class declaration.

• Can be implemented outside the class declaration

  • To do this you need to use Class_name::method_name to tell the compiler that you are implementing a method for a specific class.

  • The compiler can then type check the method implementation when it sees them.

• Can separate specification from implementation. This makes the class much easier to manage.

  • .h/.hpp file for the class declaration

  • cpp file for the class implementation

Examples

• Implementing member methods INSIDE the class declaration

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  class Account
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  {
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  private:
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      double balance;
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  public:
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      void set_balance(double bal) { balance = bal; }
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      double get_balance() { return balance; }
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  };
  

• Implementing member methods OUTSIDE the class declaration

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  class Account
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  {
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  private:
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      double balance;
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  public:
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      void set_balance(double_bal);
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      double get_balance();
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  };
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  void Account::set_balance(double bal) { balance = bal; }
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  double Account::get_balance() { return balance; }
  

  Need to tell the compiler that you are implementing a method for a specific class. The compiler can then type check the method implementation when it sees them.

• Separating specification from implementation

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  /* Account.h (Specification) */
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  /* Include guard */
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  #ifndef ACCOUNT_H
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  #define ACCOUNT_H
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  // Account class declaration
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  class Account
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  {
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  private:
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      double balance;
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  public:
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      void set_balance(double bal);
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      double get_balance();
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  };
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  #endif
  

  The include guard can be replaced by #pragma once. But, do check first if your compiler support this directive, because some don't!

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  /* Account.cpp (Implementation) */
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  #include "Account.h"
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  void Account::set_balance(double bal) { balance = bal; }
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  double Account::get_balance() { return balance; }
  

  Notice that we included accoun.h and it's in double quotes. Includes with double quotes tell the compiler to include header files that are local to this project. The compiler knows where those are.

  Includes with angled brackets with no extensions (e.g., #include <iostream>) are used to include system header files and the compiler knows where these are located.

  Main file

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  /* main.cpp */
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  #include <iostream>
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  #include "Account.h"
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  int main()
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  {
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      Account jack_account;
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      jack_account.set_balance(1000.00);
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      double bal = jack_account.get_balance();
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      std::cout << bal << std::endl;  // 1000
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      return 0;
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  }
  

  Include .h files and never include .cpp files!

  When the program is compiled, both the main.cpp and the account.cpp files are compiled and then linked to produce the executable.