Chapter 6: Classes

Chapter Goals

• To be able to implement your own classes
• To master the separation of interface and implementation
• To understand the concept of encapsulation
• To design and implement accessor and mutator member functions
• To understand object construction
• To learn how to distribute a program over multiple source files

Discovering Classes

• If you find yourself defining a number of related variables that all refer to the same concept, define a class that abstracts the concept and contains these variables as data fields.
• Example: Suppose you are reading information about computers:
  • Model Name
  • Price
  • Score between 0 and 100
• Sample data:

  ACMA P600
  995
  77
  
  Alaris Nx868
  798
  57
  
  AMAX Powerstation 600
  999
  75

• We are trying to find the product for which the value (score/price) is highest. Sample code excerpt:

  if (next_score / next_price > best_score / best_price)
  {  best_name = next_name;
     best_score = next_score;
     best_price = next_price;
  }

Discovering Classes (bestval.cpp)

• Each of these sets of variables describes a product (a best product, and the next product read in).
  
• We need to develop a Product class.

Interface

• To define a class, we first need to specify an interface.
• The interface consists of all member functions (methods) we want to apply to objects of that type.
• The interface is specified in the class definition.
• Example: Product class member functions:
  • Make a new product.
  • Read in a product object.
  • Compare two products and find out which one is better.
  • Print a product.
• Product class header:

  class Product {
  public:
     Product();
     void read();
     bool is_better_than(Product b) const;
     void print() const;
  private:
     implementation details
  };

• The first member function is a constructor: the function that is used to initialize new objects.
• This constructor has no parameters so it is called a default constructor.
• Default constructors are automatically used when you construct an object without parameters.

  Product best; /* default construction */

• As a general rule, every class should have a default constructor.
• The read() member function is a mutator function - an operation that modifies the object.
• The last two member functions are accessor functions - functions that query the object for some information without changing it.
• The keyword const indicates that the implicit parameter object is not modified by the member function.

class Class_name {
public:
   constructor declarations
   member function declarations
private:
   data fields
};

class Point {
public:
   Point (double xval, double yval);
   void move(double dx, double dy);
   double get_x() const;
   double get_y() const;
private:
   double x;
   double y;
};

• To use objects in our programs, we only need to use the interface.
• The interface definition of the class just declare the constructors and member functions.
  

Interface (product1.cpp)

Encapsulation

• Classes are broken into public and private sections. To use objects in our program we only need to use the interface. To enable any function to access the interface functions, they are placed in the public section.
• The data items are defined in the private sections of the class definition.
• Example: Every product object has a name field, a price field and a score field.

  class Product {
  public:
     Product();
     void read();
     bool is_better_than(Product b) const;
     void print() const;
  private:
     string name;
     double price;
     int score;
  };

• Only constructors and member functions of the class can access private data.
• Hiding data is called encapsulation. Encapsulation forces indirect access to the data, guaranteeing that it cannot accidentally be put in an incorrect state.
• Example:

  class Time {
  public:
     Time();
     Time(int h, int m, int s);
     void add_seconds(long s);
  
     long seconds_from(Time t) const;
     int get_seconds() const;
     int get_minutes() const;
     int get_hours() const;
  private:
     int hours;   /* conjecture */
     int minutes; /* conjecture */
     int seconds; /* conjecture */
  };

• Suppose the programmer could access data fields of the Time class directly. This allows to create invalid time:

  Time liftoff(19, 30, 0);
  /* liftoff is delayed by six hours */
  /* won't compile, but suppose it did */
  liftoff.hours = liftoff.hours + 6;

• Does this mean liftoff is at 25:30:00?
  
• The constructors and add_seconds function guarantee that all times are always valid, thanks to the encapsulation mechanism.

Member Functions

• Each member function advertised (declared) in the class interface must be implemented separately.
• A Class_name:: prefix makes it clear that we are defining a member function for that class.

  void Product::read()
  {  cout << "Please enter the model name: ";
     getline(cin, name);
     cout << "Please enter the price: ";
     cin >> price;
     cout << "Please enter the score: ";
     cin >> score;
     string remainder; /* read the remainder of the line */
     getline(cin, remainder);
  }

• It is perfectly legal to have member functions in other classes as well; it is important to specify exactly which function goes with which class.
• A member function is called object.function_name().

  next.read();

• When we defining an accessor function, we must supply the keyword const following the closing parenthesis of the parameter list.
  
• When the keyword const is used in the declaration, it must also be placed in the member function definition.

  void Product::print() const
  {  cout << name
          << " Price: " << price
          << " Score: " << score << "\n";
  }

• The keyword const indicates that the member function will not modify the object that called it.
• When you refer to a data field in a member function, such as name or price, it denotes the data field of the object for which the member function was called:

  best.print();

  The Product::print() function prints best.name, best.score and best.price.
• The object to which a member function is applied is called the implicit parameter.
• A parameter explicitly mentions in the function definition is called an explicit parameter.

  bool Product::is_better_than(Product b) const
  {  if (b.price == 0) return false;
     if (price == 0) return true;
     return score / price > b.score / b.price;
  }
  

  if (next.is_better_than(best)) ...

return_type Class_name::function_name(parameter1, ..., parametern) [const]opt
{
   statements
}

void Point::move(double dx, double dy)
{  x = x + dx; y = y + dy;
}
double Point::get_x() const
{  return x;
}

Default Constructors

• The purpose of a constructor is to initialize all data fields of an object.
• A constructor always has the same name as the class.

  Product::Product()
  {  price = 0;
     score = 0;
  }

• Most default constructors initialize the data fields to zero, but not always.
  • The Time class initializes to current time.
  • Non-numeric data (e.g. string data) may have another value.

Default Constructors (product2.cpp)

Constructors with Parameters

• It's possible to create a class with more than one constructor.

  Employee Fred; /* default construction */
  Employee Harry("Harry Hacker", 40000); /* alternate construction */

• All constructors have the same name as the class, but have different parameter lists.

  class Employee {
  public:
     Employee();
     Employee(string employee_name, double initial_salary);
     
     void set_salary(double new_salary);
   
     string get_name() const;
     double get_salary() const;
  private:
     string name;
     double salary;
  };

• Whenever two functions have the same name but are distinguished by their parameter types, the function name is overloaded.
• The second constructor sets the data fields of the new object based on the parameters.

  Employee::Employee(string employee_name, double initial_salary)
  {  name = employee_name;
     salary = initial_salary;
  }

Class_name::Class_name(parameter1, ..., parametern)
{ statements
}

Point::Point(double xval, double yval)
{ x = xval; y = yval;
}

• Sometimes a constructor gets more complex because one of the data fields is itself an object of another class with its own constructor.
• Example: suppose we modify the Employee class so that each Employee has scheduled work hours.

  class Employee {
  public:
     Employee(string employee_name, double initial_salary,
        int arrive_hour, int leave_hour);
     . . .
  private:
     string name;
     double salary;
     Time arrive;
     Time leave;
  };

  The constructor must initialize the time fields using the Time() constructor.

  Employee::Employee(string employee_name, double initial_salary,
     int arrive_hour, int leave_hour)
  {
     name = employee_name;
     salary = initial_salary;
     arrive = Time(arrive_hour, 0, 0);
     leave = Time(leave_hour, 0, 0);
  }

Accessing Data Fields

• Only member functions of a class are allowed to access the private data fields of objects of that class. 

  void raise_salary(Employee& e, double percent)
  {  e.salary = e.salary * (1 + percent / 100 ); /* Error */
  }

• All other functions must go through the public interface of the class. Data fields must be accessed by accessor and mutator functions.

  void raise_salary(Employee& e, double percent)
  {  e.set_salary(e.get_salary() * (1 + percent / 100 )); /* OK */
  }

• Not every data member needs accessor functions (the Product class did not have a get_score() function).
• Not every get_ function needs a matching set_ function (the Time class can get_minutes() but not set_minutes()).
• Remember that implementation is supposed to be hidden - just because a class has member functions named get_ or set_ does not necessarily explain how the class is designed.

  class Time {
  public:
     Time();
     Time(int h, int m, int s);
     void add_seconds(long s);
  
     long seconds_from(Time t) const;
     int get_seconds() const;
     int get_minutes() const;
     int get_hours() const;
  private:
     long time_in_secs;
  };

• The data representation is an internal implementation detail of the class that is invisible to the class user.

  Time::Time(int hour, int min, int sec)
  {  time_in_secs = 60 * 60 * hour + 60 * min + sec;
  }
   
  int Time::get_minutes() const
  {  return (time_in_secs / 60) % 60;
  }
   
  int Time::seconds_from(Time t) const
  {  return time_in_secs - t.time_in_secs;
  }

Comparing Member Functions with Nonmember Functions

• Consider the nonmember function

  void raise_salary(Employee& e, double percent)
  {  double new_salary = e.get_salary() * (1 + percent / 100);
     e.set_salary(new_salary);
  }

  versus the member function

  void Employee::raise_salary(double percent)
  {  salary = salary * (1 + percent / 100);
  }

• A nonmember function is called with two explicit parameters.

  raise_salary(harry, 10);

• A member function is called using the dot notation, with one explicit parameter and one implicit parameter.

  harry.raise_salary(10);

• A member function can invoke another member function on the implicit parameter without using the dot notation.

  void Employee::print() const
  {  cout << "Name: " << get_name()
          << "Salary: " << get_salary()
          << "\n";
  }

• employee.cpp
• Member functions automatically (by default) pass the implicit parameter by reference.
• Nonmember functions automatically (by default) pass their parameters by value.

Separate Compilation

• When your code gets large or you work in a team, you will want to split your code into separate source files.
  • Saves time: instead of recompiling the entire program, only recompile files that have been changed.
  • Group work: separate programmers work on separate files.
• The header file (e.g. product.h) contains:
  • definitions of constants
  • definitions of classes
  • declarations of nonmember functions
  • declarations of global variables
• The source file (e.g. product.cpp) contains
  • definitions of member functions
  • definitions of nonmember functions
  • definitions of global variables

Separate Compilation (product.h)

Separate Compilation (product.cpp)

Separate Compilation (prodtest.cpp)

• Special code (preprocessor directives) for the compiler must be put in a header file to prevent the file from being compiled twice.

  #ifndef PRODUCT_H
  #define PRODUCT_H
  . . .
  #endif

• The source file includes its own header file.

  #include "product.h"

• The source file does not contain a main function because many programs may use this class.
• Shared constants are placed in the header file.
• To share global variables among source files, declare them in a header file as extern.

  extern GraphicWindow cwin;

  The source file contains the definition.

  GraphicWindow cwin;