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Blitz Introduction to C++ Objects

1. A "Date" concept
2. Programmatic control
3. Switching to numeric representation
4. Procedural approach
5. Step in the right direction
6. Problems with procedural approach
7. Solution: use date as a variable
8. Fundamental properties of built-in types
9. C++ program fundamentals
10. User-defined type
11. class Date implementation
12. Overloaded operator <<
13. Conclusion

1. A "Date" concept

• Consider a program that uses dates:

  
      #include <iostream>
      int main()
      {
          std::cout << "3/8/2010" << '\n';
          std::cout << "7/20/2011" << '\n';
          return 0;
      }
  
  

  Hardcoded dates such "3/8/2010" aren't very useful.

   

2. Programmatic control

• We want programmatic control of dates:

  • Display dates

  • Input dates from a user

  • Store dates in a file or a database

  • Perform date calculations and comparisons

  • Use calendars, determine weekdays, holidays, etc.

   

3. Switching to numeric representation

• Conversion functions such as serial Julian date can help:

  
      #include <iostream>
      int serial_julian_date( int Month, int Day, int Year );
      int serial_2_month( int nDate );
      int serial_2_day( int nDate );
      int serial_2_year( int nDate );
      int main()
      {
          int month = 3;
          int day = 8;
          int year = 2010;
          // Compute serial date
          int serial_date = serial_julian_date( month, day, year );
          // Modify serial date:
          serial_date = serial_date + 100; // add 100 days
          month = serial_2_month( serial_date );
          day = serial_2_day( serial_date );
          year = serial_2_year( serial_date );
          // Display new date in human-friendly format:
          std::cout << month << '/' << day << '/' << year << '\n';
          return 0;
      }
  
  

   

4. Procedural approach

• Functions such as

  
      int serial_julian_date( int Month, int Day, int Year );
      int serial_2_month( int nDate );
      int serial_2_day( int nDate );
      int serial_2_year( int nDate );
  
  

  perform nice job converting to and from different date formats.

   

5. Step in the right direction

• Since we found a way to replace "3/8/2010" with int representation, making calculations and comparing dates becomes easy:

  
          serial_date = serial_date + 100; // add 100 days
  
          if ( serial_date < today ) {     // compare dates
              // the date is in the past ("less than today")
              // ...
          }
  
          int age = serial_2_year( today ) - serial_2_year( serial_date );
  
  

   

6. Problems with procedural approach

• However,

  • Program contains too many small steps.

  • Program requires up to 4 variables to handle each date: month, day, year, and serial_date.

  • No protection of any kind exists to prohibit illegal date modifications:

  
          int month = 2;
          int day = 31;    // the day is outside of allowable range for February
          int year = 2010;
  
  

   

7. Solution: use date as a variable

• 
      int main()
      {
          Date birthday;
  
          birthday.month = 3;
          birthday.day = 8;
          birthday.year = 2010;
  
          std::cout << birthday << '\n';
      }
  
  

• Benefits: all logic is done around a single variable named birthday.

• Questions:

  • What is Date?

  • What is birthday.month?

  • What happens here: std::cout << birthday?

   

8. Fundamental properties of built-in types

• Each C++ built-in type is defined by its size and a set of supported operations:

9. C++ program fundamentals

• A program is a set of variables and functions.

• Variables store data.

• Functions define computer operations.

• Functions provide convenient way to encapsulate various computations.

10. User-defined type

• C++ programmer describes user-defined types using keyword class:

  
  #include <iostream>
  
  class Date {
      // member variables
      int serial_date;
  public:
      // member functions
      void set_date( int month, int day, int year );
      int get_month();
      int get_day();
      int get_year();
  };
  
  int main()
  {
      Date birthday; // instantiate object of type "Date"
      birthday.set_date( 2, 8, 1999 ); // member function calls
      std::cout << birthday.get_month();
      return 0;
  }
  
  

   

11. class Date implementation

• The implementation of Date class uses previously defined serial date conversion functions:

  
      #include <iostream>
  
      int serial_julian_date( int Month, int Day, int Year );
      int serial_2_month( int nDate );
      int serial_2_day( int nDate );
      int serial_2_year( int nDate );
  
      class Date {
          // member variables
          int serial_date;
      public:
          // member functions
          void set_date( int month, int day, int year );
          int get_month();
          int get_day();
          int get_year();
      };
  
  

   

12. Overloaded operator <<

• C++ allows operator syntax for user-defined types:

  
      #include <iostream>
  
      int serial_julian_date( int Month, int Day, int Year );
      int serial_2_month( int nDate );
      int serial_2_day( int nDate );
      int serial_2_year( int nDate );
  
      class Date {
          // member variables
          int serial_date;
      public:
          // member functions
          void set_date( int month, int day, int year ) {}
          int get_month() { return 2; }
          int get_day() { return 8; }
          int get_year() { return 1999; }
      };
  
      void operator<<( std::ostream&, Date dt )
      {
          std::cout << dt.get_month();
          std::cout << '/';
          std::cout << dt.get_day();
          std::cout << '/';
          std::cout << dt.get_year();
      }
  
      int main()
      {
          Date birthday; // instantiate object of type "Date"
          birthday.set_date( 2, 8, 1999 ); // member function calls
          std::cout << birthday;
          return 0;
      }
  
  

   

13. Conclusion

• C++ program is a set of:

  • variable definitions and

  • functions

• Finally, another look at Hello, world! program:

  
      #include <iostream>
      int main()
      {
          // Statement using operator notation:
          std::cout << "Hello, World!";
  
          // Statement using function call:
          operator<<( std::cout, "Hello, World!" );
  
          return 0;
      }