Chapter 9 // Exercise 10 - Principles & Practice Using C++

In this exercise I am using Visual Studio Community 2017 and the header file "std_lib_facilities.h" which can be found here:

http://www.stroustrup.com/Programming/PPP2code/std_lib_facilities.h

Chapter 9 // Exercise 10

Implement leapyear() from section 9.8.

dateclass.h

//dateclass.h
#ifndef _DATECLASS_H_
#define _DATECLASS_H_

#include "std_lib_facilities.h"

enum class Month
{
 jan = 1, feb, mar, apr, may, jun, jul, aug, sep, oct, nov, dec
};

//forward declaration
Month returnMonth(int month);

//9.8 The Date Class
class Date
{
public:
 Date();
 ~Date();
 Date(int y, Month m, int d);  //check for valid date & initialise

          //non-modifying operations:
 int day() const { return d; };
 Month month() const { return m; }
 int year() const { return y; }

 //modifying operations:
 void add_day(int n);
 void add_month(int n);
 void add_year(int n);

private:
 int y;
 Month m;
 int d;

 bool lastDay = false;
 bool endYear = false;
};

//helper functions
bool is_date(int y, Month m, int d);  //true for valid date
bool leapyear(int y);    //true if y is leap year

ostream& operator<<(ostream& os, Date& d);
istream& operator>>(istream& is, Date& dd);

bool operator==(const Date& a, const Date& b);
bool operator!=(const Date& a, const Date& b);

#endif // !_DATECLASS_H_

dateclass.cpp

//dateclass.cpp

#include "dateclass.h"

//default Date Constructor
Date::Date() 
{
 y = 2001;
 m = Month::jan;
 d = 1;
}

//deconstructor
Date::~Date() {}

//Date Constructor - initialise the day
Date::Date(int y, Month m, int d)
{
 //check that y m d is a valid date
 if (!is_date(y, m, d))
  cout << "Error, invalid date." << endl;
 else
 {
  //if date is valid, initalise the date
  Date::y = y;
  Date::m = m;
  Date::d = d;
 }

 return;
}

//increment date by n days
void Date::add_day(int n)
{
 //if day goes above 31, increase month, set day to 1
 //if month goes above 12, increase year, set month to 1
 for (int i = 0; i < n; ++i)
 {
  //wrap days
  if (Date::d == 31)
   lastDay = true;
  Date::d = (Date::d == 31) ? 1 : ++Date::d;  //if day is equal to 31, make it 1, otherwise ++

  if (lastDay)
  {
   //wrap month
   lastDay = false;
   int mon = (static_cast<int>(Date::m) == 12) ? 1 : (static_cast<int>(Date::m) + 1); //if month is equal to 12, make it 1, otherwise ++
   Date::m = returnMonth(mon);
   if (static_cast<int>(Date::m) == 12)
    endYear = true;

   if (endYear)
   {
    //just increase year by one
    endYear = false;
    ++Date::y;
   }

  }

 }
}

void Date::add_month(int n)
{
 for (int i = 0; i < n; ++i)
 {
  //if month is equal to 12, make it 1, otherwise ++
  int mon = (static_cast<int>(Date::m) == 12) ? 1 : (static_cast<int>(Date::m) + 1);
  Date::m = returnMonth(mon);
  if (static_cast<int>(Date::m) == 12)
   endYear = true;

  if (endYear)
  {
   //just increase year by one
   endYear = false;
   ++Date::y;
  }
 }
}

void Date::add_year(int n)
{
 for (int i = 0; i < n; ++i)
  ++Date::y;
}

//switch to return correct type of Month
Month returnMonth(int month)
{
 switch (month)
 {
 case 1:
  return Month::jan;
  break;
 case 2:
  return Month::feb;
  break;
 case 3:
  return Month::mar;
  break;
 case 4:
  return Month::apr;
  break;
 case 5:
  return Month::may;
  break;
 case 6:
  return Month::jun;
  break;
 case 7:
  return Month::jul;
  break;
 case 8:
  return Month::aug;
  break;
 case 9:
  return Month::sep;
  break;
 case 10:
  return Month::oct;
  break;
 case 11:
  return Month::nov;
  break;
 case 12:
  return Month::dec;
  break;
 default:
  cout << "Bad month" << endl;
 }
}

//true for valid date
bool is_date(int y, Month m, int d)
{
 //check that y is valid
 if (y < 1900 || y > 2018)
 {
  cout << "Error, invalid year." << endl;
  return false;
 }

 //check that m is valid
 if (m < Month::jan || m > Month::dec)
  return false;

 //check that d is valid
 if (d <= 0)
  return false; //d must be positive

 int daysInMonth = 31;  //most months have 31 days

 switch (m)
 {
  case Month::feb:
   //if leapyear, make it 29, if not make it 28
   daysInMonth = (leapyear(y)) ? 29 : 28;
   break;
  case Month::apr: case Month::jun: case Month::sep: case Month::nov:
   daysInMonth = 30; //the rest have 30 days
   break;
 }

 if (daysInMonth < d)
  return false;

 return true;
}

//checks for leap year (gregorian calender)
bool leapyear(int y)
{
 //if divisible by 4 but not 100 - then leap year
 if (y % 4 == 0 && y % 100 != 0)
  return true;
 //if divisible by 4, 100 and 400 - then leap year
 if (y % 4 == 0 && y % 100 == 0 && y % 400 == 0)
  return true;

 else
  return false;
}

//compares dates
bool operator==(const Date& a, const Date& b)
{
 return a.year() == b.year()
  && a.month() == b.month()
  && a.day() == b.day();
}

bool operator!=(const Date& a, const Date& b)
{
 return !(a == b);
}

//print to screen
ostream& operator<<(ostream& os, Date& d)
{
 return os << d.day() << ", " << static_cast<int>(d.month()) << ", " << d.year() << endl;
}

//write into Date
istream& operator>>(istream& is, Date& dd)
{
 int y, m, d;
 char ch1, ch2, ch3, ch4;
 is >> ch1 >> y >> ch2 >> m >> ch3 >> d >> ch4;
 if (!is)
  return is;
 if (ch1 != '(' || ch2 != ',' || ch3 != ',' || ch4 != ')')
 {
  //format error
  is.clear(ios_base::failbit);  //set the failbit
  return is;
 }

 dd = Date(y, Month(m), d);  //update dd
 return is;
}

main.cpp

//main.cpp

#include "std_lib_facilities.h"
#include "dateclass.h"

int main()
{
 //intialise dates
 Date today(2016, Month::feb, 29);
 Date tomorrow(today);

 //increase day by one
 tomorrow.add_day(1);
 tomorrow.add_month(2);
 tomorrow.add_year(2);

 //print out results
 cout << "Today: " << today << endl;
 cout << "Tomorrow: " << tomorrow << endl;

 keep_window_open();

 return 0;
}

At first I wasn't sure how to go about this. However after googling "what determines a leap year" there is some very handy psuedocode on wikipedia to determine whether a year is a leap year or not.

https://en.wikipedia.org/wiki/Leap_year

Turns out a leap year can be determined if the year is divisible by 4 or 100. However centurial years must be divisible by 400 to be a leap year.

Please be aware in this exercise though that the add_day() function still only wraps at 31 days as a later exercise instructs us to create this function so I haven't fixed it yet.