preincrement and postincrement operator overloading problem

[Mathsniper]

#include <iostream>

using namespace std;

class Date {
	friend ostream &operator<<( ostream &, const Date & ); 
public:
	Date( int m = 1, int d = 1, int y = 1);
	void setDate( int, int, int );
	Date &operator++();
	Date operator++( int );
	const Date &operator+=( int );
	bool leapYear( int );
	bool endOfMonth( int );
private:
	int day;
	int month;
	int year;

	static const int days[];
	void helpIncrement();
};

const int Date::days[] = { 0, 31, 28, 31, 30, 31, 30,
						   31, 31, 30, 31, 30, 31 };

Date::Date( int m, int d, int y ) { setDate( m, d, y ); }

void Date::setDate( int mm, int dd, int yy )
{
	month = ( mm >= 1 && mm <= 12 ) ? mm : 1;
	year = ( yy >= 1900 && yy <= 2100 ) ? yy : 1900;
	if ( month == 2 && leapYear( year ) )
		day = ( dd >= 1 && dd <= 29 ) ? dd : 1;
	else
		day = ( dd >= 1 && dd <= days[ month ] ) ? dd : 1;
}

Date &Date::operator++()
{
	helpIncrement();
	return *this;
}

Date Date::operator++( int )
{
	Date temp = *this;
	helpIncrement();
	return temp;
}

const Date &Date::operator+=( int additionDays )
{
	for ( int i = 0; i < additionDays; i++ )
		helpIncrement();
	return *this;
}

bool Date::leapYear( int y )
{
	if ( y % 400 == 0 || ( y % 100 != 0 && y % 4 == 0) )
		return true;
	else
		return false;
}

bool Date::endOfMonth( int d )
{
	if ( month == 2 && leapYear( year ) )
		return d == 29;
	else
		return d == days[ month ];
}

void Date::helpIncrement()
{
	if ( endOfMonth( day ) && month == 12 ) {
		day = 1;
		month = 1;
		++year;
	}
	else if ( endOfMonth( day ) ) {
		day = 1;
		++month;
	}
	else
		++day;
}

ostream &operator<<( ostream &output, const Date &d )
{
	static char *monthName[ 13 ] = { "", "January",
		"February", "March", "April", "May", "June",
		"July", "August", "September", "October",
		"November", "December" };
	output << monthName[ d.month ] << ' '
		   << d.day << ", " << d.year;
	return output;
}

int main()
{
	Date d1, d2( 12, 27, 1992 ), d3( 0, 99, 3451 );
	cout << "d1 is " << d1
		 << "\nd2 is " << d2
		 << "\nd3 is " << d3 << "\n\n";
	
	cout << "d1 += 5 is " << ( d1 += 5 ) << "\n\n";

	d3.setDate( 12, 20, 1999 );
	cout << "d3 is " << d3;
	cout << "\n++d3 is " << ++d3 << "\n\n";
	
	Date d4( 4, 16, 2006 );

	cout << "Testing the preincrement operator:\n"
		 << "  d4 is " << d4 << "\n"
		 << "++d4 is " << ++d4 << "\n"
		 << "  d4 is " << d4 << "\n\n";
	
	cout << "Testing the postincrement operator:\n"
		 << "  d4 is " << d4 << "\n"
		 << "d4++ is " << d4++ << "\n"
		 << "  d4 is " << d4 << endl;

	return 0;
}

The output

d1 is January 1, 1900
d2 is December 27, 1992
d3 is January 1, 1900

d1 += 5 is January 6, 1900

d3 is December 20, 1999
++d3 is December 21, 1999

Testing the preincrement operator:
  d4 is April 17, 2006
++d4 is April 17, 2006
  d4 is April 17, 2006

Testing the postincrement operator:
  d4 is April 18, 2006
d4++ is April 17, 2006
  d4 is April 18, 2006

The d4 data is very strange...... What's the problem with it?

[laserlight]

I suggest that you do a:

cout << "Testing the preincrement operator:\n"
	<< "  d4 is " << d4 << "\n"
cout << "++d4 is " << ++d4 << "\n";
cout << "  d4 is " << d4 << "\n\n";

cout << "Testing the postincrement operator:\n"
	<< "  d4 is " << d4 << "\n"
cout << "d4++ is " << d4++ << "\n"
cout << "  d4 is " << d4 << endl;

The problem is that using the increment operators on d4 in the same statement as you print d4 causes d4 to be changed at points that you dont expect.

[Mathsniper]

It means one `cin` or `cout` have to correspond one class object?

[SlyMaelstrom]

No, what you have to take into account, as laserlight was saying is order of precendence in operators. The increment operators take a higher precedence to the left shift operator, and there for, by the time your << operator takes the variable, it's already been incremented.