preincrement and postincrement operator overloading problem

Code:

#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

Code:

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?

I suggest that you do a:

Code:

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.

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

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.