A date::prev member function

From ljl8:

void date::prev()
{
	if (day == 1) {
		if (month == 1) {
			--year;
			month = 12;
			day = length[month];
		} else {
			--month;
			day = length[month];
		}
	} else {
		--day;
	}
}

void date::prev()
{
	if (day > 1) {
		--day;
	} else {
		if (month > 1) {
			--month;
		} else {
			month = 12;
			--year;
		}
		day = length[month];
	}
}

Only need to write day = length[month]; once, if you write it in the right place:

void date::prev()
{
	if (day == 1) {
		if (month == 1) {
			--year;
			month = 12;
		} else {
			--month;
		}
		day = length[month];
	} else {
		--day;
	}
}

Move the else closer to the corresponding if

void date::prev()
{
	if (day == 1) {
		if (month == 1) {
			--year;
			month = 12;
		} else {
			--month;
		}
		day = length[month];
	} else {
		--day;
	}
}

An if/else consists of two parts.
Write the smaller part (--day;) on top, to decrease the distance between the if and its else.
I did the same thing to the other if/else too.

void date::prev()
{
	if (day > 1) {
		--day;
	} else {
		if (month > 1) {
			--month;
		} else {
			month = 12;
			--year;
		}
		day = length[month];
	}
}

Don’t repeat the data in the array

The number 31 should appear only in the array, not in the code.
From an151:

void date::prev() // Move this date object into the past
{
	if (day > 1) {
		--day;
	} else {

		if (month > 1) {
			--month;
			day = length[month]; // last day of previous month
		} else {
			month = 12;
			day = 31; // dec 31
			--year;
		}
	}
}

void date::prev() // Move this date object into the past
{
	if (day > 1) {
		--day;
	} else {
		if (month > 1) {
			--month;
			day = length[month]; // last day of previous month
		} else {
			month = 12;
			--year;
			day = length[month]; // last day of previous month
		}
	}
}

Then remove the duplication as in the first example.

Don’t write the 12 either.

In all of the above examples, we should call the static member function date::monthsInYear (exercise 4i) that returns the number of months in the static data member date::length:

void date::prev() // Move this date object into the past
{
	if (day > 1) {
		--day;
	} else {
		if (month > 1) {
			--month;
		} else {
			month = monthsInYear();
			--year;
		}
		day = length[month]; // last day of previous month
	}
}