More Overloaded Operators
Some other operations make sense for the Time class. For example, you might want to subtract one time from another or multiply a time by a factor. This suggests overloading the subtraction and multiplication operators. The technique is the same as for the addition operator: you create operator-() and operator*() methods. That is, you add the following prototypes to the class declaration:
Time operator-(const Time & t) const;
Time operator*(double n) const;
Listing 11.7 shows the new header file.
// mytime2.h -- Time class after operator overloading
#ifndef MYTIME2_H_
#define MYTIME2_H_
class Time
{
private:
int hours;
int minutes;
public:
Time();
Time(int h, int m = 0);
void AddMin(int m);
void AddHr(int h);
void Reset(int h = 0, int m = 0);
Time operator+(const Time & t) const;
Time operator-(const Time & t) const;
Time operator*(double n) const;
void Show() const;
};
#endif
Then you add definitions for the new methods to the implementation file, as shown in Listing 11.8.
// mytime2.cpp -- implementing Time methods
#include <iostream>
#include "mytime2.h"
Time::Time()
{
hours = minutes = 0;
}
Time::Time(int h, int m )
{
hours = h;
minutes = m;
}
void Time::AddMin(int m)
{
minutes += m;
hours += minutes / 60;
minutes %= 60;
}
void Time::AddHr(int h)
{
hours += h;
}
void Time::Reset(int h, int m)
{
hours = h;
minutes = m;
}
Time Time::operator+(const Time & t) const
{
Time sum;
sum.minutes = minutes + t.minutes;
sum.hours = hours + t.hours + sum.minutes / 60;
sum.minutes %= 60;
return sum;
}
Time Time::operator-(const Time & t) const
{
Time diff;
int tot1, tot2;
tot1 = t.minutes + 60 * t.hours;
tot2 = minutes + 60 * hours;
diff.minutes = (tot2 - tot1) % 60;
diff.hours = (tot2 - tot1) / 60;
return diff;
}
Time Time::operator*(double mult) const
{
Time result;
long totalminutes = hours * mult * 60 + minutes * mult;
result.hours = totalminutes / 60;
result.minutes = totalminutes % 60;
return result;
}
void Time::Show() const
{
std::cout << hours << " hours, " << minutes << " minutes";
}
With these changes made, you can test the new definitions with the code shown in Listing 11.9.
// usetime2.cpp -- using the third draft of the Time class
// compile usetime2.cpp and mytime2.cpp together
#include <iostream>
#include "mytime2.h"
int main()
{
using std::cout;
using std::endl;
Time weeding(4, 35);
Time waxing(2, 47);
Time total;
Time diff;
Time adjusted;
cout << "weeding time = ";
weeding.Show();
cout << endl;
cout << "waxing time = ";
waxing.Show();
cout << endl;
cout << "total work time = ";
total = weeding + waxing; // use operator+()
total.Show();
cout << endl;
diff = weeding - waxing; // use operator-()
cout << "weeding time - waxing time = ";
diff.Show();
cout << endl;
adjusted = total * 1.5; // use operator+()
cout << "adjusted work time = ";
adjusted.Show();
cout << endl;
return 0;
}
Here is the output of the program in Listings 11.7, 11.8, and 11.9:
weeding time = 4 hours, 35 minutes
waxing time = 2 hours, 47 minutes
total work time = 7 hours, 22 minutes
weeding time - waxing time = 1 hours, 48 minutes
adjusted work time = 11 hours, 3 minutes
Introducing Friends
As you’ve seen, C++ controls access to the private portions of a class object. Usually, public class methods serve as the only access, but sometimes this restriction is too rigid to fit particular programming problems. In such cases, C++ provides another form of access: the friend. Friends come in three varieties:
• Friend functions
• Friend classes
• Friend member functions
By making a function a friend to a class, you allow the function the same access privileges that a member function of the class has. We’ll look into friend functions now, leaving the other two varieties to Chapter 15, “Friends, Exceptions, and More.”
Before seeing how to make friends, let’s look into why they might be needed. Often, overloading a binary operator (that is, an operator with two arguments) for a class generates a need for friends. Multiplying a Time object by a real number provides just such a situation, so let’s study that case.
In the previous Time class example, the overloaded multiplication operator is different from the other two overloaded operators in that it combines two different types. That is, the addition and subtraction operators each combine two Time values, but the multiplication operator combines a Time value with a double value. This restricts how the operator can be used. Remember, the left operand is the invoking object. That is,
A = B * 2.75;
translates to the following member function call:
A = B.operator*(2.75);
But what about the following statement?
A = 2.75 * B; // cannot correspond to a member function
Conceptually, 2.75 * B should be the same as B * 2.75, but the first expression cannot correspond to a member function because 2.75 is not a type Time object. Remember, the left operand is the invoking object, but 2.75 is not an object. So the compiler cannot replace the expression with a member function call.
One way around this difficulty is to tell everyone (and to remember yourself) that you can only write B * 2.75 but never write 2.75 * B. This is a server-friendly, client-beware solution, and that’s not what OOP is about.
However, there is another possibility—using a nonmember function. (Remember, most operators can be overloaded using either member or nonmember functions.) A nonmember function is not invoked by an object; instead, any values it uses, including objects, are explicit arguments. Thus, the compiler could match the expression
A = 2.75 * B; // cannot correspond to a member function
to the following nonmember function call:
A = operator*(2.75, B);
The function would have this prototype:
Time operator*(double m, const Time & t);
With the nonmember overloaded operator function, the left operand of an operator expression corresponds to the first argument of the operator function, and the right operand corresponds to the second argument. Meanwhile, the original member function handles operands in the opposite order—that is, a Time value multiplied by a double value.
Using a nonmember function solves the problem of getting the operands in the desired order (first double and then Time), but it raises a new problem: Nonmember functions can’t directly access private data in a class. Well, at least ordinary nonmember functions lack access. But there is a special category of nonmember functions, called friends, that can access private members of a class.