Creating Friends

The first step toward creating a friend function is to place a prototype in the class declaration and prefix the declaration with the keyword friend:

friend Time operator*(double m, const Time & t);  // goes in class declaration

This prototype has two implications:

• Although the operator*() function is declared in the class declaration, it is not a member function. So it isn’t invoked by using the membership operator.

• Although the operator*() function is not a member function, it has the same access rights as a member function.

The second step is to write the function definition. Because it is not a member function, you don’t use the Time:: qualifier. Also you don’t use the friend keyword in the definition. The definition should look like this:

Time operator*(double m, const Time & t)  // friend not used in definition
{
    Time result;
    long totalminutes = t.hours * mult * 60 +t. minutes * mult;
    result.hours = totalminutes / 60;
    result.minutes = totalminutes % 60;
    return result;
}

With this declaration and definition, the statement

A = 2.75 * B;

translates to the following and invokes the nonmember friend function just defined:

A = operator*(2.75, B);

In short, a friend function to a class is a nonmember function that has the same access rights as a member function.

Actually, you can write this particular friend function as a non-friend by altering the definition so that it switches which value comes first in the multiplication:

Time operator*(double m, const Time & t)
{
    return t * m;     // use t.operator*(m)
}

The original version accessed t.minutes and t.hours explicitly, so it had to be a friend. This version only uses the Time object t as a whole, letting a member function handle the private values, so this version doesn’t have to be a friend. Nonetheless, there are reasons to make this version a friend, too. Most importantly, it ties the function in as part of the official class interface. Second, if you later find a need for the function to access private data directly, you only have to change the function definition and not the class prototype.