C++ Primer Plus

(You may, perhaps, enjoy the conceit that our little program has a support team.) The program could avoid aborting by checking the values of x and y before calling the hmean() function. However, it’s not safe to rely on a programmer to know (or care) enough to perform such a check.

Returning an Error Code

A more flexible approach than aborting is to use a function’s return value to indicate a problem. For example, the get(void) member of the ostream class ordinarily returns the ASCII code for the next input character, but it returns the special value EOF if it encounters the end-of-file. This approach doesn’t work for hmean(). Any numeric value could be a valid return value, so there’s no special value available to indicate a problem. In this kind of situation, you can use a pointer argument or a reference argument to get a value back to the calling program and use the function return value to indicate success or failure. The istream family of overloaded >> operators uses a variant of this technique. By informing the calling program of the success or failure, you give the program the option of taking actions other than aborting. Listing 15.8 shows an example of this approach. It redefines hmean() as a bool function whose return value indicates success or failure. It adds a third argument for obtaining the answer.

Listing 15.8. error2.cpp

//error2.cpp -- returning an error code

#include

#include   // (or float.h) for DBL_MAX

bool hmean(double a, double b, double * ans);

int main()

{

    double x, y, z;

    std::cout << "Enter two numbers: ";

    while (std::cin >> x >> y)

    {

        if (hmean(x,y,&z))

            std::cout << "Harmonic mean of " << x << " and " << y

                << " is " << z << std::endl;

        else

            std::cout << "One value should not be the negative "

                << "of the other - try again.\n";

        std::cout << "Enter next set of numbers : ";

    }

    std::cout << "Bye!\n";

    return 0;

}

bool hmean(double a, double b, double * ans)

{

    if (a == -b)

    {

        *ans = DBL_MAX;

        return false;

    }

    else

    {

        *ans = 2.0 * a * b / (a + b);

        return true;

    }

}

Here’s a sample run of the program in Listing 15.8:

Enter two numbers: 3 6

Harmonic mean of 3 and 6 is 4

Enter next set of numbers : 10 -10

One value should not be the negative of the other - try again.

Enter next set of numbers : 1 19

Harmonic mean of 1 and 19 is 1.9

Enter next set of numbers : q

Bye!

Program Notes

In Listing 15.8, the program design allows the user to continue, bypassing the effects of bad input. Of course, the design does rely on the user to check the function return value, something that programmers don’t always do. For example, to keep the sample programs short, the listings in this book don’t check to see if cout was successful in handling output.

You could use either a pointer or a reference for the third arguments. Many programmers prefer using pointers for arguments of the built-in types because that makes it obvious which argument is being used for the answer.

Another variation on the idea of storing a return condition somewhere is to use a global variable. The function with the potential problem could set the global variable to a particular value if things go wrong, and the calling program could check the variable. This is the method used by the traditional C math library, which uses the global variable errno for this purpose. You have to make sure, of course, that some other function doesn’t try to use a global variable of the same name for some other purpose.

The Exception Mechanism

Now let’s see how you can handle problems by using the exception mechanism. A C++ exception is a response to an exceptional circumstance that arises while a program is running, such as an attempt to divide by zero. Exceptions provide a way to transfer control from one part of a program to another. Handling an exception has three components:

• Throwing an exception

• Catching an exception with a handler

• Using a try block

A program throws an exception when a problem shows up. For example, you can modify hmean() in Listing 15.7 to throw an exception instead of call the abort() function. A throw statement, in essence, is a jump; that is, it tells a program to jump to statements at another location. The throw keyword indicates the throwing of an exception. It’s followed by a value, such as a character string or an object, that indicates the nature of the exception.