C++ Primer Plus

Listing 15.6 offers a short program for testing the new class.

Listing 15.6. nested.cpp

// nested.cpp -- using a queue that has a nested class

#include

#include

#include "queuetp.h"

int main()

{

    using std::string;

    using std::cin;

    using std::cout;

    QueueTP cs(5);

    string temp;

    while(!cs.isfull())

    {

        cout << "Please enter your name. You will be "

                "served in the order of arrival.\n"

                "name: ";

        getline(cin, temp);

        cs.enqueue(temp);

    }

    cout << "The queue is full. Processing begins!\n";

    while (!cs.isempty())

    {

        cs.dequeue(temp);

        cout << "Now processing " << temp << "...\n";

    }

    return 0;

}

Here is a sample run of the program in Listings 15.5 and 15.6:

Please enter your name. You will be served in the order of arrival.

name: Kinsey Millhone

Please enter your name. You will be served in the order of arrival.

name: Adam Dalgliesh

Please enter your name. You will be served in the order of arrival.

name: Andrew Dalziel

Please enter your name. You will be served in the order of arrival.

name: Kay Scarpetta

Please enter your name. You will be served in the order of arrival.

name: Richard Jury

The queue is full. Processing begins!

Now processing Kinsey Millhone...

Now processing Adam Dalgliesh...

Now processing Andrew Dalziel...

Now processing Kay Scarpetta...

Now processing Richard Jury...

Exceptions

Programs sometimes encounter runtime problems that prevent them from continuing normally. For example, a program may try to open an unavailable file, or it may request more memory than is available, or it may encounter values it cannot abide. Usually, programmers try to anticipate such calamities. C++ exceptions provide a powerful and flexible tool for dealing with these situations. Exceptions are a relatively recent addition to C++, so some older compilers haven’t implemented them. Also some compilers turn this feature off by default, so you may have to use the compiler options to turn it on.

Before examining exceptions, let’s look at some of the more rudimentary options available to programmers. As a test case, let’s look at a function that calculates the harmonic mean of two numbers. The harmonic mean of two numbers is defined as the inverse of the average of the inverses. This can be reduced to the following expression:

2.0 × x × y / (x + y)

Note that if y is the negative of x, this formula results in division by zero, a rather undesirable operation. Many newer compilers handle division by zero by generating a special floating-point value that represents infinity; cout displays this value as Inf, inf, INF, or something similar. Other compilers may produce programs that crash when division by zero occurs. It is best to write code that behaves in the same controlled fashion on all systems.

Calling abort()

One way to handle this is to have the function call the abort() function if one argument is the negative of the other. The abort() function has its prototype in the cstdlib (or stdlib.h) header file. A typical implementation, if called, sends a message such as “abnormal program termination” to the standard error stream (the same as the one used by cerr) and terminates the program. It also returns an implementation-dependent value that indicates failure to the operating system or, if the program was initiated by another program, to the parent process. Whether abort() flushes file buffers (that is, memory areas used to store material for transfers to and from files) depends on the implementation. If you prefer, you can use exit(), which does flush file buffers, but without displaying a message. Listing 15.7 shows a short program that uses abort().

Listing 15.7. error1.cpp

//error1.cpp -- using the abort() function

#include

#include

double hmean(double a, double b);

int main()

{

    double x, y, z;

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

    while (std::cin >> x >> y)

    {

        z = hmean(x,y);

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

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

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

    }

    std::cout << "Bye!\n";

    return 0;

}

double hmean(double a, double b)

{

    if (a == -b)

    {

        std::cout << "untenable arguments to hmean()\n";

        std::abort();

    }

    return 2.0 * a * b / (a + b);

}

Here’s a sample run of the program in Listing 15.7:

Enter two numbers: 3 6

Harmonic mean of 3 and 6 is 4

Enter next set of numbers : 10 -10

untenable arguments to hmean()

abnormal program termination

Note that calling the abort() function from hmean() terminates the program directly, without returning first to main(). In general, different compilers issue different abort messages. Here is another such message:

This application has requested the Runtime to terminate it

in an unusual way. Please contact the application's support

team for more information.