C++ Primer Plus

Let’s trace through the events in the sample run that occur after the values 10 and -10 are passed to the hmean() function. The if test causes hmean() to throw an exception. This terminates execution of hmean(). Searching back, the program determines that hmean() was called from within a try block in main(). It then looks for a catch block with a type that matches the exception type. The single catch block present has a char * parameter, so it does match. Detecting the match, the program assigns the string "bad hmean() arguments: a = -b not allowed" to the variable s. Next, the program executes the code in the handler. First, it prints s, which is the caught exception. Then it prints instructions to the user to enter new data. Finally, it executes a continue statement, which causes the program to skip the rest of the while loop and jump to its beginning again. The fact that the continue statement takes the program to the beginning of the loop illustrates the fact that handler statements are part of the loop and that the catch line acts like a label directing program flow (see Figure 15.2).

Figure 15.2. Program flow with exceptions.

You might wonder what happens if a function throws an exception and there’s no try block or else no matching handler. By default, the program eventually calls the abort() function, but you can modify that behavior. We’ll return to this topic later in this chapter.

Using Objects as Exceptions

Typically, functions that throw exceptions throw objects. One important advantage of this is that you can use different exception types to distinguish among different functions and situations that produce exceptions. Also an object can carry information with it, and you can use this information to help identify the conditions that caused the exception to be thrown. Also in principle a catch block could use that information to decide which course of action to pursue. Here, for example, is one possible design for an exception to be thrown by the hmean() function:

class bad_hmean

{

private:

    double v1;

    double v2;

public:

    bad_hmean(int a = 0, int b = 0) : v1(a), v2(b){}

    void mesg();

};

inline void bad_hmean::mesg()

{

    std::cout << "hmean(" << v1 << ", " << v2 <<"): "

              << "invalid arguments: a = -b\n";

}

A bad_hmean object can be initialized to the values passed to hmean(), and the mesg() method can be used to report the problem, including the values. The hmean() function can use code like this:

if (a == -b)

    throw bad_hmean(a,b);

This calls the bad_hmean() constructor, initializing the object to hold the argument values.

Listings 15.10 and 15.11 add a second exception class, bad_gmean, and a second function, called gmean(), that throws a bad_gmean exception. The gmean() function calculates the geometric mean of two numbers, which is the square root of their product. This function is defined if both arguments are non-negative, so it throws an exception if it detects negative arguments. Listing 15.10 is a header file that holds the exception class definitions, and Listing 15.11 is a sample program that uses that header file. Note that the try block is followed by two consecutive catch blocks:

try {                  // start of try block

    ...

}// end of try block

catch (bad_hmean & bg)    // start of catch block

{

   ...

}

catch (bad_gmean & hg)

{

   ...

} // end of catch block

If, say, hmean() throws a bad_hmean exception, the first catch block catches it. If gmean() throws a bad_gmean exception, the exception falls through the first catch block and gets caught by the second.

Listing 15.10. exc_mean.h

// exc_mean.h  -- exception classes for hmean(), gmean()

#include

class bad_hmean

{

private:

    double v1;

    double v2;

public:

    bad_hmean(double a = 0, double b = 0) : v1(a), v2(b){}

    void mesg();

};

inline void bad_hmean::mesg()

{

    std::cout << "hmean(" << v1 << ", " << v2 <<"): "

              << "invalid arguments: a = -b\n";

}

class bad_gmean

{

public:

    double v1;

    double v2;

    bad_gmean(double a = 0, double b = 0) : v1(a), v2(b){}

    const char * mesg();

};

inline const char * bad_gmean::mesg()

{

    return "gmean() arguments should be >= 0\n";

}

Listing 15.11. error4.cpp

//error4.cpp – using exception classes

#include

#include // or math.h, unix users may need -lm flag

#include "exc_mean.h"

// function prototypes

double hmean(double a, double b);

double gmean(double a, double b);

int main()

{

    using std::cout;

    using std::cin;

    using std::endl;

    double x, y, z;

    cout << "Enter two numbers: ";

    while (cin >> x >> y)

    {

        try {                  // start of try block

            z = hmean(x,y);

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

                << " is " << z << endl;

            cout << "Geometric mean of " << x << " and " << y

                << " is " << gmean(x,y) << endl;

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

        }// end of try block

        catch (bad_hmean & bg)    // start of catch block

        {

            bg.mesg();

            cout << "Try again.\n";

            continue;

        }

        catch (bad_gmean & hg)

        {

            cout << hg.mesg();

            cout << "Values used: " << hg.v1 << ", "