Flow of Program Control When the User Enters a Denominator of Zero

Now consider the case in which the user inputs the numerator 100 and the denominator 0. In line 13, quotient determines that the denominator is zero, which indicates an attempt to divide by zero. Line 14 throws an exception, which we represent as an object of class DivideByZeroException (Fig. 17.1).

To throw an exception, line 14 in Fig. 17.2 uses keyword throw followed by an operand of the type of exception to throw. Normally, a throw statement specifies one operand. (In Section 17.3, we discuss how to use a throw statement with no operand.) The operand of a throw can be of any type (but it must be copy constructable). If the operand is an object, we call it an exception object—in this example, the exception object is of type DivideByZeroException. However, a throw operand also can assume other values, such as the value of an expression that does not result in an object of a class (e.g., throw x > 5) or the value of an int (e.g., throw 5). The examples in this chapter focus exclusively on throwing objects of exception classes.

As part of throwing an exception, the throw operand is created and used to initialize the parameter in the catch handler, which we discuss momentarily. The throw statement in line 14 creates a DivideByZeroException object. When line 14 throws the exception, function quotient exits immediately. So, line 14 throws the exception before function quotient can perform the division in line 17. This is a central characteristic of exception handling: If your program explicitly throws an exception, it should do so before the error has an opportunity to occur.

Because we enclosed the call to quotient (line 34) in a try block, program control enters the catch handler (lines 37–41) that immediately follows the try block. This catch handler serves as the exception handler for the divide-by-zero exception. In general, when an exception is thrown within a try block, the exception is caught by a catch handler that specifies the type matching the thrown exception. In this program, the catch handler specifies that it catches DivideByZeroException objects—this type matches the object type thrown in function quotient. Actually, the catch handler catches a reference to the DivideByZeroException object created by function quotient’s throw statement (line 14), so that the catch handler does not make a copy of the exception object.

The catch’s body (lines 39–40) prints the error message returned by function what of base-class runtime_error—i.e., the string that the DivideByZeroException constructor (lines 11–12 in Fig. 17.1) passed to the runtime_error base-class constructor.