Exception Handling Basics in Software

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Exception Handling Basics
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Display 185 Throwing an Exception Inside a Function (part 2 of 2)
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Enter numerator: 5 Enter denominator: 10 5/10 = 05 End of Program
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Enter numerator: 5 Enter denominator: 0 Error: Division by zero! Program aborting
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The meaning of throw (DivideByZero) in the declaration of safeDivide is discussed in the next subsection
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s EXCEPTION SPECIFICATION
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If a function does not catch an exception, it should at least warn programmers that any invocation of the function might possibly throw an exception If there are exceptions that might be thrown but not caught in the function de nition, those exception types should be listed in an exception speci cation, which is illustrated by the following exception specification function declaration from Display 185:
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double safeDivide(int top, int bottom) throw (DivideByZero);
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As illustrated in Display 185, the exception speci cation should appear in both the function declaration and the function de nition If a function has more than one function declaration, then all the function declarations must have identical exception speci cations The exception speci cation for a function is also sometimes called the throw throw list list If more than one possible exception can be thrown in the function de nition, the exception types are listed separated by commas, as illustrated in what follows:
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void someFunction( ) throw (DivideByZero, SomeOtherException);
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Exception Handling
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All exception types listed in the exception speci cation are treated normally When we say the exception is treated normally we mean it is treated as we have described before this subsection In particular, you can place the function invocation in a try block followed by a catch block to catch that type of exception and if the function throws the exception (and does not catch it inside the function), then the catch block following the try block will catch the exception If there is no exception speci cation (no throw list) at all (not even an empty one), then the code behaves the same as if all possible exception types were listed in the exception speci cation; that is, any exception that is thrown is treated normally What happens when an exception is thrown in a function but is not listed in the exception speci cation (and not caught inside the function) This is neither a compile time error nor a runtime error In such cases the function unexpected( ) is called You can change the behavior of the function unexpected, but the default behavior is to call the function terminate( ), which ends the program In particular, notice that if an exception is thrown in a function but is not listed in the exception speci cation (and not caught inside the function), then it will not be caught by any catch block in your program but will instead result in an invocation of unexpected( ) whose default behavior is to end your program Keep in mind that the exception speci cation is for exceptions that get outside the function If they do not get outside the function, they do not belong in the exception speci cation If they get outside the function, they belong in the exception speci cation no matter where they originate If an exception is thrown in a try block that is inside a function de nition and is caught in a catch block inside the function de nition, then its type need not be listed in the exception speci cation If a function de nition includes an invocation of another function and that other function can throw an exception that is not caught, then the type of the exception should be placed in the exception speci cation You might think that the possibility of throwing an exception that is not caught and is not on the throw list should be checked by the compiler and produce a compiler error However, because of the details of exceptions in C++, it is not possible for the compiler to perform the check The check must be done at runtime1 To say that a function should not throw any exceptions that are not caught inside the function, use an empty exception speci cation like so:
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void someFunction( ) throw ( );
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By way of summary:
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void someFunction( ) throw (DivideByZero, SomeOtherException); //Exceptions of type DivideByZero or SomeOtherException are //treated normally All other exceptions invoke unexpected( )
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This is not true in all programming languages It depends on the details of how the exception speci cation details are de ned for the language
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