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Uploaded byPranali Chaudhari
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Exception handling

Exceptions are runtime errors that a program may encounter. There are two types: synchronous from faults in input data, and asynchronous from external events. Exception handling uses try, throw, and catch keywords. Code that may cause exceptions is placed in a try block. When an exception occurs it is thrown, and the catch block handles it to prevent program termination. Multiple catch blocks can handle different exception types, and a catch-all block uses ellipses to catch any exception.

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EXCEPTION HANDLING Pranali P Chaudhari
Contents  Errors and Exception  Exception Handling Mechanism  Try, Throw and Catch  Re-throwing an Exception  Specifying Exceptions
Quiz 1  What is an error?  An error is a term used to describe any issue that arises unexpectedly and results in incorrect output.  What are the different types of errors?  Logical error:  Occur due to poor understanding of problem or solution procedure.  Syntactic error:  Arise due to poor understanding of the language itself.  What is an exception?  Exceptions are run time anomalies or unusual conditions that a program may encounter while executing.
Exception Handling  Exceptions are of two types:  Synchronous exceptions  The exceptions which occur during the program execution due to some fault in the input data are known as synchronous exceptions.  For example: errors such as out of range, overflow, underflow.  Asynchronous exceptions.  The exceptions caused by events or faults unrelated (external) to the program and beyond the control of the program are called asynchronous exceptions.  For example: errors such as keyboard interrupts, hardware malfunctions, disk failure.
Exception Handling Mechanism  Exception handling mechanism provides a means to detect and report an exception circumstances.  Find the problem (Hit the exception)  Inform that an error has occurred (Throw the exception)  Receive the error information (Catch the exception)  Take corrective actions (Handle the exception)  The error handling consists of two segments
Exception Handling Mechanism  The exception handling mechanism is built upon three keywords:  Try  Is used to preface a block of statements which may generate exceptions.  Throw  When an exception is detected, it is thrown using a throw statement in the try block.  Catch  A catch block defined by the keyword catch catches the exception thrown by the throw statement in the try block and handles it appropriately.
Exception Handling Mechanism  When the try block throws an exception the program control leaves the try block and enters the catch statement of the catch block.  If the type of object thrown matches the arg type in the catch statment the catch block is executed.  Otherwise the program is terminated with the help of abort( ) function.
Try block throwing an exception int main() { int a,b; cout<<“enter the values of a and b :”; cin>>a; cin>>b; int x = a-b; try { if(x != 0) { cout<<“Result (a/x) =“ << a/x; } else { throw(x); } } catch(int i) { cout<<“Exception Caught : x = “ << x << “n”; } return 0; }
Exceptions thrown by functions  Mostly exceptions are thrown by functions that are invoked from within the try blocks.  The point at which the throw is executed is called the throw point.
Exceptions thrown by functions void divide(int x, int y, int z) { if((x-y) != 0) { int R = z/(x-y); cout << “Result = “ << R << “n”; } else { throw (x-y); } }
Exceptions thrown by functions int main() { try { divide(10,20,30); divide(10,10,20); } catch(int i) { cout << “n Exception caught” ; } return 0; }
Throwing Mechanism  When an exception is desired to be handled is detected, it is thrown using the throw statement.  Throw statement has one of the following forms:  throw(exception);  throw exception;  throw;  The operand object exception may be of any type, including constants.
Catching Mechanism  A catch block looks like a function definition: catch(type arg) { // statements for managing exceptions. }  The type indicates the type of exception that catch block handles.  The catch statement catches an exception whose type matches with the type of catch argument.
Multiple Catch Statements  Multiple catch statements can be associated with a try block.  When an exception is thrown, the exception handlers are searched for an appropriate match.  The first handler that yields the match is executed.  After executing the handler, the controls goes to the first statement after the last catch block for that try.
Multiple Catch Statements void test(int x) { try { if (x==1) throw x; else if(x==0) throw ‘x’; else if(x== -1) throw 1.0; cout<<“nEnd of try-block”; } } catch(char c) // catch 1 { cout<<“nCaught a character”; } catch(int m) // catch 2 { cout<<“nCaught an integer”; } catch(double d) // catch 3 { cout<<“nCaught a double”; } cout<<“n End of try-catch block”;
Multiple Catch Statements int main( ) { cout<<“n x = =1”; test(1); cout<<“n x = = 0”; test(0); cout<<“n x = = -1”; test(-1); cout<<“n x = =2”; test(2); return 0; } x == 1 Caught an integer End of try-catch system x == 0 Caught a character End of try-catch system x == -1 Caught a double End of try-catch system x == 2 End of try-block End of try-catch system
Catch all Exceptions  Sometimes it is not possible to anticipate all possible types of exceptions and therefore not able to design independent catch handlers to catch them.  A catch statement can also force to catch all exceptions instead of a certain type alone.  Syntax: catch (…) { // statements for processing all exceptions. }
Catch all Exceptions void test(int x) { try { if (x==1) throw x; else if(x==0) throw ‘x’; else if(x== -1) throw 1.0; cout<<“nEnd of try- block”; } } catch(…) { cout<<“n Caught an exception”; int main( ) { cout<<“nTesting generic catch”; test(1); test(0); test(-1); test(2); return 0; }
Re-throwing an Exception  A handler can re-throw the exception caught without processing it.  This can be done using throw without any arguments.  Here the current exception is thrown to the next enclosing try/catch block.  Every time when an exception is re-thrown it will not be caught by the same catch statements rather it will be caught by the catch statements outside the try catch block.
Re-throwing an Exception void divide(double x, double y) { cout<<“Inside Function”; try { if(y = =0.0) throw y; else cout<<“Division = “ <<x/y<<“n”; } catch(double) { cout<<“nCaught double inside function”; throw; } cout<<“n End of function”; } int main() { cout<<“n Inside main”; try { divide(10.5, 2.0); divide(20.0, 0.0); } catch(double) { cout<<“n Caught double inside main”; } cout<<“n End of main”; return 0; }
Specifying Exceptions  It is possible to restrict a function to throw only certain specified exceptions.  This is done by adding a throw list clause to the function definition. type function(arg-list) throw (type-list) { ....... ....... }  The type-list specifies the type of exceptions that may be thrown.  Throwing other type of exceptions cause abnormal termination of program.
Specifying Exceptions void test(int x) throw (int, double) { if (x==0) throw ‘x’; else if(x==1) throw x; else if(x== -1) throw 1.0; cout<<“n End of function block”; } int main( ) { try { cout<<“nTesting throw restrictions”; cout<<“n x==0”; test(0); cout<<“n x==1”; test(1); cout<<“n x== -1”; test(-1); cout<<“n x== 2”; test(2); } Catch(char c) { cout<<“n Caught a character”; } Catch(int m) { cout<<“n Caught a integer”; } Catch(double d) { cout<<“n Caught a double”; } Cout<<“n End of try catch block”; return 0;
Summary  ______ are peculiar problems that a program may encounter at run time.  Exceptions are of two types _____ and ______.  An exception is caused by a faulty statement in ____ block, which is caught by _____ block.  We can place two or more catch blocks to catch and handle multiple types of exceptions. (True/ False).  It is also possible to make a catch statement to catch all types of exception. (True/ False)  We cannot restrict a function to throw a specified exceptions. (True /
Short Answer Questions  What is an exception?  Exceptions are run time anomalies or unusual conditions that a program may encounter while executing.  How is exception handled in C++?  In C++ the exception is handled using the three keywords try, throw and catch. Or try-catch mechanism.  What are the advantages of using exception handling mechanism in a program?  The purpose of exception handling mechanism is to provide a means to detect and report an exceptional circumstances so that appropriate action can be taken and prevent abnormal termination of program.
Short Answer Questions  When should a program throw an exception?  There are some situation when a program come across unexpected errors and cause abnormal termination of program. To handle such errors and prevent program from termination exceptions are thrown and handled.  What should be placed inside the try block?  The statement that may generate an exception are placed in the try block.  When do we use multiple catch handlers?  Multiple catch handlers are used in a situation where a program has more than one condition to throw and exception.
Short Answer Questions  Explain under what circumstances the following statements would be used:  throw;  Re-throwing an exception.  void fun1(float x) throw()  Prevent a function from throwing any exception.  catch( ... )  Used to catch all types of exceptions.
References  Object Oriented Programming with C++ by E. Balagurusamy.
END OF UNIT ....