EXCEPTION HANDLING AND EVENT HANDLING

Exception handling is a special processing that may required after detection of an exception. Exception handling code unit is called an exception handler. There are many program that allow program to trap input/output errors(including EOF). An exception is any usual event, either erroneous or not, detectable by either hardware or software, that may require special processing.

In a language without exception handling, when an exception occurs, control goes to the operating system, where a message is displayed and the program is terminated.

In a language with exception handling, program are allowed to trap some exceptions, thereby providing the possibility of fixing the problem and continuing.

Advantages of built-in exception handling :

• Error detection code is tedious to write and it clutters the program
• Exception handling encourages programmers to consider many different possible errors
• Exception propagation allows a high level of reuse of exception handling code

Exception handling control flow

C++ exception handlers,

• Exception Handlers Form:

               try {

— code that is expected to raise an exception

}

catch (formal parameter) {

— handler code

}

…

catch (formal parameter) {

— handler code

}

Catch function,

• catch is the name of all handlers–it is an overloaded name, so the formal parameter of each must be unique
• The formal parameter need not have a variable,

It can be simply a type name to distinguish the handler it is in from others

• The formal parameter can be used to transfer information to the handler
• The formal parameter can be an ellipsis, in which case it handles all exceptions not yet handled

Throwing exceptions,

• Exceptions are all raised explicitly by the statement:

               throw [expression];

• The brackets are metasymbols
• A throw without an operand can only appear in a handler; when it appears, it simply re-raises the exception, which is then handled elsewhere
• The type of the expression disambiguates the intended handler

Unhandled exception,

• An unhandled exception is propagated to the caller of the function in which it is raised
• This propagation continues to the main function
• If no handler is found, the default handler is called

Event handling, an event is a notification that something specific has occurred, such as a mouse click on a graphical button. Event handler is a segment of code that is executed in response to an event.