Exceptions. Text: Chapter19, Big C++

Transcription

1 Exceptions Text: Chapter19, Big C++

2 What are exceptions? In functions/methods, you write a sequence of statements to perform some task, and statements are executed in that sequence Some expected/un-expected errors might occur while execution, and because of which execution can not proceed further, and are called exceptions possibly because, these are exceptions to main thread of execution

3 What are exceptions? Exceptions are typically logical or run-time errors Both either cause program to malfunction or crash leading to undesirable behavior of a program... may prove to be fatal.. may be corrupting result, data files or so.

4 Types of Errors User input errors Device errors Physical limitations Software Component failures

5 Exception handling You should provide alternate path of execution for exceptions in your programs. called Exception Handling Importance becomes high as complexity of systems increases Though normally ignored in class-room exercises, exceptional handling is a critical and equally important component in any real life application

6 Exception handling Exceptions need to be handled properly Detect the cause of problem If possible give user a chance to correct it and continue execution Or terminate execution with proper reason of not able to continue

7 What run-time error can occur in following program? Program crashes if b is zero Crashing is not good thing, probably program may terminate after displaying appropriate message, or Still better, if user is prompted again for input saying that inputted value is zero and c can not be computed. ex01a.cpp

8 This is probably better.. ex01b.cpp

9 Computing got bit complex.... and you moved computing of c in a function

10 Computing got bit complex.... and you moved computing of c in a function ex01c.cpp

11 This exception handling has some concerns Though, traditionally, if has been the only way of handling exception, but It is quiet limited, makes codes lengthy and difficult to read C++ provides sophisticated exception handling mechanism

12 Conceptual view of exception handling in C++

13 It goes like this try (out) something, if this exceptional case happens Do this Otherwise if that exceptional case happens Do this And so on

14 It might take a while to appreciate elegance of code below, but should give an idea how it is done ex01d.cpp

15 It might take a while to appreciate elegance of code below, but should give an idea how it is done ex01d.cpp

16 Program still might have crashed, if anything else goes wrong, by providing default catch catches all exceptions

17 try throw - catch Are three magical words in exceptional handling Semantics understanding of this is all important

18 General syntax of try-catch block try { //statements } catch (datatype1 identifier) { //exception handling code } catch (datatype2 identifier) { //exception handling code } catch (...) { //exception handling code }

19 How does it works? Any of the statement or function call in try block throws an exception it can not handle the situation and surrenders Throwing an exception is throwing an object of a data type Thrown object is caught by some catch block catch with the matching data type is catches the object.. works as handler of the situation exception handler

20 try-catch block If no exception is thrown in a try block All catch blocks for that try block are ignored Execution resumes after the last catch block If an exception is thrown in a try block Remaining statements in that try block are ignored

21 try-catch block The program searches catch blocks in order of their occurrences, looking for an appropriate exception handler If the type of thrown exception matches the parameter type in one of the catch blocks: Code of that catch block executes Remaining catch blocks are ignored A catch block with an ellipse (three dots) catches any type of exception

22 try-catch block Consider the following catch block: catch (int x) { //exception handling } In this catch block: The identifier x acts as parameter. In fact, it is called a catch parameter The data type int specifies that this catch block can catch an exception of type int. A catch block can have atmost one catch block parameter

23 Order of catch blocks When an exception is thrown, matching catch block is executed Order of matching is done from top to bottom Recall that ellipse ( ) catches any exception, therefore if needed, need to be placed at the end! No type conversion is done while matching catch, not even int to long, however Object of derived class matches with catch with base class parameter

24 Which catch would be caught? Ex_03.cpp

25 Which catch would be caught?

26 Thanks