Rossella Lau Lecture 9, DCO10105, Semester B, DCO10105 Object-Oriented Programming and Design  Lecture 9: Application with Exception Handling  Popular usage of polymorphism in applications  Exception handling -- By Rossella Lau

Rossella Lau Lecture 9, DCO10105, Semester B,  Polymorphism – many types  Same classes can play as different types (of ancestors) Static binding with pointer  Same expressions can perform different operations Dynamic binding with pointer and virtual function Functions of sub-class can override the function with identical signature in the base class Samples in bindingPlayer3.cpp : donald->print() can invoke print() in Donald and print() in Dewey according to the actual object’s type Polymorphism

Rossella Lau Lecture 9, DCO10105, Semester B, Popular usage of polymorphism  Data in the same family class can be placed into a single container  E.g., vector publications;  To add a publication: publications.push_back(new Publication);  Book is a Publication: publications.push_back(new Book);  Then, a simple loop can print different details: for ( int i=0; i < publications.size(); i++) publications[i]->print();

Rossella Lau Lecture 9, DCO10105, Semester B, The sample application: Mini book shop

Rossella Lau Lecture 9, DCO10105, Semester B, Details of the Publication family

Rossella Lau Lecture 9, DCO10105, Semester B, The sample program set  The application: bookshop.cpp  Classes:  Publication.h  Book.h  Magazine.h  Catalog.h  OrderItem.h  Order.h  Sales.h  InputException.h

Rossella Lau Lecture 9, DCO10105, Semester B, Input check  Before a datum can be input, one should also guarantee the input device works properly  In C++, some user input can put the input device (istream) into a fail state and it cannot work properly; see slide 6-8 of Lecture 2  Then, a user input involves data domain check  A popular process: uses a loop to prompt user until a correct input is entered bool done = false; do { … // user prompt … // accept input … // device check // domain check if (……) // correct done = true; else … // error } while (!done)

Rossella Lau Lecture 9, DCO10105, Semester B, Exception handling  Error state or invalid input is an undesirable situation that makes program logic complicated, and sometimes disturbs the logic of normal operations  OOP uses exception handling to manage undesirable situations  System exceptions: logic_error & runtime_error Some popular derived sub-classes: invalid_argument, out_of_range, overflow_error, underflow_error  User defined exceptions: a user can use basic data types as exceptions or create its own exception classes

Rossella Lau Lecture 9, DCO10105, Semester B, Exceptions  Some system functions will throw exception(s) to allow the calling function to notify there is an exception (unexpected result) from the function  type funID(…) throw exception1, exception2, … {}  A user can also throw exceptions to have systematic exception handling: throw exception;  To manage the exception: use try -- catch

Rossella Lau Lecture 9, DCO10105, Semester B, A simple example try { cin >> input; if (!cin) throw InputException(); //domain check if (0 = input) done = true; else throw InputException("Invalid choice!"); } catch (InputException ie) { cerr << ie.what() << endl; const int DEFAULT_LINE_LENGTH = 100; cin.clear(); cin.ignore (DEFAULT_LINE_LENGTH, '\n'); }

Rossella Lau Lecture 9, DCO10105, Semester B, The sample user defined exception class InputException { private: string message; public: InputException(string str="Input Error!"): message(str) {} string what() {return message; } };  System exception classes usually provide a service function what() to return the error message

Rossella Lau Lecture 9, DCO10105, Semester B, Exceptions  Exception: undesirable event detectable during program execution  If exceptions occurred during execution  Programmer-supplied code terminated the program or  Program terminated with an appropriate error message  Can add exception-handling code at point where an error can occur (Malik’s slide: 16-4)  Example 16-1,2: Ch16_Ex1.cpp, Ch16_Ex2.cpp

Rossella Lau Lecture 9, DCO10105, Semester B, Handling Exceptions within a Program (continued)  Function assert:  Checks if an expression meets certain condition(s)  If conditions are not met, it terminates the program  Example: division by 0  If divisor is zero, assert terminates the program with an error message (Malik’s slide: 16-5)  Example 16-3: Ch16_Ex3.cpp

Rossella Lau Lecture 9, DCO10105, Semester B, C++ Mechanisms of Exception Handling  The try/catch block handles exceptions  Exception must be thrown in a try block and caught by a catch block  C++ provides support to handle exceptions via a hierarchy of classes (Malik’s slide: 16-6)

Rossella Lau Lecture 9, DCO10105, Semester B, try/catch Block  Statements that may generate an exception are placed in a try block  The try block also contains statements that should not be executed if an exception occurs  The try block is followed by one or more catch blocks (Malik’s slide: 16-7)

Rossella Lau Lecture 9, DCO10105, Semester B, try/catch Block (continued)  The catch block:  Specifies the type of exception it can catch  Contains an exception handler  If the heading of a catch block contains... (ellipses) in place of parameters  Catch block can catch exceptions of all types (Malik’s slide: 16-8)

Rossella Lau Lecture 9, DCO10105, Semester B,  General syntax of the try/catch block: (Malik’s slide: 16-9)

Rossella Lau Lecture 9, DCO10105, Semester B, try/catch Block (continued)  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 (Malik’s slide: 16-10)  Exercise: 16:2

Rossella Lau Lecture 9, DCO10105, Semester B, try/catch Block (continued)  The program searches catch blocks in order, 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 (Malik’s slide: 16-11)

Rossella Lau Lecture 9, DCO10105, Semester B, Throwing an Exception  For try/catch to work, the exception must be thrown in the try block  General syntax to throw an exception is: throw expression; where expression is a constant value, variable, or object (Malik’s slide: 16-12)  Example 16-5,6,7: Ch16_Exp5,6,7.cpp

Rossella Lau Lecture 9, DCO10105, Semester B, Throwing an Exception (continued)  The object being thrown can be  Specific object  Anonymous object  In C++  An exception is a value  throw is a reserved word (Malik’s slide: 16-13)

Rossella Lau Lecture 9, DCO10105, Semester B, Throwing an Exception: (Malik’s slide: 16-14)

Rossella Lau Lecture 9, DCO10105, Semester B, Order of catch Blocks  Catch block can catch  All exceptions of a specific type  All types of exceptions  A catch block with an ellipses (three dots) catches any type of exception  In a sequence of try/catch blocks, if the catch block with an ellipses is needed  It should be the last catch block of that sequence (Malik’s slide: 16-15)  Exercise: 16:3

Rossella Lau Lecture 9, DCO10105, Semester B, Using try/catch Blocks in a Program: (Malik’s slide: 16-16)

Rossella Lau Lecture 9, DCO10105, Semester B, Using C++ Exception Classes  C++ provides support to handle exceptions via hierarchy of classes  The function what returns the string containing exception object thrown by C++’s built-in exception classes  The class exception is:  The base class of the exception classes provided by C++  Contained in the header file exception (Malik’s slide: 16-17)

Rossella Lau Lecture 9, DCO10105, Semester B, Using C++ Exception Classes (continued)  Two classes derived from exception:  logic_error  runtime_error  logic_error and runtime_error are defined in header file stdexcept  The class invalid_argument deals with illegal arguments used in a function call (Malik’s slide: 16-18)

Rossella Lau Lecture 9, DCO10105, Semester B, Using C++ Exception Classes (continued)  The class out_of_range deals with the string subscript out_of_range error  The class length_error handles the error if  A length greater than the maximum allowed for a string object is used (Malik’s slide: 16-19)  Example 16-8,9: Ch16_Ex8.cpp, Ch16_Ex9.cpp

Rossella Lau Lecture 9, DCO10105, Semester B, Using C++ Exception Classes (continued)  If the operator new cannot allocate memory space  It throws a bad_alloc exception  The class runtime_error deals with errors that occur only during program execution  Classes overflow_error and underflow_error  Deal with arithmetic overflow and under-flow exceptions  Derived from the class runtime_error (Malik’s slide: 16-20)

Rossella Lau Lecture 9, DCO10105, Semester B, Creating Your Own Exception Classes  Programmers can create exception classes to handle exceptions not covered by C++’s exception classes and their own exceptions  C++ uses the same mechanism to process the exceptions you define as for built-in exceptions  You must throw your own exceptions using the throw statement  Any class can be an exception class (Malik’s slide: 16-21)  Example : Ch16_Ex10-13.cpp

Rossella Lau Lecture 9, DCO10105, Semester B, Rethrowing and Throwing an Exception  When an exception occurs in a try block  Control immediately passes to one of the catch blocks  A catch block either  Handles the exception or partially processes the exception and then rethrows the same exception OR  Rethrows another exception for the calling environment to handle (Malik’s slide: 16-22)

Rossella Lau Lecture 9, DCO10105, Semester B, Rethrowing and Throwing an Exception (continued)  The general syntax to rethrow an exception caught by a catch block is: throw; (in this case, the same exception is rethrown) or: throw expression; where expression is a constant value, variable, or object (Malik’s slide: 16-23)

Rossella Lau Lecture 9, DCO10105, Semester B, Rethrowing and Throwing an Exception (continued)  The object being thrown can be  A specific object  An anonymous object  A function specifies the exceptions it throws in its heading using the throw clause (Malik’s slide: 16-24)  Example 16-14,15: Ch16_Ex15.cpp, Ch16_Ex15.cpp

Rossella Lau Lecture 9, DCO10105, Semester B, Exception Handling Techniques  When an exception occurs, the programmer usually has three choices:  Terminate the program  Include code to recover from the exception  Log the error and continue (Malik’s slide: 16-25)

Rossella Lau Lecture 9, DCO10105, Semester B, Terminate the Program  In some cases, it is best to let the program terminate when an exception occurs  For example, if the input file does not exist when the program executes  There is no point in continuing with the program  The program can output an appropriate error message and terminate (Malik’s slide: 16-26)

Rossella Lau Lecture 9, DCO10105, Semester B, Fix the Error and Continue  In some cases, you will want to handle the exception and let the program continue  For example, if a user inputs a letter in place of a number  The input stream will enter the fail state  You can include the necessary code to keep prompting the user to input a number until the entry is valid (Malik’s slide: 16-27)  Example 16-16: Ch16_Ex16.cpp

Rossella Lau Lecture 9, DCO10105, Semester B, Log the Error and Continue (continued)  For example, if your program is designed to run a nuclear reactor or continuously monitor a satellite  It cannot be terminated if an exception occurs  When an exception occurs  The program should write the exception into a file and continue to run (Malik’s slide: 16-28)

Rossella Lau Lecture 9, DCO10105, Semester B, Summary  Polymorphism provides dynamic binding which reduces programming efforts for different data types within the same family  To manage an application better, exception handling should be employed

Rossella Lau Lecture 9, DCO10105, Semester B, Reference  Malik: 16  Sample program set (mini book shop):  bookShop.cpp, Publication.h, Book.h, Magazine.h, Catalog.h, OrderItem.h, Order.h, Sales.h, InputException.h -- END --