Exceptions in C++ Object Oriented Programming.pptx

Exceptions in C++ Object Oriented Programming.pptx

• 1.
  Exception Handling
• 2.
  Exception Handling • Generalmechanism for handling abnormal conditions • Predefined exceptions: Constraint violations, I/O errors, other illegalities • User-defined exceptions • Exception handlers specify remedial actions or proper shutdown. • Exception handling (EH) allows a programmer to provide code in the program to handle run-time errors or exceptional situations – this improves reliability
• 3.
  EXCEPTIONS Exceptions are runtime anomalies or unusual conditions that a program may encounter during execution. Conditions such as • Division by zero • Access to an array outside of its bounds • Running out of memory • Running out of disk space It was not a part of original C++. It is a new feature added to ANSI C++.
• 4.
  EXCEPTION HANDLING • Exceptionsare of 2 kinds – Synchronous Exception: • Out of rage • Over flow • Asynchronous Exception: – Error that are caused by causes beyond the control of the program • Keyboard interrupts • In C++ only synchronous exception can be handled.
• 5.
  Exception handling mechanism •Find the problem (Hit the exception) • Inform that an error has occurred (Throw the exception) • Receive the error information (Catch the exception) • Take corrective action (handle the exception) • It is basically build upon three keywords – Try – Throw – Catch
• 6.
  Example #include <iostream> using namespacestd; int main () { try { throw 10; } catch (int e) { cout << “We have a problem!!” << endl; } return 0; } Output : We have a problem!!!
• 7.
  Contd. • The keywordtry is used to preface a block of statements which may generate exceptions. • When an exception is detected, it is thrown using a throw statement in the try block. • A catch block defined by the keyword catch catches the exception and handles it appropriately. • The catch block that catches an exception must immediately follow the try block that throws the exception.
• 8.
  Contd. • Exceptions areobjects used to transmit information about a problem. • If the type of the object thrown matches the arg type in the catch statement, the catch block is executed. • If they do not match, the program is aborted • Often, 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. • Once an exception is thrown to the catch block, control cannot return to the throw point.
• 9.
  THROWING MECHANISM • Thethrow statement can have one of the following 3 forms – throw(exception) – throw exception – throw //used to re-throw a exception • The operand object exception can be of any type, including constant. • It is also possible to throw an object not intended for error handling. • Throw point can be in a deeply nested scope within a try block or in a deeply nested function call. • In any case, control is transferred to the catch statement
• 10.
  THROWING MECHANISM • Thethrow expression accepts one parameter as its argument and this is passed to the exception handler. • You can have a number of throw statements at different parts of your try block with different values being thrown try { // code if ( x ) throw 10; // code if (y) throw 20; //code }
• 11.
  CATCHING MECHANISM • Thetype indicates the type of exception the catch block handles. • The parameter arg is an optional parameter name • The catch statement catches an exception whose type matches with the type of the catch argument. • If the parameter in the catch statement is named, then the parameter can be used in the exception handling code. • If a catch statement does not match the exception it is skipped. • More than one catch statement can be associated with a try block. • The exception handler can be identified by the keyword catch. • catch always takes only one parameter.
• 12.
  Contd. • The firsthandler that yields a match is executed. • If several catch statement matches the type of an exception the first handler that matches the exception type is executed. • This way we can chain multiple exception handlers and only the one with the correct parameter type gets executed.
• 13.
  Contd. try { throw exception; } catch(type1 arg) { //catch block 1 } catch(type2 arg) { // catch block 2 } …… catch(typeN arg) { // catch block N }
• 14.
  Catch all exceptions try { try{ // code here } catch (int n) { throw; } } catch (...) { cout << "Exception occurred"; }
• 15.
  RETHROWING AN EXCEPTION •A handler may decide to rethrow the exception caught without processing it. • In such a case we have to invoke throw without any arguments as shown below throw; • This causes the current exception to be thrown to the next enclosing try/catch sequence and is caught by a catch statement listed after the enclosing try block
• 16.
  Example void myFunction() { try { throw "hello";// throw a char * } catch(const char *) { // catch a char * cout << "Caught char * inside myFunctionn"; throw ; // rethrow char * out of function } // myFunction } } int main() { cout << "Startn"; try { myFunction(); } catch(const char *) { cout << "Caught char * inside mainn"; } cout << "End"; return 0; }
• 17.
  SPECIFYING EXCEPTION • Itis possible to restrict a function to throw certain specific exceptions by adding a throw list clause to the function definition. • The type-list specifies the type of exception that may be thrown. • Throwing any other kind of exception will cause abnormal program termination. • If you want to prevent a function from throwing any exception, you may do so by making the type-list empty. type function( type function(arg-list) throw(type list) { … … … }
• 18.
  Contd. • void f1(int)throw(int ,char, float) • void f2(int) throw(int ) • A variation on the previous example is: • void f3(int) throw(); // empty parentheses • This declaration guarantees that no exception is generated by the function f3. • If a function exits through any exception that is not allowed by an exception specification, it results in a call to the predefined function unexpected(). • By default, unexpected() calls terminate() which by default exits the program.
• 19.
  Copyright © 2003Pearson Education, Inc. Slide 19 Type Conversion • No automatic type conversions are done with exceptions – if double is in the exception specification, an int cannot be thrown unless int is also in the exception specification