Exception Handling Introduction Try Throw Catch

Exception Handling Use exception handling to write programs that are –Clearer –More robust –More fault-tolerant Exception examples –Failure of new to obtain enough memory –An out-of-bounds array subscript –Arithmetic overflow –Division by zero –Invalid function parameters

Error-handling Code Typically error-handling code is put where errors are expected to occur –Advantage Programmers can easily check to if error handling has been done in the immediate vicinity of the code –Disadvantage Code may become cluttered with too much error processing

C++’s Exception Handling Using C++’s exception handling removes most of the error handling code from the main program –Improves program readability and modifiability –Can handle “synchronous errors” Such as division by zero –Cannot handle “asynchronous situations” Such as network message arrivals Mouse clicks

C++’s Exception Handling Exception handling is used to –Allow the system to recover from an exception –Or (in the case of an unrecoverable error) clean up the mess & shut down gracefully

The Basics: try, throw, & catch Code that may generate an exception is encased in a try block –Within the try bock an exception is thrown After the try block, a catch block will catch and handle the exception, so that the program can return to its normal state –Program will search for a catch block with the same parameter type as the exception type If an exception is not handled, the program will terminate

Exception Class & Function class DBZException { public: DBZException() :message("ERROR: division by zero"){} void print() const {cout<<message<<endl;} private: const char *message; }; double quotient( int num, int den ){ if ( den == 0 ) throw DBZException(); return static_cast (num)/den; } (See DBZException.cpp.txt)DBZException.cpp.txt

Main int main(){ int num1, num2; double result; cout << "Enter two integers (EOF to end): "; while ( cin >> num1 >> num2 ) { try { result = quotient( num1, num2 ); cout<<"The quotient is: "<<result<<endl; } catch ( DBZException ex ) { ex.print(); } cout<<"\nEnter two integers (EOF to end): "; } return 0; }

Output Even thought the user made a mistake, the program can continue without an abend (ABnormal END) Enter two integers (EOF to end): 3 4 The quotient is: 0.75 Enter two integers (EOF to end): 4 0 ERROR: division by zero Enter two integers (EOF to end): 4 5 The quotient is: 0.8

End-of-File Marker File ends with a end-of-file (EOF) marker –EOF key combinations on different systems: UNIX systems: d IBM PC and compatibles: z Macintosh: d

Throwing an Exception A throw indicates that an exception occurred –The throw operand can be of any type –Can throw objects not intended for error handling When an operand is thrown –A temporary copy is made & initialized When this operand is caught –It initializes exception handler’s parameter When the exception handler completes execution and exits, the temporary object is destroyed

Throwing an Exception II If a handler is not found within a try block –The program will search for a matching handler in the next nested try block, until it is found All exceptions thrown outside a try block will terminate the program All objects declared in a try block will be destroyed before an exception is thrown from that block

New Exception Class class DBZException { public: DBZException():message("ERROR: division by zero"){} void print() const {cout<<message<<endl; } private: const char *message;}; class NIFException { public: NIFException() :message("ERROR: numerator is five"){} void print() const { cout<<message<<endl; } private: const char *message; };

Functions void checkNumeratorIs5(int n){ if(n==5) throw NIFException(); if(n==100) throw int(); } double quotient( int num, int den ){ if ( den == 0 ) throw DBZException(); try{ checkNumeratorIs5(num); cout<<"No exception in QUOTIENT()"<<endl;} catch(int e){ cout<<"Type INT exception thrown"<<endl;} return static_cast ( num ) / den; }

Main int main(){ int num1, num2; double result; cout << "Enter two integers (EOF to end): "; while ( cin >> num1 >> num2 ) { try{ result = quotient( num1, num2 ); cout<<"The quotient is:"<<result<<endl;} catch ( DBZException e ) { e.print(); } catch ( NIFException e ) { e.print(); } cout<<"\nEnter two integers (EOF to end): "; } return 0;}

Output Enter two integers (EOF to end): 1 2 No exception in QUOTIENT() The quotient is: 0.5 Enter two integers (EOF to end): 5 2 ERROR: numerator is five Enter two integers (EOF to end): Type INT exception thrown The quotient is: 25 (See throwing.cpp.txt)throwing.cpp.txt

Catching an Exception The catch block is used to 1.Catch an exception 2.Execute the code within the block The parameter in a catch statement can be named or unnamed –The handler catch(…) will catch any exception If no matching parameter type is found for a thrown object, the program terminates

Catching an Exception II A catch block can throw another kind of exception or re-throw itself –These exceptions will not be caught by any catch blocks listed after the current catch block –They will be caught after the next outer try block –If not caught, they will terminate the program

Exception Class class DBZException { public: DBZException() :message("ERROR: division by zero"){} void print() const {cout<<message<<endl;} private: const char *message; }; (See catching.cpp.txt)catching.cpp.txt

Functions void check(int n){ if(n==100) throw int(); } double quotient( int num, int den ){ if ( den == 0 ) throw DBZException(); try{ check(num); } catch( int ){ //unnamed parameter cout<<"Type INT exception thrown"<<endl; throw; //re-throw exception } return static_cast ( num ) / den; }

void main(){ int num1, num2; double result; cout << "Enter two integers (EOF to end): "; try{ while ( cin >> num1 >> num2 ) { try { result = quotient( num1, num2 ); cout<<"The quotient is:"<<result<<endl;} catch ( DBZException e ) { e.print(); throw float(); } catch (...) { cout<<"UNDEFINED ERROR I"<<endl; } cout<<"\nEnter two integers : ";} } catch(...){ cout<<"UNDEFINED ERROR II"<<endl;} }

Output Enter two integers (EOF to end): Type INT exception thrown UNDEFINED ERROR I Enter two integers (EOF to end): ERROR: division by zero UNDEFINED ERROR II (program terminates)

Class Exercise 1 See exercise1.txtexercise1.txt