Exception Handling

1. Exception Handling Lesson #11 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek

2. Content • Exception and Exception Handling • Structure of exception handling in C++ • Exception in Templates • Group of exceptions • Scope

3. Choices upon an error • Terminate immediately • Ignore the error • Set an error flag, check by the caller

4. What is an Exception? • An error generated by the program at run-time which would normally terminate the execution of the program. • Such as, memory exhaustion, subscript range errors, or division by zero.

5. What is Exception Handling? • A mechanism that allows a program to detect and possibly recover from errors during execution. • The alternative is to terminate the program.

6. Common Exceptions • out-of-bound array subscript • arithmetic overflow • divide by zero • out of memory reference • invalid function parameters • etc.

7. Benefits of Exception Handling • improved readability • easier to modify code • more robust code

8. Without Exception Handling #include <iostream.h> #include <stdlib.h> double hmean(double a, double b); int main(void) { double x, y, z; cout << "Enter two numbers: "; while (cin >> x >> y) { z = hmean(x,y); cout << "Harmonic mean of " << x << " and " << y << " is " << z << "\n"; cout << "Enter next set of numbers <q to quit>: "; }

9. Without Exception Handling cout << "Bye!\n"; return 0; } double hmean(double a, double b) { if (a == -b) { cout << "untenable arguments to hmean()"<<endl; abort(); } return 2.0 * a * b / (a + b); }//ex11-1.cpp

11. With Exception Handling #include <iostream.h> #include <stdlib.h> double hmean(double a, double b); int main(void) { double x, y, z; cout << "Enter two numbers: "; while (cin >> x >> y) { try { z = hmean(x,y); } // end of try block

12. With Exception Handling catch (const char * s) // start of exception handler { cout << s << "\n"; cout << "Enter a new pair of numbers: "; continue; } // end of handler cout << "Harmonic mean of " << x << " and " << y << " is " << z << "\n"; cout << "Enter next set of numbers <q to quit>: "; } cout << "Bye!\n"; return 0; }

13. With Exception Handling double hmean(double a, double b) { if (a == -b) throw "bad hmean() arguments: a = -b not allowed"; return 2.0 * a * b / (a + b); }//ex11-2.cpp

15. try { statement-list } catch(exception) { statement-list } catch(exception) { statement-list } Structure of Exception Handling

16. Procedure for exception handling try{ func();//1.The program call in try{} } catch(Class anObj){… //3.Transfer the execution to the program } func(){ throw exObj; //2.The function throws an exception and transfer the //execution to the catch block,and assign the exObj //to anObj }

17. Naming of an Exception #include <iostream.h> class ZeroDivide { const char* errmsg; public: ZeroDivide() : errmsg("Error: dividing by zero") { } void print() { cout << errmsg; } }; float divide(int n1, int n2); void main() { int a, b; cout <<"Input two integers:";

18. Naming cin >> a >> b; try { float c = divide(a,b); cout << "a / b = " << c << '\n'; } catch (ZeroDivide err) { err.print(); } cout <<“\n”<< "end of program\n"; } float divide(int n1, int n2) { if (n2 == 0) throw ZeroDivide(); return (float) n1 / n2; }//ex11-3.cpp Results: Input two integers:34 0 Error: dividing by zero end of program

19. Example #include <iostream.h> const unsigned ArraySize = 50; int array[ArraySize]; class RangeError{}; void InsertValue(unsigned i, int value) { if (i>=ArraySize) throw RangeError(); array[i] = value; } void TestTheArray() { unsigned j ; int anInt; cout <<"Enter a subscript and a value:";

20. Example cin >> j >> anInt; InsertValue(j, anInt); } void main() { try { TestTheArray(); } catch (const RangeError &) { cout <<"Range Error in main()!"<< endl; } }//ex11-4.cpp

22. Passing Data with Exceptions #include <iostream.h> class array { int* v, lower, upper; public: class SubscriptError { int index; public: SubscriptError(int i) : index(i) { }; int get() { return index; }; }; array(int l, int u):lower(l), upper(u){ v = new int [upper-lower +1]; }; int& operator[](int); ~array(){delete [] v;} };

23. int& array::operator[](int i) { if (i>=lower && i<=upper) return *(v+i-lower); throw SubscriptError(i); return *v; }; void fun(array& a) { try { a[100]; } catch (array::SubscriptError s) { cerr <<"illegal subscript:" <<s.get() << '\n'; } } void main() { array ar(20, 40); fun (ar); }//ex-5.cpp

25. More than One Exception const MAX = 500; class array { int* v, lower, upper; public: //… class SizeError {}; array(int l, int u):lower(l), upper(u){ int size = upper-lower+1; if (size<0 || size>MAX) throw SizeError(); else { v = new int[size]; } }; }; If we add another exception SizeError, how is it done?

26. More than One Exception void main() { int l, u; try { cout << "Input two bounds for the array:"; cin >> l>>u; array ar(l, u); fun (ar); } catch (array::SizeError ){ cerr <<"illegal size:" <<u - l +1 << '\n'; } }//ex11-6.cpp

28. Unwinding the Exception Stack • Stack unwinding: • When a program throw an exception, the destructor is called for each object that was constructed when the program entered the try block.

29. Unwinding the Exception Stack • When an exception is thrown during the construction of a composite object, destructors will be invoked only for those data members that have already been constructed.

30. Unwinding #include <iostream.h> #include <string.h> class Test{}; class VString{ public: VString(){ }; VString(char *s){ }; ~VString(){cout <<"VString Des!"<<endl;}; }; class Transcript{ public: Transcript(){ throw Test(); }; ~Transcript(){cout <<"Transcript Des!"<<endl;}; };

31. Unwinding class Student{ VString lastName; Transcript records; public: Student(){}; ~Student(){cout <<"Student Des!"<<endl;}; }; void main() { try{ VString collegeName("NJIT"); Student S; } catch (...) {cout << "exception!"<<endl; } }//ex11-7.cpp

33. Multiple Handlers • Most programs performing exception handling have to handle more than one type of exception. A single try block can be followed by multiple handlers(catch), each configured to match a different exception type.

34. Example #include <stdio.h> #include <iostream.h> #include <fstream.h> class RangeError { }; //Exception class InputFileError { }; //Exception class LongArray { public: LongArray( unsigned sz = 0 ); ~LongArray(); long Get( unsigned i ) const; private: long * data; // array of long integers unsigned size; // allocation size };//cis601source/chap8/except/except2.cpp

35. Example LongArray::LongArray( unsigned sz ) { size = sz; data = new long[size]; } LongArray::~LongArray() { delete [] data;} long LongArray::Get( unsigned i ) const { if( i >= size ) throw RangeError(); return data[i]; }

36. Example unsigned ArraySize = 50; void ReadFile( ifstream & infile, LongArray & L ) { infile.open( "INDATA.TXT" ); if( !infile ) throw InputFileError(); } void GetArrayElement( const LongArray & L ) { unsigned i; cout << "Enter subscript: "; cin >> i; long n = L.Get( i ); }

37. Example int main() { try { ifstream infile; LongArray L( ArraySize ); ReadFile( infile, L ); GetArrayElement( L ); cout << "Program completed normally.\n"; } catch( const RangeError & R ) { cout << "Subscript out of range\n"; } catch( const InputFileError & F ) { cout << "Unable to open input file\n"; } catch( ... ) { cout << "Unknown exception thrown\n"; } return 0; }//except2.cpp

38. Results Enter subscript: 23 Program completed normally. Enter subscript: 56 Subscript out of range

39. Ex: RangeError Class #include <iostream.h> #include <stdlib.h> #include <fstream.h> #include "fstring.h" #include "range.h" class LongArray { public: LongArray( unsigned sz = 0 ){size = sz; data = new long [sz];}; ~LongArray() {delete [] data;}; unsigned GetSize() const; long Get( unsigned i ) const; void Put( unsigned i, long item ); private: long * data; // array of long integers unsigned size; // allocation size }; //cis601source/chap8/except/except3.cpp

40. Range.h #include <iostream.h> #include <string.h> const unsigned FileNameSize = 50; // Make this >= longest filename on target system. class RangeError { public: RangeError( const char * fname, unsigned line, unsigned subscr ) { strncpy(fileName, fname, FileNameSize); lineNumber = line; value = subscr; }

41. Range.h friend ostream & operator <<( ostream & os, const RangeError & R ) { os << "\nRangeError exception thrown: " << R.fileName << ", line " << R.lineNumber << " value = " << R.value << endl; return os; } private: char fileName[FileNameSize+1]; unsigned lineNumber; unsigned value; };

42. The name of the current source file. __FILE__ expands to a string surrounded by double quotation marks. RangeError inline unsigned LongArray::GetSize() const { return size;} inline void LongArray::Put( unsigned i, long item ) { if( i >= size ) throw RangeError( __FILE__ ,__LINE__, i ); data[i] = item; } inline long LongArray::Get( unsigned i ) const { if( i >= size ) throw RangeError( __FILE__ ,__LINE__, i ); return data[i]; } The line number in the current source file. The line number is a decimal constant.

43. RangeError unsigned GetArraySize() { unsigned n; cout << "Number of array elements? "; cin >> n; return n; } void FillArray( LongArray & L ) { int i; try { for( i = 0; i < L.GetSize(); i++ ) L.Put( i, rand() ); } catch( const RangeError & R ) { cout << R; throw R; } }

44. RangeError void GetArrayElement( const LongArray & L ) { int ok = 0; while( !ok ) { unsigned i; cout << "Enter an array subscript (0-"<< ( L.GetSize()-1 ) << "): "; cin >> i; long n; try { n = L.Get( i ); ok = 1; cout << "Element contains " << n << endl; } catch( const RangeError & R ) { cout << R; cout << "Caught at: " << __FILE__<< ", line " << __LINE__ << endl; throw; } } }

45. RangeError int main() { try { LongArray L( GetArraySize() ); FillArray( L ); GetArrayElement( L ); } catch( ... ) { cout << "Exception caught in main().\n"; return 1; } return 0; }//except3.cpp

47. Exception with no Catch • If no catch matches the exception generated by the try block, the search continues with the next enclosing try block. • If no catch is found, then error!

48. void fun1(array& a) { ... try { fun2(a); ... } catch (array::SubscriptError) { ... } ... }

49. void fun2(array& a) { ... try { ... // use array a } catch (array::SizeError) { ... } ... }

50. Exception within an Exception • If the same exception type is raised while handling the exception of a try block, then the new exception is handled by the outer try block.