Chapter 17 Templates and Exceptions Part 2

1. Chapter 17Templates and ExceptionsPart 2 Dale/Weems/Headington

2. An Exception is… An unusual, often unpredictable event, detectable by software or hardware, that requires special processing; also, in C++, a variable or class object that represents an exceptional event. An exception handler is a section of program code that is executed when a particular exception occurs.

3. The throw Statement Throw: to signal the fact that an exception has occurred; also called raise Throw Statement throwExpression

4. Throw examples (1) • throw 5; • string str = “Invalid customer age”; throw str; • class SalaryError { }; … SalaryError sal; throw sal;

5. Throw examples (2) • Use of anonymous (unnamed) object class SalaryError {}; … throw SalaryError(); • A wrong way throw SalaryError;

6. The try-catch Statement How one part of the program catches and processes the exception that another part of the program throws. TryCatchStatement try Block catch (FormalParameter) Block catch (FormalParameter) Block FormalParameter DataType [VariableName] …

7. Example of a try-catch Statement try { // Statements that process personnel data and may throw // exceptions of type int, string, and SalaryError } catch ( int ) { // Statements to handle an int exception } catch ( string s ) {

8. try-catch Continued cout << s << endl; // Prints "Invalid customer age" // More statements to handle an age error } catch ( SalaryError ) { // Statements to handle a salary error } catch (…) { // call all other types of exceptions }

9. Execution of try-catch A statement throws an exception No statements throw an exception Exception Handler Control moves directly to exception handler Statements to deal with exception are executed Statement following entire try-catch statement

10. Selecting an Exception Handler • The computer: • matches data types of throw clause with the data types in the catch clauses • Searches in a “north-to-south” order • Selects first handler whose data type matches that of the thrown exception • Ellipse parameters are a “wild card” and catch all. Place the “catch all” handler last.

11. Selecting an Exception Handler What constitutes a match? • Suppose an object of type T is thrown. • A catch block that takes an argument of type C is a match if: • T and C are the same type. • C adds a reference qualifies (e.g. int can be caught by int&) • C is a base class of publicly derived class T • Note: matching process more restrictive than for function calls. Ex: int can’t be caught by float or char

12. More on Selecting Exception Handlers • The parameter’s name is needed only if statements in the body of the exception handler use that variable. • It is a good idea to use only • user-defined classes (and structs) as exception types, • one type per exception • descriptive identifiers

13. An example class SalaryError // Exception class {}; class BadRange // Exception class {}; … if ( condition ) throw SalaryError(); … if ( condition ) throw BadRange();

14. Nonlocal Exception Handlers • It is more common for the throw to occur inside a function that is calledfrom within a try-clause than for the throw to be located within the try-catch statement

15. Throwing an Exception to be Caught by the Calling Code void Func3() { try { Func4(); } catch ( ErrType ) { } } void Func4() { if ( error ) throw ErrType(); } Function call Normal return Return from thrown exception

16. Call Call Call Call Call Call Call Call Immediate return Immediate return Passing an Exception up the Chain of Function Calls Program terminates immediately main main No ErrType handler No ErrType handler Func1 Func1 No ErrType handler ErrType handler Func2 Func2 Immediate return Immediate return No ErrType handler No ErrType handler Func3 Func3 Immediate return No ErrType handler No ErrType handler Func4 Func4 No ErrType handler throw ErrType(); No ErrType handler throw ErrType(); Immediate return Immediate return Function Func1 has a handler for ErrType No function has a handler for ErrType

17. Re-Throwing an Exception • The throw expression is optional. throw; • Re-throwing an exception in C++ allows partial exception handling.

18. An example (1) void WriteToFile( parameters ) { … // Open a file for output try { while ( condition ) { DoSomething( arguments ); // May throw a BadData exception … // Write to output file } }

19. An example (2) catch ( BadData ) { … // Write massage to output file and // close it throw; // Re-throw the exception } … // Continue processing // and close the output file }

20. A Solution // quotient.cpp -- Quotient program #include<iostream>#include <string> using namespace std; int Quotient( int, int ); class DivByZero // Exception class {}; int main() { int numer; // Numerator int denom; // Denominator

21. cout << "Enter numerator and denominator: "; cin >> numer >> denom; while (cin){ try { cout << "Their quotient: " << Quotient(numer, denom) << endl; } catch ( DivByZero ) {cout << "*** Denominator can't be 0" << endl; } cout << "Enter numerator and denominator: "; cin >> numer >> denom; } return 0;}

22. int Quotient( /* in */ int numer, // The numerator /* in */ int denom ) // The denominator { if (denom == 0) throw DivByZero(); return numer / denom; }

23. Standard Exceptions • Exceptions Thrown by the Language • new, dynamic_cast, typeid, exception specification • Exceptions Thrown by Standard Library Routines • Facilities inherited from the C language • Facilities designed specifically for C++

24. Exception thrown by new float* arr; try { arr = new float[50000]; } catch ( bad_alloc ) { cout << “*** Out of memory. Can’t allocate array.” << endl; return 1; // return to calling function } … // Continue. Allocation succeeded

25. Exceptions thrown by C++ libraries (1) • Exceptions thrown by string classes void SomeFunc( parameters ) { string s1, s2; try { … s2 = s1.substr(pos, len); // May throw out_of_range() s1 = s1 + s1 + s2; // May throw length_error() … }

26. Exceptions thrown by C++ libraries (2) catch ( out_of_range ) { cout << “Exception: out_of_range in SomeFunc” << endl; throw; // Re-throw exception to a caller } catch ( length_error ) { cout << “Exception: length_error in SomeFunc” << endl; throw; // Re-throw exception to a caller } … // Continue if no errors } • Exceptions thrown by string classes

27. Dividing by ZERO Apply what you know: int Quotient( /* in */ int numer, // The numerator /* in */ int denom ) // The denominator { if (denom != 0) return numer / denom; else // What to do?? }

28. The end of Chapter 17 Part 2