Chapter 17 Templates and Exceptions Part 2 Dale/Weems/Headington

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.

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

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

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

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) FormalParameter DataType [VariableName] …

Example of a try-catch Statement { // 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 )

try-catch Continued // call all other types of exceptions 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

Execution of try-catch 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

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.

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

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

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

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

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

Passing an Exception up the Chain of Function Calls main Program terminates immediately main No ErrType handler No ErrType handler Call Immediate return Func1 Func1 Call ErrType handler No ErrType handler Call Call Func2 Func2 Immediate return No ErrType handler Immediate return No ErrType handler Call Call Immediate return Func3 Func3 Immediate return No ErrType handler No ErrType handler Call Call 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

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

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 }

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

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

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; } cin >> numer >> denom; } return 0; }

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

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++

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

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() }

Exceptions thrown by C++ libraries (2) Exceptions thrown by string classes 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” throw; // Re-throw exception to a caller … // Continue if no errors

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?? }

The end of Chapter 17 Part 2