1. Chapter 17 Templates and Exceptions Part 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
throw Expression

4. Throw examples (1) throw 5; string str = “Invalid customer age”;
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)
FormalParameter
DataType [VariableName] …

7. 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 )

8. 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

9. 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

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();
throw BadRange();

14. 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

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. 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

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; }
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)
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

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