1. Andy Wang Object Oriented Programming in C++ COP 3330
Exception Handling
Andy Wang
Object Oriented Programming in C++
COP 3330

2. Exception Handling
A type of error checking, available in many programming languages
Exception
Some sort of problem or error that occurs during a program’s execution
Many error situations could occur besides those that we usually check for (usually related to things like user input)
Exception handler
A piece of code that resolves an exception situation
Typical error check often intermixed with the tasks of a program (if statements, etc)
Exception handlers are intended to be separate from the main tasks

3. Why?
Intermixing program logic with the error-checking code can sometimes make programs hard to read, debug, etc.
Many potential problems are very infrequent
Exception handlers are separate from main tasks of a program
Can improve clarity and modifiability
Exception handling can improve a program’s fault tolerance

4. When?
This does not mean that exception handlers should be used in all cases
Sometimes conventional error checking is more appropriate
Exception handling best for problems that occur infrequently
Errors that will result in termination of the program
Not for user input checking
Good for setting up uniform techniques for error handling when many programmers and multiple modules are involved

5. How? Reserved words in C++: try throw, catch try blocks catch blocks
Consists of keyword try and a block of code inside { }
Encloses the statements that might cause exceptions
catch blocks
1+ catch blocks follow a try block (also in { })
Each catch block is an exception handler
A cache block has a single parameter (with type listed)

6. How? If an exception occurs in a try block
The try block immediately ends
Program attempts to match the exception to one of the catch handlers (based on type of item thrown)
If a match is found, the code in the catch block executes
Only one catch block will be matched, if any
Program control resumes after the last catch block
If no exception occur in a try block, the catch blocks are skipped
A point where an exception occurs is the throw point
Keyword throw used to throw an exception to be caught

7. How?
In C++, there is a standard library with pre-built exception classes
#include <exception>
using std::exception;
When a function intends to throw an exception back to the caller (not handled internally), it’s good to tell the caller what to expect via a throw list when declaring a function
void someFunction throw (DivideByZero, OtherException);
This can be used to limit the type of exceptions that a function is allowed to throw

8. How? The first function can throw no exceptions to the outside
void someFunction1() throw(); // empty throw list
void someFunction2(); // no throw list
The first function can throw no exceptions to the outside
The second can throw exceptions of any kind

9. Simple Example
eptions/except.cpp

10. Main.cpp
#include <iostream> using namespace std; int main() { int cookies, people; double cpp; try { cout << “Enter number of people: “; cin >> people; cout << “Enter number of cookies: “; cin >> cookies if (cookies == 0) throw people; if (cookies < 0) throw static_cast<double>(people);

11. Main.cpp
cpp = cookies/static_cast<double>(people); cout << cookies << “ cookies.\n” << people << “ people.\n” << “You have “ << cpp << “ cookies per person.\n”; } catch(int e) { cout << e << “ people, and no cookies!\nGo buy some cookies!\n”; catch(double t) { cout << “Second catch block type double – do we reach it?\n”; cout << “End of program.\n”; return 0;

12. Negative Number Example
04.cpp

13. 18-04.cpp
#include <iostream> #include <string> using namespace std; class NegativeNumber { public: NegativeNumber() {} NegativeNumber(string m): message(m) {} string getMessage() const ( return message; } private: string message; }; class DivideByZero {};

14. 18-04.cpp
int main() { int pencils, erasers; double ppe; try { cout << “How many pencils do you have?\n”; cin >> pencils; if (pencils < 0) throw NegativeNumber(“pencils”); cout << “How many erasers do you have?\n”; cin >> erasers; if (erasers < 0) throw NegativeNumber(“erasers”); if (erasers != 0) ppe = pensils/static_cast<double>(erasers); else throw DivideByZero();

15. 18-04.cpp
cout << “Each eraser must last through “ << ppe << pencils.\n”; } catch(NegativeNumber e) { cout << “Cannot have a negative number of “ << e.getMessage() << endl; catch(DivideByZero) { cout << “Do not make any mistakes.\n”; cout << “End of program.\n”; return 0;

16. Safe Divide Example
05.cpp

17. 18-05.cpp
#include <iostream> #include <cstdlib> using namespace std; class DivideByZero {}; double safeDevide(int top, int bottom) throw (DivideByZero) { if (bottom == 0) throw DivideByZero(); return top/static_cast<double>(bottom); }

18. 18-05.cpp
int main() { int numerator, denominator; double quotient; cout << “Enter numerator:\n”; cin >> numerator; cout << “Enter denominator:\n”; cin >> denominator; try { quotient = safeDivide(numerator, denominator); } catch(DivideByZero) { cout << “Error: Division by zero!\n”; << “Program aborting.\n”; exit(0); cout << numerator << “/” << denominator << “ = “ << quotient << endl; return 0;