1. CSC 270 – Survey of Programming Languages
C++ Lecture 6 – Exceptions

2. Difference from Java Fewer exceptions
No null pointer exception
No divide by zero exception
Undefined behavior instead
No finally block (try, catch, finally)
Catch all statements
Standard: inherit from std::exception or std::runtime_error (for what)
Use catch (...) for all
Use catch(const char* Message) for a string
No concept of checked exceptions

3. See no divide by zero error
#include <iostream>
#include <string>
using namespace std;
int main(void) {
int pencils, erasers;
double ppe; //pencils per eraser
cout << "How many pencils do you" << " have?\n";
cin >> pencils;
cout << "How many erasers do you" << " have?\n";
cin >> erasers;
ppe = pencils / (double) erasers;
cout << "Each eraser must last through "
<< ppe << " pencils." << endl;
cout << "End of program." << endl;
return(0); }

4. Just Throw a String #include <iostream> #include <string>
using namespace std;
int
main (void) {
int pencils, erasers;
double ppe; //pencils per eraser
cout << "How many pencils do you" << " have?\n";
cin >> pencils;
cout << "How many erasers do you" << " have?\n";
cin >> erasers;
try
if (erasers == 0)
throw "divided by zero"; }
ppe = pencils / (double) erasers;
cout << "Each eraser must last through " << ppe << " pencils." << endl ;
cout << "End of program." << endl;
catch (const char *Message)
cout << Message << endl;
return (0);

5. Or throw object of Existing Class
#include <stdexcept> #include <iostream> #include <string> using namespace std; int main (void) { int pencils, erasers; double ppe; //pencils per eraser cout << "How many pencils do you" << " have?\n"; cin >> pencils; cout << "How many erasers do you" << " have?\n"; cin >> erasers; try if (erasers == 0) throw runtime_error("a message"); } ppe = pencils / (double) erasers; cout << "Each eraser must last through " << ppe << " pencils." << endl; cout << "End of program." << endl; catch (runtime_error e) cout << e.what() << endl; return (0);

6. Throw an error object you create Step 1: Create an error class
Create a small class named as the error type
Empty or filled is fine
using namespace std;
class NegativeNumber {
public:
NegativeNumber(void) {}
NegativeNumber(string theMessage)
: message(theMessage) {}
string getMessage() const {return message; }
private:
string message; };
class DivideByZero {};

7. ExceptionDemo.cpp creation of the Error classes
#include <iostream> #include <string> using namespace std; class NegativeNumber { public: NegativeNumber(void) {} NegativeNumber(string theMessage) : message(theMessage) {} string getMessage() const {return message; } private: string message; }; class DivideByZero {};

8. Throw an error object you create Step 2: Throw when you find error
Use the throw command
Follow it by an object of an error class
Throw will
Stop executing the method it is in
give that object back to the calling program
If it is not caught, it will crash the program.

9. int main(void) { int pencils, erasers; double ppe; //pencils per eraser try { cout << "How many pencils do you" << " have?\n"; cin >> pencils; if (pencils < 0) throw NegativeNumber("pencils");

10. Catch the error
Surround the call to the method that may throw an error with a try { }
After the try, add a catch block
Catch ( what you want to catch and its var name){ }
Inside the catch block, you can access the variables inside the error object

11. cout << "How many erasers do you" << " have
cout << "How many erasers do you" << " have?\n"; cin >> erasers; if (erasers < 0) throw NegativeNumber("erasers"); if (erasers != 0) ppe = pencils / (double) erasers; else throw DivideByZero(); cout << "Each eraser must last through " << ppe << " pencils." << endl; }

12. catch(NegativeNumber e) { cout << "Cannot have a negative number of " << e.getMessage() << endl; } catch(DivideByZero) { cout << "Do not make any mistakes!" << endl; cout << "End of program." << endl; return(0);

13. Summary At error – throw exception At use of the method: String
Existing exception
Your own class empty
Your own class with data
At use of the method:
Try { } surrounding all that should not be done for error
Catch ( ) { } do only if error
If standard error, use what method
If custom use your method
… to catch all