Lecture 11

Lecture 11, Thu 05/07

Exceptions

Exception handling is a mechanism for specifying and handling error conditions that can occur.
The code that deals with the error condition (called the exception handler) is said to catch the exception.

Example

int main() {
	int value;
	try {
		cout << "Enter a positive number: ";
		cin >> value;

		if (value < 0)
			throw value;

		cout << "The number entered is: " << value << endl;
	} catch (int e) {
		cout << "The number entered is not positive." << endl;
	}
	
	cout << "Statement after try/catch block" << endl;
	return 0;
}

Everything executes within the try block if execution is normal. Every try block must be accompanied by one or more catch blocks.
- Only when value < 0 would we throw something in the example above.
- Statements after throwing an exception within the try block is ignored, and code within the catch block of the exception type is executed.
- Regardless if an exception was thrown or not, execution resumes after the try/catch block.
We can throw any value (basic types, structs, classes, …).
The code within the catch block is called the exception handler
- It’s possible for try / catch statements to be embedded (try/catch statements in a try block and/or catch block)
- Observe the output when running the following code:

int main() {
    int value;
    try {
        try {
            cout << "in try block within try block" << endl;
            throw 100;
        } catch (int d) {
            cout << "In catch block within try block: " << d << endl;
        }
        cout << "Enter a positive number: ";
        cin >> value;

        if (value < 0)
            throw value;

        cout << "The number entered is: " << value << endl;
    } catch (int e) {
        cout << "The number entered is not positive." << endl;
        try {
            cout << "In try block within catch" << endl;
            string message = "some error message";
            throw message;
        } catch (string s) {
            cout << "in catch block within catch block:" << s << endl;
        }
        cout << "exiting catch (int e) block" << endl;
    }
	
    cout << "Statement after try/catch block" << endl;
    return 0;
}

Throwing / Catching Multiple Exceptions

Example

class NegativeValueException {};
class EvenValueException {};

// …
if (value < 0)
	throw NegativeValueException();
if (value % 2 == 0)
	throw EvenValueException();
// …

catch(NegativeValueException e) {
	cout << "The number entered is not positive." << endl;
} catch (EvenValueException e) {
	cout << "The number entered is even." << endl;
}

The exceptions are checked one-by-one until the first match occurs.
If we don’t want to (or wouldn’t know how to) handle the exception where it occurred, we can “pass it up” to the caller and let them handle the exception.

Another Example

class DivideByZeroException {};

// Note that the throw (DivideByZeroException) is not required
// since C++ does not support "checked" exceptions.
double divide(int numerator, int denominator) throw (DivideByZeroException) {
	if (denominator == 0)
		throw DivideByZeroException();
	return numerator / (double) denominator;
}

int main() {
	try {
		cout << divide(1,1) << endl;
		cout << divide(1,0) << endl;
		cout << divide(2,2) << endl;
	} catch (DivideByZeroException) { // variable name is optional
		cout << "Error: cannot divide by zero" << endl;
	}
}

The program terminates if a thrown exception isn’t caught / handled before it is passed by main().

Inheritance and Exceptions

We can create a hierarchy of exceptions that inherit from a base class.
If an object is thrown and a catch block with one of its base classes is reached…
- then the code within the base class’ catch block is executed.

Example:

class A {};
class B : public A {};
class C {};

int main() {
	int value;
	try {
		cout << "Enter a positive number: ";
		cin >> value;
	
		if (value < 0)
			throw B();
	} catch (C) {
		cout << "Exception of type C caught." << endl;
	} catch (A) {
		cout << "Exception of type A caught." << endl;
	} catch (B) {
		cout << "Exception of type B caught." << endl;
	}
	return 0;
}

Note: Exceptions are checked top-to-bottom. The compiler may provide a warning if A exists before B since an exception of type B will never get caught if thrown in this case.