Exceptions CSCE 121 J. Michael Moore Based on slides created by Carlos Soto.

Handling runtime errors Check for unexpected conditions cout << “Enter a month (1-12): ”; int month; cin >> month; if (month < 1 || month > 12) { cout << “Invalid month.” << endl; }

Handling runtime errors: If statements? if (month < 0 || month > 12) { cout << “Invalid month.” << endl; } else { cout << “Enter a day (1-31): ”; int day; cin >> day; if (day < 1 || day > 31) { cout << “Invalid day.” << endl; cout << “Enter a year (1-2016): ”; //... Code cluttered with error handling! Lose track of what you are trying to accomplish! This can get out of hand…

Exceptions Rationale Prior to exceptions Code became cluttered with dealing with errors. Hard to focus on the underlying problem.

Goal Write code in a general way to solve the problem. Add structure to deal with error that does not obscure the code solving the problem.

Exceptions: Throw, Try and Catch When an unexpected condition happens, you “throw” an exception The Try block is the part of your code where an exception might occur You “catch” exceptions and deal with them in an exception handler

Exceptions: Throw, Try and Catch When an unexpected condition happens, “throw” an exception We’ll talk about how to throw an exception later where it makes sense. (i.e. after we talk about functions) Generally only within a function we write.

Exceptions: Throw, Try and Catch The Try block is the part of your code where an exception might occur

Handling runtime errors Go ahead and use without worrying about validity. What happens if month’s value is invalid? try { cout << “Enter a month (1-12): ”; int month; cin >> month; int month6 = halfYear(month); cout << “6 months after ” << month << “ is “ << month6; } Exception can happen here.

Exceptions: Try, Throw and Catch You “catch” exceptions in an exception handler (aka catch block) This is like a function that takes the exception as an argument

Handling runtime errors try { cout << “Enter a month (1-12): ”; int month; cin >> month; int month6 = halfYear(month); cout << “6 months after ” << month << “ is “ << month6; } catch (int exception) { cout << “Invalid month.” << endl; Normal Case. Exceptional Case handled in a separate block.

Notes on Exceptions We can have multiple separate catch blocks (aka exception handlers) for multiple datatypes Allows us keep track of different types of errors

Handling runtime errors catch (int exception) { if (exception == 1) { cout << “Error 1” << endl; } catch (char exception) { if (exception == ‘a’) { cout << “Error 2” << endl; The catch block only runs if the try block throws an exception. Then after it finishes, the program continues execution after the catch block. It takes an exception variable as an argument. We’ll see more about that when we cover functions. Let’s see what that looks like for more than one exception (go to code)

More Later Basics here More useful with functions We’ll revisit this topic later. In reality, we never throw something in the same function where the try block is. For now, use try and catch. Later we’ll throw exceptions.