You now have a complete calculator. It lets you enter an operator and a number and it accumulates the effects of these inputs in a static local variable of a function. It uses the switch statement to determine what to do to the accumulator, based on an operator character entered by the user. When it gets an invalid operator, it throws a Standard C++ library exception (runtime_error) that it handles with a new and separate catch of runtime_error in main(). It reports the exception with a new function in the ErrorHandlingModule.