Overview of C Exceptions

1. Overview of C++ Exceptions Normal program control flow is halted At the point where an exception is thrown The program call stack �unwinds� Stack frame of each function in call chain �pops� Variables in each popped frame are destroyed Goes until an enclosing try/catch scope is reached Control passes to first matching catch block Can handle the exception and continue from there Can free some resources and re-throw exception

2. C++ Exceptions Example Consider a program using a Number class We�ve not covered class member functions in detail yet Class member functions are much like �plain-old� functions But are given a hidden extra pointer to the object (�this�) One of the class member functions is operator /= Divides value object contains by another passed value Problem: what if someone passes 0 to this function?

3. Motivation for C++ Exceptions void Number::operator/=(const double denom) { if (denom == 0.0) { // what do we do here? } m_value /= denom; } Need to handle divide by zero case Otherwise bad things happen Program crash Wrong results Could set value to NaN Special �not-a-number� value But, caller might fail to check for it Could return a �special� value Might be a valid return value Could still be ignored Exceptions offer a better alternative

4. C++ Exception Syntax void foo() throw (int) { throw 2; } catch (int &i) { cout << i << endl; } catch (...) { cout << �another exception� << endl; } Can throw any type Can specify what a function (only) throws in it�s declaration Can catch and use exceptions in code �Default� catch block

5. What�s in a Stack Frame? In general, structure is common A chunk of memory representing the state of an active function call Pushed on program call stack at run-time g++ -s gives assembler output can be used to deduce exact structure for a given platform Contains: The previous frame pointer The return address for the call (i.e., just after the point from which it was called) Parameters passed to the function Automatic (stack) variables for the function