Motivation for C++ Exceptions

1. Void Number:: operator/= (const double denom) { if (denom == 0.0) { // what to do here? } m_value /= denom; } Need to handle cases where program cannot behave normally E.g., zero denominator for division Otherwise bad things happen Program crashes Wrong results Could set value to Number::NaN I.e., a special “not-a-number” value Must avoid using a valid return value… … which can be difficult (e.g., for int) Anyway, caller might fail to check for it Exceptions offer a better alternative Motivation for C++ Exceptions

2. void foo() throw (int) { throw 2; } try { foo(); } catch (int &i) { cout << “caught ” << i << endl; } catch (...) { cout << “another exception” << endl; } Can throw any type A function’s declaration can list types it can throw Otherwise the function may throw any defined type Can catch and inspect or use exceptions “Default” catch block is indicated by three dots Catches every type C++ Exception Syntax

3. C++11 standardizes a hierarchy of exception classes To access these classes #include <stdexcept> Two main kinds (subclasses) of exception Run time errors (overflow errors and underflow errors) Logic errors (invalid arguments, length error, out of range) Several other subclasses of exception Bad memory allocation Bad cast Bad type id Bad exception You can also declare other subclasses of these Using the class and inheritance material in later lectures C++11 Exceptions

4. 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 • This goes on 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 • Let’s look at the call stack and how it behaves • Good way to explain how exceptions work (in some detail) • Also a good way to understand normal function behavior

5. In general, the 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 generated machine code (in assembly language) can be used to deduce exact structure for a given platform Each stack frame contains: A pointer to the previous stack frame The return address (i.e., just after point from which function was called) The parameters passed to the function (if any) Automatic (local) variables for the function Sometimes called “stack variables” A More Detailed Look at Stack Frames automatic variables parameters previous frame pointer return address

6. int main (int argc, const char *argv[]) { Number n(8.1); try { n /= 0.0; cout << n << endl; } catch (int) { return 1; } return 0; } Stack frame for function main Pushed on program call stack With stack variable n With parameters argc and argv Note that parameters are initialized at frame creation Variables are not necessarily initialized at frame creation May occur later in called function Illustrating the Call Stack (1/8) n m_value main argc argv

7. int main (int argc, const char *argv[]) { Number n(8.1); try { n /= 0.0; cout << n << endl; } catch (int) { return 1; } return 0; } Number constructor called Stack frame created Illustrating the Call Stack (2/8) n Number::Number(const double n) this n m_value main argc argv

8. Number::Number(const double n) :m_value(n) { } Enter Number constructor m_value is set in object How do we know which m_value to set? Implicit this pointer Illustrating the Call Stack (3/8) n Number::Number(const double n) this n m_value main argc argv

9. Number::Number(const double n) :m_value(n) { } Return from constructor Its stack frame is popped Returns to main Illustrating the Call Stack (4/8) n m_value main argc argv

10. int main (int argc, char **argv) { Number n(8.1); try { n /= 0.0; cout << n << endl; } catch (int) { return 1; } return 0; } Call operator/= on n New stack frame created Copy 0.0 into n Setthis Illustrating the Call Stack (5/8) denom Number::operator/=(const double denom) this n m_value main argc argv

11. Test denom against 0.0 Test succeeds throw integer 0 Illustrating the Call Stack (6/8) void Number::operator/=(const double denom) { if (denom == 0.0) { throw 0; } m_value /= denom; } denom Number::operator/=(const double denom) this n m_value main argc argv

12. throw unwinds stack Skips over rest of function Returns to caller Illustrating the Call Stack (7/8) void Number::operator/=(const double denom) { if (denom == 0.0) { throw 0; } value_ /= denom; } n m_value main argc argv

13. int main (int argc, char **argv) { Number n(8.1); try { n /= 0.0; cout << n << endl; } catch (int) { return 1; } return 0; } We’ve reached an enclosing try/catch scope So stack unwinding stops Control jumps to first matching catch block Skips over intervening coutstatement (no output!) Illustrating the Call Stack (8/8) n m_value main argc argv

14. try { // can throw exceptions } catch (Derived &d) { // Do something } catch (Base &d) { // Do something else } catch (...) { // Catch everything else } Control jumps to first matching catch block Order matters if multiple possible matches Especially with inheritance-related exception classes Put more specific catch blocks before more general ones Put catch blocks for more derived exception classes before catch blocks for their respective base classes Catching Exceptions in C++

15. try { // can throw exceptions } catch (Derived &d) { // Do something } catch (Base &d) { // Do something else } catch (...) { // Catch everything else } catch(...) Catches any type throw; Throws nothing (cannot catch) Often preferable to exit(1) But usually it’s better to throw something E.g., a special type E.g., at least an int More About Catching Exceptions in C++

16. // can throw anything void Foo::baz(); // promises not to throw void Foo::baz() throw(); // promises to only throw int void Foo::baz() throw(int); // only char or int void Foo::baz() throw(char,int); Added in C++98, deprecated in C++11 Make promises to the calling function Empty throw clause promises no exceptions Can list multiple types (comma separated) By default, a function can throw whatever it wants Throw clause in function signature limits what that function body can throw But can call code that leaves off throw clause Exception Specifications (do not use them)

17. Use exceptions to handle any cases where the program cannot behave normally Do not use exceptions as a way to control program execution under normal operating conditions Don't let a thrown exception propagate out of the main function uncaught Instead, always catch any exceptions that propagate up Then return a non-zero value to indicate program failure Don’t use or rely on exception specifications A false promise if you declare them, unless you have fully checked all the code used to implement that interface No guarantees that they will work for templates, because a template parameter could leave them off and then throw Rules of Thumb for Using C++ Exceptions