CSE 332: C++ exceptions 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

CSE 332: C++ exceptions 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?

CSE 332: C++ exceptions 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

CSE 332: C++ exceptions 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

CSE 332: C++ exceptions 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 previous frame pointer return address parameters local (automatic) variables

CSE 332: C++ exceptions Illustrating the Call Stack int main (int argc, 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 variables i and c –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 argcargv n m_value main

CSE 332: C++ exceptions Initializing an Object (Constructor Call) int main (int argc, 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 argcargv n m_value main this Number:: Number(const double n) n

CSE 332: C++ exceptions Constructor Executes Number::Number(const double num) :m_value(n) { } Enter Number constructor –m_value is set in n How do we know which m_value to set? –Implicit this pointer argcargv n m_value main this Number::Number(const double num) num

CSE 332: C++ exceptions argcargv n m_value main Constructor Finishes Number::Number(const double num) :m_value(n) { } Return from constructor –Stack frame is popped –Return to main

CSE 332: C++ exceptions Calling Operator /= 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 –Set this argcargv n m_value main this Number:: operator/=(const double denom) denom

CSE 332: C++ exceptions Exception Thrown Test denom against 0.0 –Test succeeds –throw integer 0 void Number::operator/=(const double denom) { if (denom == 0.0) { throw 0; } m_value /= denom; } argcargv n m_value main this Number::operator/=(const double denom) denom

CSE 332: C++ exceptions argcargv n m_value main Live Exception Unwinds Stack Skips over rest of function Control returns to caller Continues until try/catch scope void Number::operator/=(const double denom) { if (denom == 0.0) { throw 0; } m_value /= denom; }

CSE 332: C++ exceptions Catching the Exception 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 cout statement argcargv n m_value main

CSE 332: C++ exceptions A Few More Details 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 catch(...) –catches any type throw; –does not throw a type –essentially a call to abort()

CSE 332: C++ exceptions Exception Specifications // can throw anything void Foo::bar(); // promises not to throw void Foo::bar() throw(); // promises to only throw int void Foo::bar() throw(int); // throws only char or int void Foo::bar() throw(char,int); Make promises to the caller Allow stronger type checking enforced by the compiler By default, a function can throw anything it wants A throw clause in a function’s signature –Limits what can be thrown –A promise to calling function A throw clause with no types –Says nothing will be thrown Can list multiple types –Comma separated

CSE 332: C++ exceptions Good Programming Style with C++ Exceptions Don’t use exceptions for normal program flow –Only use where normal flow isn’t possible Don’t let exceptions leave main or constructors –Violates “normal” initialization and termination Always throw some type –So the exception can be caught Use exception specifications widely –Helps caller know possible exceptions to catch