CSC 213 Code Chapter 13, Fall 2007

Exception Handeling 1 Throwing an Exception throw indicates that an exception has occurred… ! Usually has one operand of any type… # The operand can be an object… exception object # Conditional expression can be thrown… throw condition ? expression_1 : expression_2 ! Code referenced in a try block can throw an exception… ! Exception caught by closest exception handler… ! Control exits current try block and goes to catch handler (if it exists) Exception does not terminate the program… ! However, it terminates the block where the exception occurred… Exception Handeling 2 ... try { // Code to be tried… throw exception_type; } catch (type parameter_name) { // Code to be executed in case of exception 1… } catch (...) { // Code to be executed for all exception … } ... Catching an Exception Exception handlers are defined in catch blocks… ! Format: catch( exceptionType parameterName){ EExxcceeppttiioonn hhaannddlliinngg ccooddee...... } ! Caught if argument type matches throw type… ! If not caught then terminate called which (by default) calls abort… Catch all exceptions… catch(...[JAN1]) ! You do not know what type of exception occurred… ! There is no parameter name, thus cannot reference an object… ! Not to be placed in front of other catch blocks… None of the other blocks will ever be executed… Exception Handeling 3 If no handler matches thrown object ! Searches next enclosing try block… # If none found, terminate called… ... try { if ( number1 == 0 ) throw 0 ; try { if ( number1 < 0 ) throw -1 ; } catch ( char * str ) { cout << "Exception: " << str << endl; } } catch ( int i) { cout << " Exception: " << i << endl; } ... # If found, control resumes after last catch block… # If several handlers match thrown object, first one found is executed… Exception Handeling 4 class ABC { public: ABC() : baseMsg ("Class Exception...") {} const char *who() const {return baseMsg;} private: const char *baseMsg; }; ... try { if ( number1 == 0 ) throw ABC(); } catch ( ABC obj1 ) { cout << "Exception occurred: " << obj1.who() << endl; } ... cccaaatttccchhh pppaaarrraaam mmeeettteeerrr m mmaaatttccchhheeesss ttthhhrrrooow wwnnn ooobbbjjjeeecccttt w wwhhheeennn::: ! They are of the same type… Exact match required, nnoo pprroom moottiioonnss//ccoonnvveerrssiioonnss aalllloowweedd… ... try { if ( number1 == 0 ) throw 1.0[JAN2] ; } catch ( int i ) { cout << " Exception: " << i << endl; } ... ! The catch parameter is a class of the thrown object… Exception Handeling 5 class ABase { public: ABase() : baseMsg ("Base Exception...") {} const char *who() const {return baseMsg;} private: const char *baseMsg; }; class ADerived : public Abase { public: ADerived() : drvMsg ("Derived Exception...") {} const char *who() const {return drvMsg;} private: const char *drvMsg; }; ! The catch parameter is a base class of the thrown object… ... ADerived dObj; ... try { if ( number1 == 0 ) throw ADerived(); if ( number1 == 1 ) throw dObj; } catch ( ABase[JAN3] bObj) { cout << "Exception occurred: " << bObj.who() << endl; } ... Exception Handeling 6 ... ADerived *dPtr=&dObj, &dRef=dObj; ... try { if ( number1 == 1 ) throw dPtr; if ( number1 == 2 ) throw dRef; } catch ( ABase *bPtr) { cout << "Exception 1 occurred: " << bPtr->who() << endl; } catch ( ABase &bRef) { cout << "Exception 2 occurred: " << bRef.who() << endl; } ... ... try { if ( number1 == 0 ) throw 1.0 ; } catch ( ... ) { cout << " Exception occurred...\n"; } ... ! The catch parameter is a base-class pointer/ reference type and the thrown object is a derived-class pointer/ reference type… ! The catch handler is catch( ... ) Exception Handeling 7 catch handler should delete space allocated by new and close any opened files… ! Resources may have been allocated when exception thrown… catch handlers can throw exceptions… ! It can only be processed by outer try blocks… ... try { if ( number1 == 0 ) throw 0 ; try { if ( number1 < 0 ) throw -1 ; } catch ( int i) { cout << "Exception 1: " << i << endl; throw "Inner exception..."; } catch ( char * str ) { cout << "Exception 2: " << str << endl; } } catch ( int i) { cout << "Exception 3: " << i << endl; } catch ( char * str ) { cout << "Exception 4: " << str << endl; } ... Exception Handeling 8 Rethrowing an Exception It is possible that the handler that caches the exception may decide that it cannot process it… Or it may simply want to release some resources before letting someone else handle the exception… The handler can simply rethrow the exception with Throw ; Rethrown exception detected by next enclosing try block… ... try { if ( number1 == 0 ) throw 0 ; try { if ( number1 < 0 ) throw -1 ; } catch ( int i) { cout << "Exception 1: " << i << endl; throw;[JAN4] } catch ( char * str ) { cout << "Exception 2: " << str << endl; } } catch ( char * str ) { cout << "Exception 4: " << str << endl; } catch[JAN5] ( int i) { cout << "Exception 3: " << i << endl; } ...[JAN6] Exception Handeling 9 Calls Calls Calls Terminates Terminates Terminates Stack unwinding Function-call stack unwound when exception thrown and not caught in a particular scope… ! Tries to catch exception in next outer try/catch block… ! Function in which exception was not caught terminates… # Local variables destroyed… # Control returns to place where function was called… ! If control returns to a try block, attempt made to catch exception… # Otherwise, further unwinds stack… ! If exception not caught, terminate called… main() catch(); function1() function2() Function3() throw(); EExxcceeppttiioonn RRaaiisseedd EExxcceeppttiioonn H Haannddlleedd Exception Handeling 10 #include <iostream.h> void function3(int m) { if (m < 0) throw "Exception thrown in function 3"; cout << "Function Three\n"; } void function2(int k) { function3(k); cout << "Function Two\n"; } void function1(int i) { function2(i); cout << "Function One\n"; } void main() { try { function1(10); cout << "------------------" << endl; function1(-10); } catch ( char * str) { cout << "Exception: " << str << endl; } } Function Three Function Two Function One ------------------ Exception: Exception thrown in function 3 Exception Handeling 11 Constructors and Exception Handling Constructors don't have a return type, thus it's not possible to use return codes… So what happens when an error is detected in the constructor? $ Partially constructed object or Zombie object  #include <iostream.h> #include <string.h> class XYZ { public: XYZ(int i=10) {iVal=i;} private: int iVal; }; class ABC { public: ABC(char *str) {[JAN7] msg = new char[strlen(str) +1]; if (msg != 0) strcpy(msg,str); } void sayIt() { cout << msg << endl; } private: XYZ x1[J8]; char *msg; }; void main() { ABC abc("I Love C++"); abc.sayIt(); } Exception Handeling 12 Put the object into a "deffect" state by setting an internal status… class XYZ { public: XYZ(int i=10) {iVal=i;} private: int iVal; }; class ABC { public: ABC(char *str) { status = 0; // Fully constructed. msg = new char[strlen(str) +1]; if (msg != 0) strcpy(msg,str); else status = -1; // Partially constructed. } int isZombie() {return status;} private: XYZ x1; char *msg; int status; }; void main() { ABC abc("Hello World"); if (abc.isZombie()) cout << "Failed to construct\n"; ... ... } Exception Handeling 13 Throw an exception from the constructor passing some information… Destructors are called for automatic objects… class XYZ { public: XYZ(int i=10) {iVal=i;} ~XYZ[JAN9]() { cout << "XYZ destructor\n"; } private: int iVal; }; class ABC { public: ABC(char *str) { msg = new char[strlen(str) +1]; if (msg == 0) throw[JAN10] "No Sapce Left\n"; strcpy(msg,str); } private: XYZ x1; char *msg; }; void main() { try { ABC abc("Hello World"); ... } catch (char *exMsg) { cout << exMsg << endl; } } Exception Handeling 14 class XYZ { public: XYZ(int i=10) {iVal=i;} ~XYZ() { cout << "XYZ destructor\n"; } private: int iVal; }; class ABC { public: ABC(char *str) { msg = new char[strlen(str) +1]; if (msg == 0) throw this; strcpy(msg,str); } void what() { cout << "Error: No space left..." << endl; } private: XYZ x1; char *msg; }; void main() { try { ABC abc("Hello World"); ... } catch (ABC *obj) { obj->what(); } } Exception Handeling 15 Calls Calls Calls Terminates Destructors and Exception Handling The C++ rule is that you must be careful throwing an exception from a destructor … During stack unwinding, all the local objects are destructed... If one of those destructors throws an exception, it confuses the C++ runtime system… ! Should it ignore the “last thrown exception” and continue where it was originally headed? Or vice versa… So the C++ language guarantees that it will call terminate() at this point, and terminate() kills the process. main() catch(); function1() function2() Function3() throw(); EExxcceeppttiioonn RRaaiisseedd EExxcceeppttiioonn N Noott H Haannddlleedd EExxcceeppttiioonn RRaaiisseedd bbyy ddeessttrruuccttoorr Exception Handeling 16 class XYZ { public: XYZ(int i=10) {iVal=i;} ~XYZ() { cout << "Destructor: " << iVal << endl; if (iVal == 20) throw "XYZ Destructor Exception\n"; } private: int iVal; }; void function3() { XYZ x3(30); throw "Exception thrown in function 3"; } void function2() { XYZ x2(20); function3(); } void function1() { XYZ x1(10); function2(); } void main() { try { function1(); } catch ( char * str) { cout << "Exception: " << str << endl; } }