Stack Unwinding - Object Oriented Programming - Lecture Slides, Slides of Object Oriented Programming

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Object-Oriented Programming (OOP) Lecture No. 44

Example

class DivideByZero { public: DivideByZero() { } }; int Quotient(int a, int b){ if(b == 0){ throw DivideByZero(); } return a / b; }

main Function

int main() { try{ … quot = Quotient(a,b); … } catch(DivideByZero) { … } return 0; }

Stack Unwinding

• The flow control of throw is referred to as stack unwinding
• Stack unwinding is more complex than return statement
• Return can be used to transfer the control to the calling function only
• Stack unwinding can transfer the control to any function in nested function calls

Stack Unwinding

• All the local objects of a executing block are destroyed when an exception is thrown
• Dynamically allocated memory is not destroyed automatically
• If no catch handler catches the exception the function terminate is called, which by default calls function abort

Example

void function1() { ... throw Exception(); … } void function2() {… function1();… } int main() { try{ function2(); } catch( Exception ) { } return 0; }

Function-Call Stack

function2()

main()

function1()

function2() main()

main()

Stack Unwinding

• The stack unwinding is also performed if we have nested try blocks

Example

int main( ) { try { try { throw 1; } catch( float ) { } } catch( int ) { } return 0; }

Stack Unwinding

• Firstly the catch handler with float parameter is tried
• This catch handler will not be executed as its parameter is of different type – no coercion
• Secondly the catch handler with int parameter is tried and executed

Catch Handler

• We can modify this to use the object to carry information about the cause of error
• The object thrown is copied to the object given in the handler
• Use the reference in the catch handler to avoid problem caused by shallow copy

Example

class DivideByZero { int numerator; public: DivideByZero(int i) { numerator = i; } void Print() const{ cout << endl << numerator << “ was divided by zero”; } };

Example

int Quotient(int a, int b) {

if(b == 0){ throw DivideByZero(a); } return a / b;

}

Body of main Function

for ( int i = 0; i < 10; i++ ) { try { GetNumbers(a, b); quot = Quotient(a, b); ... } catch(DivideByZero & obj) { obj.Print(); i--; } } cout << “\nSum of ten quotients is ” << sum;

Output

Enter two integers 10 10 Quotient of 10 and 10 is 1 Enter two integers 10 0 10 was divided by zero ... // there will be sum of exactly ten quotients

Catch Handler

• The object thrown as exception is destroyed when the execution of the catch handler completes

Avoiding too many Catch

Handlers

• There are two ways to catch more then one object in a single catch handler - Use inheritance - Catch every exception

Inheritance of Exceptions

MathError

DivideByZero

IntergerOutOfRange

Grouping Exceptions

try{ ... } catch(DivideByZero){ ... } catch(IntergerOutOfRange){ ... } catch (InputStreamError){ }

Example–With Inheritance

try{

...

}

catch (MathError){

}

catch (InputStreamError){

}

Catch Every Exception

• C++ provides a special syntax that allows to catch every object thrown

catch ( ... )

{

//...

}

Re-Throw

• A function can catch an exception and perform partial handling
• Re-throw is a mechanism of throw the exception again after partial handling

throw; /without any expression/

Example

int main ( ) { try { Function(); } catch(Exception&) { ... } return 0; }

Example

void Function( ) { try { /Code that might throw an Exception/ } catch(Exception&){ if( can_handle_completely ) { // handle exception } else { // partially handle exception throw; //re-throw exception } } }

Order of Handlers

• Order of the more then one catch handlers can cause logical errors when using inheritance or catch all