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; }

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

Function-Call Stack

function2()

main()

function1()

function2() main()

main()

Stack Unwinding

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

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

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;

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

Grouping Exceptions

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

Example–With Inheritance

try{

...

}

catch (MathError){

}

catch (InputStreamError){

}