C++ Exception Handling

Exceptions are certain disastrous error conditions that occur during the execution of a program. They could be errors that cause the programs to fail or certain conditions that lead to errors. If these run time errors are not handled by the program, OS handles them and program terminates abruptly, which is not good.

To avoid this, C++ provides Exception Handling mechanism.

The three keywords for Exception Handling are:

Try, Catch and Throw.

The program tries to do something. If it encounters some problem, it throws an exception to another program block which catches the exception.

Ex:

void main()

{

int no1, no2;

try

{

cout << “Enter two nos:”;

cin >> no1 >> no2;

if (no2 == 0)

throw “Divide by zero”;

else

throw no1/no2;

}

catch (char *s)

{

cout << s;

}

catch (int ans)

{

cout << ans;

}

}

We know that divide by zero is an exception. If user enters second no as zero, the program throws an exception, which is caught and an error message is printed else the answer is printed.

Rethrowing an expression from within an exception handler can be done by calling throw, by itself, with no exception. This causes current exception to be passed on to an outer try/catch sequence. An exception can only be rethrown from within a catch block. When an exception is rethrown, it is propagated outward to the next catch block.

Consider following code:

#include <iostream>

using namespace std;

void MyHandler()

{

try

{

throw “hello”;

}

catch (const char*)

{

cout <<”Caught exception inside MyHandlern”;

throw; //rethrow char* out of function

}

}

int main()

{

cout<< “Main start”;

try

{

MyHandler();

}

catch(const char*)

{

cout <<”Caught exception inside Mainn”;

}

cout << “Main end”;

return 0;

}

O/p:

Main start

Caught exception inside MyHandler

Caught exception inside Main

Main end

Thus, exception rethrown by the catch block inside MyHandler() is caught inside main();