C++ Exception Handling Interview Preparation Guide

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 MyHandler\n”;
throw; //rethrow char* out of function
}
}
int main()
{
cout<< “Main start”;
try
{
MyHandler();
}
catch(const char*)
{
cout <<”Caught exception inside Main\n”;
}
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();

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.

Exceptions: 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. So C++ provides Exception Handling mechanism.

Errors that occur due to incorrect input data or incorrect handling of array indices (“out of range index”), memory overflow are known as Synchronous Exceptions

The errors that are caused by events that are beyond control of the program are Asynchronous Exceptions. E.g. Keyboard interrupts

The benefits of Exception Handling are:

1. Program is not terminated abruptly
2. User will understand what errors are occurring in the program.

The three keywords for Exception Handling are:
Try, Catch and Throw.

terminate() is a library function which by default aborts the program
It is called whenever the exception handling mechanism cannot find a handler for a thrown exception.

unexpected() is called when a function with an exception specification throws an exception of a type that is not listed in the exception specification for the function
A function declaration without a specification like throw(char*) may throw any type of exception, and one with throw() is not allowed to throw exceptions at all.

By default unexpected() calls terminate().

Exception handling in C++ is implemented by using the try{} and catch(){} statements.

When a try block throws an exception, the program leaves the try block and enters the catch statement of the catch block.

If they type of the object thrown matches the arg type in the catch block, catch block is executed for handling the code.

If they are not caught, abort() function is executed by default.

When no exception is deteted or thrown then the control goes to the statement below the catch block.

b. throw