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Advanced C++ - Exceptions

Exception handling is programming feature helpful for programmers to handle run time error conditions.

Run time error conditions can be like division by zero, unable to allocate memory due to scarcity, trying to open a file which doesn’t exist etc. There are two types of exceptions. One is Standard exceptions provided by C++ Standard library and other type is User defined exceptions.

Now, to handle any exception, we should understand 3 main block of exception statement which are as follows:
  • throw block - in this block, we throw an exception. The operand of the throw statement decides the type of exception occurred. Once this line is executed, program jumps to catch block to handle the exception occurred.
  • catch block - is the block where we handle exception. This is also called as exception handler and this block is written immediately after the try block as shown in example. This can be written similar to a normal function with at least one argument preceding with catch keyword. We can have more than one handlers with different type of arguments. Then, only the handler that matches its type of argument with the type of the argument mentioned in throw statement is executed. Also, we can have ellipsis (…) as an argument of the handler i.e catch (…). That time, irrespective of type of argument used in throw statement, this handler is executed. We can also have nested try-catch blocks. After catch block is executed, the program execution resumes after the try-catch block but not after the throw statement.
  • try block - is nothing but that block of code where an exception can occur. This block is followed by one or more catch blocks as required to handle exceptions

Let us see few Standard exceptions examples:
  • bad_alloc
    • - this exception is thrown whenever new operator fails to allocate memory. Refer example “StdException.cpp” which is self explanatory.
  • bad_cast
    • - This is thrown when type conversions fails using dynamic_cast
  • bad_typeid
    • -This is thrown by type_id()
  • Bad_exception
    • - This is thrown when any unexpected exception occurs.

User defined exceptions can be programmed as shown in the below example: Output of that example is as follows: In this example, we show how to handle Divide by zero exception. In class DividerWithException we are throwing exception when divisor is equal 0 (we cannot devide by 0). Then in main function we are using try-catch block to handle exception when it occures and operate it. Thanks to that our program does not crash when we are trying to divide by 0, but we can show some message to user to and notify him about wrong data set pushed to DividerWithException object. Code of this example you can find our our GitHub account here: https://github.com/xmementoit/CppAdventureExamples/tree/master/advancedCpp/excpetions

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