how to use Exceptions in C++ program?

Question

hey , i am trying to inherit the exception class and make a new class called NonExistingException: i wrote the following code in my h file:

class NonExistingException : public exception
{
public:
    virtual const char* what() const throw()  {return "Exception: could not find 
     Item";}
};

in my code before i am sending something to a function i am writing

try{
    func(); // func is a function inside another class
}
catch(NonExistingException& e)
{
    cout<<e.what()<<endl;
}
catch (exception& e)
{
     cout<<e.what()<<endl;
}

inside func i am throwing an exception but nothing catches it. thanks in advance for your help.

How are you throwing? It should look like this: throw NonExistingVehicleException(); — Loki Astari
– Loki Astari, Commented Sep 22, 2010 at 8:35
You must throw the exception as "throw NonExistingException()", not "throw new NonExistingException()". — avakar
– avakar, Commented Sep 22, 2010 at 8:36
Can you show us the code where you actually throw the exception? — jwueller
– jwueller, Commented Sep 22, 2010 at 8:37
@ sellibitze: Either one will do. You obviously need to read them :-p (Joke) But no it means I am too tiered to think about it and you should read a book about the subject. But Frankly the whole point of C++ is Type and half that battle is getting const correctness done properly. — Loki Astari
– Loki Astari, Commented Sep 22, 2010 at 9:29

Loki Astari · Accepted Answer · 2010-09-22 08:44:16Z

I would do this:

// Derive from std::runtime_error rather than std::exception
// runtime_error's constructor can take a string as parameter
// the standard's compliant version of std::exception can not
// (though some compiler provide a non standard constructor).
//
class NonExistingVehicleException : public std::runtime_error
{
    public:
       NonExistingVehicleException()
         :std::runtime_error("Exception: could not find Item") {}
};

int main()
{
    try
    {
        throw NonExistingVehicleException();
    }
    // Prefer to catch by const reference.
    catch(NonExistingVehicleException const& e)
    {
        std::cout << "NonExistingVehicleException: " << e.what() << std::endl;
    }
    // Try and catch all exceptions
    catch(std::exception const& e)
    {
        std::cout << "std::exception: " << e.what() << std::endl;
    }
    // If you miss any then ... will catch anything left over.
    catch(...)
    {
        std::cout << "Unknown exception: " << std::endl;
        // Re-Throw this one.
        // It was not handled so you want to make sure it is handled correctly by
        // the OS. So just allow the exception to keep propagating.
        throw;

        // Note: I would probably re-throw any exceptions from main
        //       That I did not explicitly handle and correct.
    }
}

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