C++ exception class extension syntax

Question

I am taking a course online and came across some syntax I am not really sure I understand.

    #include <iostream>
    #include <exception>

    using namespace std;

    class derivedexception: public exception {
          virtual const char* what() const throw() {
            return "My derived exception";
  }        
    } myderivedexception;

    int main() {
          try {
            throw myderivedexception;
          }
          catch (exception& e) {
            cout << e.what() << '\n';
          }
    }

My Problem is with:

    virtual const char* what() const throw()

What does this line mean?

also, what is with the

    } myderivedexception;

in the end of the class declaration?

Please have a look at this [C++ books](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) list. — Ron, Mar 18 '18 at 18:11
You'd probably be much better served reading a [C++ book](https://stackoverflow.com/q/388242/241631) for beginners. If this course is showing you the code above without first introducing you to inheritance and virtual functions, it doesn't sound like a good course. — Praetorian, Mar 18 '18 at 18:11
Not catching the exception by `const&` is also strange. What are you going to do with that non-const reference? Modify the exception after it's been caught? That doesn't make sense. Neither does creating the `myderivedexception` object. Forget about this course. — Christian Hackl, Mar 18 '18 at 20:08

score 4 · Answer 1 · 2018-03-18T18:24:00.340

4

This line:

  virtual const char* what() const throw()

says that what is a virtual method that returns a pointer to a constant char (which means it can be used to return a string literal, or the contents of a std::string obtained by calling the string::c_str() function), is itself constant so it doesn't modify any class members, and it does not throw any exceptions.

This line:

   } myderivedexception;

creates an instance of the derivedexception class named myderivedexception. You probably do not want to do this, but instead throw an unnamed exception:

throw derivedexception();

edited Mar 18 '18 at 18:24

answered Mar 18 '18 at 18:10

Got a question about the `virtual`. Is it really needed? Isn't this already the override for the `what` method contained within `std::exception `? – Bee Mar 18 '18 at 18:17
3

No, it's not needed, using it this situation is really a question of style - some people like it. – Mar 18 '18 at 18:19
Ok, so the throw() part means that it does not throw any exceptions? Is there any depth on this? Could you initiate an object in the end of the class declaration as a "default" object? – kaptsea Mar 18 '18 at 18:32
1

"Ok, so the throw() part means that it does not throw any exceptions? " - yes.Why would you want a "default object"? – Mar 18 '18 at 18:34
Tbh, I don't, the lecturer for some weird reason decided to, plus he did not even care to explain why – kaptsea Mar 18 '18 at 18:50

C++ exception class extension syntax

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