Question

I’m trying to understand exceptions for my C++ class, and there is something that I don’t understand about this program. Why is there no object created in the exception? Why is there only the class name and parameters provided?

Here: throw( testing ("testing this message"));

#include <iostream>
#include <string>
#include <stdexcept>

using namespace std;


class testing: public runtime_error
{
public:
  testing(const string &message)
    :runtime_error(message) {}
};

int main()
{
  try {
    throw( testing ("testing this message"));
  }
  catch (runtime_error &exception) {
    cerr << exception.what() << endl;
  }
  return 0;
}

Answer 1

You’re creating a temporary testing object. I know the syntax looks a little funny because it’s unnamed. You were expecting to see testing myObj("Testing this message");, but what you get is the same thing without the variable name.

Put a breakpoint in your testing constructor and you’ll see you really are creating an object. It just doesn’t have a name in the scope you created it.

You can do this in a number of places (throws, returns, and as arguments to functions)…

return std::vector<int>(); // return an empty vector of ints

func(MyClass(1, 2, 3)); // passing `func` a `MyClass` constructed with the arguments 1, 2, and 3