运行下面的代码，将出现什么情况？（    ）

double hmean(double a, double b) {

      if (a == -b )

            throw runtime_error("Runtime error occurred");

      return 2.0*a*b/(a + b);

}

int main() {

      double x = 10;

      double y = -10;

      try {

            int result = hmean(x, y);

            cout << "hmean: " << result << endl;

      }

      catch (const runtime_error& e) {

            cout << "Caught: " << e.what() << endl;

      } catch (...) {

            cout << "Caught an unknown exception." << endl;

      }

      return 0;

}