Re: enum

From:

Roger <rogerngo@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Sun, 19 Aug 2007 23:23:27 -0700

Message-ID:

<1187591007.850200.310520@r23g2000prd.googlegroups.com>

On Aug 19, 11:03 pm, Kai-Uwe Bux <jkherci...@gmx.net> wrote:

Kai-Uwe Bux wrote:

Roger wrote:

enum status { CONTINUE, WON, LOST }

What do we really need to use enum for? I'm not sure what the use of
it is really for... Maybe I just don't quite understand. My textbook
says that these enumeration constants are integers and start at 0 if
you don't assign the first constant in the enum. So, in this case,
CONTINUE is 0, WON is 1, and LOST is 2.

Why cant we just use integers instead? Why enum???

In many cases, you can :-)

I was a little brief here. I meant, in many cases, you can use integer
constants instead of enums:

  int const CONTINUE = 0;
  int const WON = 1;
  int const LOST = 2;

However, there are some tricks that you cannot do with integer constants.
IMHO, the most important feature of enums is that they create honest to
god types and not just aliases for the underlying arithmetic type. That
implies that you can use overload resolution based on the enum type. This
is usefult, for instance, in overloading operator<< to output the enum
values in text form to streams.

#include <iostream>
#include <ostream>
#include <cassert>

enum status { down, up };

std::ostream & operator<< ( std::ostream & ostr,
                            status st ) {
  if ( st == down ) {
    return ( ostr << "down" );
  }
  if ( st == up ) {
    return ( ostr << "up" );
  }
  assert ( false );
}

int main ( void ) {
  status st = up;
  std::cout << up << '\n';
}

Sometimes, it is important or at least useful that constants have a type
of their own. In those case, enums are a nice thing to have. In other
cases, you can of course get away with integers.

And this also should have been "get away with integer constants".

Best

Kai-Uwe Bux

I think I understand this a little bit better now. Here's something I
did under Visual C++ 6.0 (yes, it's very basic... I'm still
learning :) )

#include <iostream.h>

int main() {
    enum Number { ZERO, ONE, TWO };

    cout << "Hello, try inputing 0,1, or 2. Just try!\n";

    int input;
    cin >> input;

    switch (input) {
    case ZERO:
        cout << "ZERO!\n";
        break;
    case ONE:
        cout << "ONE!\n";
        break;
    case TWO:
        cout << "TWO\n";
        break;
    default:
        cout << "TRY AGAIN!\n";
        break;

    }
    return 0;
}

So if the user inputs 0, the program will take it as "ZERO" and so
on...