[LC++]enum, #define, const, static const, or member const???

Carlo Wood carlo at alinoe.com
Mon Apr 8 22:36:06 UTC 2002

On Mon, Apr 08, 2002 at 05:59:07PM +0930, Mark Phillips wrote:
> Why is this bad?  If all the data handling routines use the
> enums appropriately, the data will all shift perfectly without
> error.

Because it is less maintainable.  How do you know that another
programmer who added code 5 months ago in the 4 megabyte big
source subtree artificial-intelligence/ didn't use your enum
in a slighty different way?  You'd have to check the correct
usage in ALL of the 150 megabytes source code...

The point that I am trying to make is that you shouldn't
use enums when you need to assign values to them in a way
that insertion of new values at any point doesn't work anymore.
Your examples worked and were correct, but you used the edges
of the subgroups.  This would break:

enum foo {
  green = 0,
  yellow = 1,
  blue = 2,
  white = 3

// USE the integer values:

if (color == 2) // we know it is blue
if (color1 - color2 == 1) // Hey, that seemed the best way to detect that color1 == green && color2 == blue
'save array to disk'
char colorNames[] = { "green", "yellow", "blue", "white" };
if (color & 1) { 'a light color' }	// Overloading the least-significant bit

Because each of these uses would break when you insert a new value and change
the values of the enums.

You could do this however:

enum Color {
  // Light Colors
  // Dark colors

> > no.  If your compiler does that then it is non conforming to ISO C++.
> > 
> > int const x = 3;		// Exported.
> > static int const y = 4;		// Not exported.
> Does that mean x will have an address by default?

Yes, pointing to the read-only section.

> I thought that
> unless the address of x is asked for, it will inline the value 3.

It will, x is constant.

> If I'm right then "int const" will behave the same as "static
> int const" except in the case of the address of x being referred
> to.  Is this right?

Hmm, yes - you seem to be right.  I guess I never left away
the 'static' unless I also add an 'extern', in that case
it is exported.  Nevertheless I remember a case where I added
non-static constants to a header file and that gave linker
errors :/ (multiple inclusions).

Carlo Wood <carlo at alinoe.com>

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