Loops for enumerated types

Question

I'm currently reading Code Complete by Steve McConnell. In section 12.6, Enumerated Types, he says that we can define first and last entries like limits:

Define the first and last entries of an enumeration for use as loop limits Defining the first and last elements in an enumeration to be Color_First, Color_Last, Country_First, Country_Last, and so on allows you to write a loop that loops through the elements of an enumeration. You set up the enumerated type by using explicit values, as shown here:

Visual Basic Example of Setting First and Last Values in an Enumerated Type

Public Enum Country
 Country_First = 0
 Country_China = 0
 Country_England = 1
 Country_France = 2
 Country_Germany = 3
 Country_India = 4
 Country_Japan = 5
 Country_Usa = 6
 Country_Last = 6
End Enum

Now the Country_First and Country_Last values can be used as loop limits:

Good Visual Basic Example of Looping Through Elements in an Enumeration

' compute currency conversions from US currency to target currency
Dim usaCurrencyConversionRate( Country_Last ) As Single
Dim iCountry As Country
For iCountry = Country_First To Country_Last
 usaCurrencyConversionRate( iCountry ) = ConversionRate( Country_Usa, iCountry )
Next

Why do we need duplicated indices on the edges for enumerated type?

Answer 1

Let's say for some reason you want to iterate over all countries. "That's easy", you say:

for (i = Country_China; i <= Country_Usa; i++) {
  // Do stuff
}

and you write that loop construct at lots of places in your code.

The next day, your product owner comes along and says "Hey EugZ, we're launching in Albania soon, can you make sure the product supports that?". "Sure", you say, and think about adding Country_Albania to your enum. Where do you add Country_Albania?

1. Before Country_China, in order to maintain the alphabetical ordering? You now need to go and rewrite every instance of your loop construct to start at Country_Albania rather than Country_China - and you may well miss one.
2. After Country_Usa, in order to maintain the existing values of the enum? Same problem.
3. You could put Country_Albania in the middle of the list of countries. While this wouldn't break the loop construct, you've implicitly and undocumentedly made Country_China and Country_Usa magic values in the enum which must always be first and last, and all sorts of things will break if anyone ever changes them. Not good.

Adding explicit values for Country_First and Country_Last avoids a lot of these and similar issues; given the small amount of work needed to do it, it's almost a no-brainer.