Implementing a range data type

Question

I have legacy code that is using an enum as a range and iterating through the range. I need to port this to a new platform and make it safer.

Note: enums are not safe to iterate through as there may be "holes" or gaps between values.

I'm looking for a safe C language pattern for a range type.

For example, given a range (RED, VIOLET, BLUE, GREEN, YELLOW, ORANGE), I want to iterate through each value, like "FOR color IN (RED, VIOLET, BLUE, GREEN, YELLOW, ORANGE)".

When I search SO and the web, I get replies about the range of a data type, such as the range of an integer.

This code will reside on an embedded system that uses an ARM7 processor.

Can there be gaps by default? You could always specify the values of the enums, like `enum { zero = 0, one = 1, two = 2};`, no? — Dan Fego, Jan 05 '12 at 17:52
The problem is when somebody inserts "four = 4" into the `enum`, then undefined behavor will result (or hard to find bugs). — Thomas Matthews, Jan 05 '12 at 18:14

ugoren · Accepted Answer · 2012-01-05T18:02:53.537

3

An enum can do just fine, as long as you let the compiler choose values. This way, there would be no holes. You could, for example, do this:

enum colors {
    FIRST_COLOR=0,
    RED=FIRST_COLOR, VIOLET, BLUE, GREEN, YELLOW, ORANGE,
    NUM_COLORS
}

enum colors color;
for (color=FIRST_COLOR; color<NUM_COLORS; color++) {
    // whatever
}

And a quote from the ANSI C standard:

If the first enumerator has no = , the value of its enumeration constant is 0. Each subsequent enumerator with no = defines its enumeration constant as the value of the constant expression obtained by adding 1 to the value of the previous enumeration constant

edited Jan 05 '12 at 18:02

answered Jan 05 '12 at 17:57

ugoren

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My compiler, IAR, is giving me *"enumerated type mixed with another type"* for `for (color = 0; color < ORANGE; color++)`, but i guess that is because of the 0. – Thomas Matthews Jan 05 '12 at 18:19
I'm used for gcc, where enums and integers can freely be mixed. I actually wish it did warn about such mixes. I think using FIRST_COLOR should satisfy any compiler. – ugoren Jan 05 '12 at 19:48
I really like the `FIRST_COLOR` trick. This makes the iteration loops more readable and less prone to "accidents" when the enum list is modified. – Thomas Matthews Jan 06 '12 at 16:09

Dave · Answer 2 · 2012-01-05T18:00:15.807

You could use an array:

 static Color colors[] = {Red, Violet, Green, Yellow, Orange};
 #define SIZE(x) sizeof(x)/sizeof(*x)

 for(x=0; x<SIZE(colors); x++){
      //do stuff with colors[x]
 }

Otherwise, the usual pattern you see is:

 enum Colors {FirstColor, Red=0, Violet, Green, Yellow, Orange, LastColor};

 for(x=FirstColor; x<LastColor; x++){
     //do stuff with x.
 }

But if the way to iterate them changes, you could always define a function nextcolor(), and:

 for(x=FirstColor;x<LastColor;x=nextcolor(x)){
     //do stuff with x.
 }

Implementing a range data type

2 Answers2