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Enumeration types in C map to integers. The normal expectation is that each enumeration type member is distinct. However, there are some nonobvious errors that are commonly made that cause multiple enumeration type members A C enumeration defines a type with a finite set of values represented by identifiers known as enumeration constants, or enumerators. An enumerator is a constant integer expression whose value is representable as an int. While the language allows multiple enumerators of the same type to have the same value, it is a common expectation that all enumerators of the same type have distinct values. However, defining two or more enumerators of the same type to have the same value can lead to some nonobvious errors.

Noncompliant Code Example

In this noncompliant code example, enumeration two enumerators of type members Color are assigned explicit values. It may not be obvious to the programmer that yellow and indigo have been declared to be identical values (6), as are green and violet (7). Probably the least dangerous error that can result from such a definition is attempting to use the enumerators as labels of a switch statement. Since all labels in a switch statement are required to be unique, the following code violates this semantic constraint and is required to be diagnosed by a conforming compiler:

enum Color { red=4, orange, yellow, green, blue, indigo=6, violet };

It may not be obvious to the programmer (though it is fully specified in the language) that yellow and indigo have been declared to be identical values (6), as are green and violet (7).

Compliant Solution

const char* color_name(enum Color col) {
  switch (col) {
  case red: return "red";
  case orange: return "orange";
  case yellow: return "yellow";
  case green: return "green";
  case blue: return "blue";
  case indigo: return "indigo";   // error: duplicate label (yellow)
  case violet: return "violet";   // error: duplicate label (green)
  }
}

Compliant Solution

To prevent the error discussed above enumeration type declarations must take one of the following formsEnumeration type declarations must do one of the following:

• provide no explicit integer assignments, as in this example:

enum Color { red, orange, yellow, green, blue, indigo, violet };

• assign a value to the first member only (the rest are then sequential), as in this example:

enum Color { red=4, orange, yellow, green, blue, indigo, violet };

• assign a value to all members so any equivalence is explicit, as in this example:

enum Color {
  red=4, 
  orange=5, 
  yellow=6, 
  green=7, 
  blue=8, 
  indigo=6, 
  violet=7
};

...

Of these three options, the first — "provide no explicit integer assignments" — is the simplest, and consequently the preferred, approach in the typical case.unless the first enumerator must have non-zero value.

Exceptions

enum Color { red, orange, yellow, green, blue, indigo, violet=indigo };

Risk Assessment

Failing to ensure that constants within an enumeration have unique values can result in unexpected results.

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