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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  Although 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 non-obvious nonobvious errors.

Noncompliant Code Example

In this noncompliant code example, two enumerators of type Color are assigned explicit values. It could 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 Because 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 };

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";   //* errorError: duplicate label (yellow) */
  case violet: return "violet";   //* errorError: duplicate label (green) */
  }
}

Compliant Solution

To prevent the error discussed above enumeration discussed of the noncompliant code example, enumeration type declarations must take one of the following forms:

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

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

• assign 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 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
};

It is also advisable to provide a comment explaining why multiple enumeration type members are being assigned the same value so that future maintainers don't mistakenly do not mistakenly identify this form as an error.

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

Exceptions

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

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

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

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ISO/IEC TR 24772 "CCB Enumerator issues"

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MISRA Rule 9.3

Bibliography

Image Removed      04. Integers (INT)      INT11-C. Take care when converting from pointer to integer or integer to pointer