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. 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 nonobvious errors.
Noncompliant Code Example
In this noncompliant code example, two enumerators of type
Color are assigned explicit values. It may not be obvious to the programmer that
indigo have been declared to be identical values (6), as are
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. 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:
To prevent the error discussed of the noncompliant code example, enumeration type declarations must take one of the following forms:
- Provide no explicit integer assignments, as in this example:
- Assign a value to the first member only (the rest are then sequential), as in this example:
- Assign a value to all members so any equivalence is explicit, as in this example:
It is also advisable to provide a comment explaining why multiple enumeration type members are being assigned the same value so that future maintainers do not mistakenly identify this form as an error.
Of these three options, providing no explicit integer assignments is the simplest and consequently the preferred approach unless the first enumerator must have a nonzero value.
INT09-C-EX1: In cases where defining an enumeration with two or more enumerators with the same value is intended, the constant expression used to define the value of the duplicate enumerator should reference the enumerator rather than the original enumerator's value. This practice makes the intent clear to both human readers of the code and automated code analysis tools that detect violations of this guideline and would diagnose them otherwise. Note, however, that it does not make it possible to use such enumerators in contexts where unique values are required (such as in a
switch statement, as discussed earlier).
Failing to ensure that constants within an enumeration have unique values can result in unexpected results.
|Axivion Bauhaus Suite|
|LANG.STRUCT.INIT.ENUM||Inconsistent Enumerator Initialization|
|LDRA tool suite|
85 S, 630 S
In an enumerator list, the "=" construct shall not be used to explicitly initialise members other than the first, unless all items are explicitly initialised
|Polyspace Bug Finder|
|CERT C: Rec. INT09-C|
Checks for situations where enumeration constants map to same value (rec. fully covered)
Search for vulnerabilities resulting from the violation of this rule on the CERT website.