This rule is part of MISRA C++:2023.
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Rule 11.6.3 - Within an enumerator list, the value of an implicitly-specified enumeration constant shall be unique
Category: Required
Analysis: Decidable,Single Translation Unit
Rationale
An implicitly-specified enumeration constant has a value one greater than its predecessor. If the first enumeration constant is
implicitly-specified, then its value is zero.
An explicitly-specified enumeration constant has the value of the associated constant expression.
If implicitly-specified and explicitly-specified constants are mixed within an enumeration list, it is possible for values to be duplicated. Such
duplication may be unintentional and may give rise to unexpected behaviour.
This rule requires that any duplication of enumeration constants be made explicit, thus making the intent clear.
Exception
An implicitly-specified enumeration constant may have the same value as an explicitly-specified enumeration constant from the
same enumeration when the explicitly-specified value is defined using only the name of another enumeration constant.
Example
The following examples are compliant as it is clear which enumeration constants have the same value:
enum E1 { A = 3, B, C = 5, D = 5 }; // Compliant
enum E2 { A = 3, B, C, D = C, E = D }; // Compliant by exception
The following examples are non-compliant as enumeration constants have the same implicit values:
enum E3 { A = 3, B, C, D = 4 }; // 'B' and 'D' have the same value
enum E4 { A = 3, B, C, D = B, E }; // 'C' and 'E' have the same value
The following example is non-compliant as the use of B + 1 means the exception does not apply.
enum E5 { A = 3, B, C, D = B + 1 }; // Non-compliant
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