Enumerations in C++ come in two forms: scoped enumerations in which the underlying type is fixed and unscoped enumerations in which the underlying type may or may not be fixed. The range of values that can be represented by either form of enumeration may include enumerator values not specified by the enumeration itself. The range of valid enumeration values for an enumeration type is defined by the C++ Standard, [dcl.enum], in paragraph 8 [ISO/IEC 14882-20172020]:
For an enumeration whose underlying type is fixed, the values of the enumeration are the values of the underlying type. Otherwise, the values of the enumeration are the values representable by a hypothetical integer type with minimal width M such that all enumerators can be represented. The width of the smallest bit-field large enough to hold all the values of the enumeration type is M. It is possible to define an enumeration that has values not defined by any of its enumerators. If the enumerator-list is empty, the values of the enumeration are as if the enumeration had a single enumerator with value 0.
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| [Becker 2009] | Section 7.2, "Enumeration Declarations" |
| [ISO/IEC 14882-20172020] | Subclause 5.2.9, "Static Cast" |
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