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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], paragraph 8 [ISO/IEC 14882-2014]:

For an enumeration whose underlying type is fixed, the values of the enumeration are the values of the underlying type. Otherwise, for an enumeration where e_min is the smallest enumerator and e_max is the largest, the values of the enumeration are the values in the range b_min to b_max, defined as follows: Let K be 1 for a two’s complement representation and 0 for a one’s complement or sign-magnitude representation. b_max is the smallest value greater than or equal to max(|e_min| − K, |e_max|) and equal to 2^M − 1, where M is a non-negative integer. b_min is zero if e_min is non-negative and −(b_max + K) otherwise. The size of the smallest bit-field large enough to hold all the values of the enumeration type is max(M, 1) if b_min is zero and M + 1 otherwise. 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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