9 Declarations [dcl.dcl]

9.7 Enumerations [enum]

9.7.1 Enumeration declarations [dcl.enum]

An enumeration is a distinct type ([basic.compound]) with named constants.
Its name becomes an enum-name within its scope.
enum class
enum struct
The optional attribute-specifier-seq in the enum-head and the opaque-enum-declaration appertains to the enumeration; the attributes in that attribute-specifier-seq are thereafter considered attributes of the enumeration whenever it is named.
A : following “enum nested-name-specifier identifier” within the decl-specifier-seq of a member-declaration is parsed as part of an enum-base.
[Note 1: 
This resolves a potential ambiguity between the declaration of an enumeration with an enum-base and the declaration of an unnamed bit-field of enumeration type.
[Example 1: struct S { enum E : int {}; enum E : int {}; // error: redeclaration of enumeration }; — end example]
— end note]
The identifier in an enum-head-name is not looked up and is introduced by the enum-specifier or opaque-enum-declaration.
If the enum-head-name of an opaque-enum-declaration contains a nested-name-specifier, the declaration shall be an explicit specialization.
The enumeration type declared with an enum-key of only enum is an unscoped enumeration, and its enumerators are unscoped enumerators.
The enum-keys enum class and enum struct are semantically equivalent; an enumeration type declared with one of these is a scoped enumeration, and its enumerators are scoped enumerators.
The optional enum-head-name shall not be omitted in the declaration of a scoped enumeration.
The type-specifier-seq of an enum-base shall name an integral type; any cv-qualification is ignored.
An opaque-enum-declaration declaring an unscoped enumeration shall not omit the enum-base.
The identifiers in an enumerator-list are declared as constants, and can appear wherever constants are required.
The same identifier shall not appear as the name of multiple enumerators in an enumerator-list.
An enumerator-definition with = gives the associated enumerator the value indicated by the constant-expression.
An enumerator-definition without = gives the associated enumerator the value zero if it is the first enumerator-definition, and the value of the previous enumerator increased by one otherwise.
[Example 2: 
enum { a, b, c=0 }; enum { d, e, f=e+2 }; defines a, c, and d to be zero, b and e to be 1, and f to be 3.
— end example]
The optional attribute-specifier-seq in an enumerator appertains to that enumerator.
An opaque-enum-declaration is either a redeclaration of an enumeration in the current scope or a declaration of a new enumeration.
[Note 2: 
An enumeration declared by an opaque-enum-declaration has a fixed underlying type and is a complete type.
The list of enumerators can be provided in a later redeclaration with an enum-specifier.
— end note]
A scoped enumeration shall not be later redeclared as unscoped or with a different underlying type.
An unscoped enumeration shall not be later redeclared as scoped and each redeclaration shall include an enum-base specifying the same underlying type as in the original declaration.
If an enum-head-name contains a nested-name-specifier, the enclosing enum-specifier or opaque-enum-declaration D shall not inhabit a class scope and shall correspond to one or more declarations nominable in the class, class template, or namespace to which the nested-name-specifier refers ([basic.scope.scope]).
All those declarations shall have the same target scope; the target scope of D is that scope.
Each enumeration defines a type that is different from all other types.
Each enumeration also has an underlying type.
The underlying type can be explicitly specified using an enum-base.
For a scoped enumeration type, the underlying type is int if it is not explicitly specified.
In both of these cases, the underlying type is said to be fixed.
Following the closing brace of an enum-specifier, each enumerator has the type of its enumeration.
If the underlying type is fixed, the type of each enumerator prior to the closing brace is the underlying type and the constant-expression in the enumerator-definition shall be a converted constant expression of the underlying type.
If the underlying type is not fixed, the type of each enumerator prior to the closing brace is determined as follows:
  • If an initializer is specified for an enumerator, the constant-expression shall be an integral constant expression.
    If the expression has unscoped enumeration type, the enumerator has the underlying type of that enumeration type, otherwise it has the same type as the expression.
  • If no initializer is specified for the first enumerator, its type is an unspecified signed integral type.
  • Otherwise the type of the enumerator is the same as that of the preceding enumerator unless the incremented value is not representable in that type, in which case the type is an unspecified integral type sufficient to contain the incremented value.
    If no such type exists, the program is ill-formed.
An enumeration whose underlying type is fixed is an incomplete type until immediately after its enum-base (if any), at which point it becomes a complete type.
An enumeration whose underlying type is not fixed is an incomplete type until the closing } of its enum-specifier, at which point it becomes a complete type.
For an enumeration whose underlying type is not fixed, the underlying type is an integral type that can represent all the enumerator values defined in the enumeration.
If no integral type can represent all the enumerator values, the enumeration is ill-formed.
It is implementation-defined which integral type is used as the underlying type except that the underlying type shall not be larger than int unless the value of an enumerator cannot fit in an int or unsigned int.
If the enumerator-list is empty, the underlying type is as if the enumeration had a single enumerator with value 0.
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 0.83
An enumeration has the same size, value representation, and alignment requirements ([basic.align]) as its underlying type.
Furthermore, each value of an enumeration has the same representation as the corresponding value of the underlying type.
Two enumeration types are layout-compatible enumerations if they have the same underlying type.
The value of an enumerator or an object of an unscoped enumeration type is converted to an integer by integral promotion.
[Example 3: 
enum color { red, yellow, green=20, blue }; color col = red; color* cp = &col; if (*cp == blue) // ... makes color a type describing various colors, and then declares col as an object of that type, and cp as a pointer to an object of that type.
The possible values of an object of type color are red, yellow, green, blue; these values can be converted to the integral values 0, 1, 20, and 21.
Since enumerations are distinct types, objects of type color can be assigned only values of type color.
color c = 1; // error: type mismatch, no conversion from int to color int i = yellow; // OK, yellow converted to integral value 1, integral promotion
Note that this implicit enum to int conversion is not provided for a scoped enumeration: enum class Col { red, yellow, green }; int x = Col::red; // error: no Col to int conversion Col y = Col::red; if (y) { } // error: no Col to bool conversion
— end example]
The name of each unscoped enumerator is also bound in the scope that immediately contains the enum-specifier.
An unnamed enumeration that does not have a typedef name for linkage purposes ([dcl.typedef]) and that has a first enumerator is denoted, for linkage purposes ([basic.link]), by its underlying type and its first enumerator; such an enumeration is said to have an enumerator as a name for linkage purposes.
[Note 3: 
Each unnamed enumeration with no enumerators is a distinct type.
— end note]
[Example 4: enum direction { left='l', right='r' }; void g() { direction d; // OK d = left; // OK d = direction::right; // OK } enum class altitude { high='h', low='l' }; void h() { altitude a; // OK a = high; // error: high not in scope a = altitude::low; // OK } — end example]
This set of values is used to define promotion and conversion semantics for the enumeration type.
It does not preclude an expression of enumeration type from having a value that falls outside this range.