Enumeration declaration

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C++ language
Declarations

An enumeration is a distinct type whose value is restricted to a range of values (see below for details), which may include several explicitly named constants ("enumerators"). The values of the constants are values of an integral type known as the underlying type of the enumeration.

An enumeration is defined by enum-specifier, which appears in decl-specifier-seq of the declaration syntax. The enum-specifier has the following syntax:

enum-key attr(optional) identifier(optional) enum-base(optional)(C++11) { enumerator-list(optional) } (1)
enum-key attr(optional) nested-name-specifier(optional) identifier enum-base(optional) ; (2) (since C++11)
1) Definition of an enumeration type.
2) Opaque enum declaration: defines the enumeration type but not its enumerators: after this declaration, the type is a complete type and its size is known. Note: an explicit specialization declaration of a scoped enumeration member of a class template is the only case where nested-name-specifier appears before identifier (since C++14)
enum-key - one of enum, enum class(since C++11), or enum struct(since C++11)
attr(C++11) - optional sequence of any number of attributes
identifier - the name of the enumeration that's being declared. If present, and if this declaration is a re-declaration, it may be preceded by nested-name-specifier(since C++11): sequence of names and scope-resolution operators ::, ending with scope-resolution operator. The name can be omitted only in unscoped enumeration declarations
enum-base(C++11) - colon (:), followed by a type-specifier-seq that names an integral type (if it is cv-qualified, qualifications are ignored) that will serve as the fixed underlying type for this enumeration type
enumerator-list - comma-separated list of enumerator definitions, each of which is either simply an identifier, which becomes the name of the enumerator, or an identifier with an initializer: identifier = constexpr. In either case, the identifier can be directly followed by an optional attribute specifier sequence. (since C++17)

There are two distinct kinds of enumerations: unscoped enumeration (declared with the enum-key enum) and scoped enumeration (declared with the enum-key enum class or enum struct).

Unscoped enumeration

enum name { enumerator = constexpr , enumerator = constexpr , ... } (1)
enum name : type { enumerator = constexpr , enumerator = constexpr , ... } (2) (since C++11)
enum name : type ; (3) (since C++11)
1) Declares an unscoped enumeration type whose underlying type is not fixed (in this case, the underlying type is an implementation-defined integral type that can represent all enumerator values; this type is not 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).
2) Declares an unscoped enumeration type whose underlying type is fixed.
3) Opaque enum declaration for an unscoped enumeration must specify the underlying type.

Each enumerator becomes a named constant of the enumeration's type (that is, name), visible in the enclosing scope, and can be used whenever constants are required. 

enum Color { red, green, blue };
Color r = red;
switch(r)
{
    case red  : std::cout << "red\n";   break;
    case green: std::cout << "green\n"; break;
    case blue : std::cout << "blue\n";  break;
}

Each enumerator is associated with a value of the underlying type. When initializers are provided in the enumerator-list, the values of enumerators are defined by those initializers. If the first enumerator does not have an initializer, the associated value is zero. For any other enumerator whose definition does not have an initializer, the associated value is the value of the previous enumerator plus one. 

enum Foo { a, b, c = 10, d, e = 1, f, g = f + c };
//a = 0, b = 1, c = 10, d = 11, e = 1, f = 2, g = 12

Values of unscoped enumeration type are implicitly-convertible to integral types. If the underlying type is not fixed, the value is convertible to the first type from the following list able to hold their entire value range: int, unsigned int, long, unsigned long, long long, or unsigned long long. If the underlying type is fixed, the values can be converted to their promoted underlying type. 

enum color { red, yellow, green = 20, blue };
color col = red;
int n = blue; // n == 21

Values of integer, floating-point, and other enumeration types can be converted by static_cast or explicit cast, to any enumeration type. If the underlying type is not fixed, the result is unspecified (until C++17)undefined behavior (since C++17) if the source value (first converted to the enumeration's underlying type if floating-point) is out of range (the range is all values possible for the smallest bit field large enough to hold all enumerators of the target enumeration). Otherwise, the result is the same as the result of implicit conversion to the underlying type.

Note that the value after such conversion may not necessarily equal any of the named enumerators defined for the enumeration. 

enum access_t { read = 1, write = 2, exec = 4 }; // enumerators: 1, 2, 4 range: 0..7
access_t rw = static_cast<access_t>(3);
assert(rw & read && rw & write);

access_t x = static_cast<access_t>(8.0); // undefined behavior since C++17
access_t y = static_cast<access_t>(8); // undefined behavior since C++17

enum foo { a = 0, b = UINT_MAX }; // range: [0, UINT_MAX]
foo x= foo(-1); // undefined behavior since C++17, even if foo's underlying type is unsigned int

The name of an unscoped enumeration may be omitted: such declaration only introduces the enumerators into the enclosing scope: 

enum { a, b, c = 0, d = a + 2 }; // defines a = 0, b = 1, c = 0, d = 2

When an unscoped enumeration is a class member, its enumerators may be accessed using class member access operators . and ->: 

struct X
{
    enum direction { left = 'l', right = 'r' };
};
X x;
X* p = &x;

int a = X::direction::left; // allowed only in C++11 and later
int b = X::left;
int c = x.left;
int d = p->left;

Scoped enumerations

enum struct|class name { enumerator = constexpr , enumerator = constexpr , ... } (1)
enum struct|class name : type { enumerator = constexpr , enumerator = constexpr , ... } (2)
enum struct|class name ; (3)
enum struct|class name : type ; (4)
1) declares a scoped enumeration type whose underlying type is int (the keywords class and struct are exactly equivalent)
2) declares a scoped enumeration type whose underlying type is type
3) opaque enum declaration for a scoped enumeration whose underlying type is int
4) opaque enum declaration for a scoped enumeration whose underlying type is type

Each enumerator becomes a named constant of the enumeration's type (that is, name), which is contained within the scope of the enumeration, and can be accessed using scope resolution operator. There are no implicit conversions from the values of a scoped enumerator to integral types, although static_cast may be used to obtain the numeric value of the enumerator. 

enum class Color { red, green = 20, blue };
Color r = Color::blue;
switch(r)
{
    case Color::red  : std::cout << "red\n";   break;
    case Color::green: std::cout << "green\n"; break;
    case Color::blue : std::cout << "blue\n";  break;
}
// int n = r; // error: no scoped enum to int conversion
int n = static_cast<int>(r); // OK, n = 21
 (since C++11)

Both scoped enumeration types and unscoped enumeration types whose underlying type is fixed can be initialized from an integer without a cast, using list initialization, if all of the following is true:

  • the initialization is direct-list-initialization
  • the initializer list has only a single element
  • the enumeration is either scoped or unscoped with underlying type fixed
  • the conversion is non-narrowing

This makes it possible to introduce new integer types (e.g. SafeInt) that enjoy the same existing calling conventions as their underlying integer types, even on ABIs that penalize passing/returning structures by value. 

enum byte : unsigned char {}; // byte is a new integer type
byte b { 42 }; // OK as of C++17 (direct-list-initialization)
byte c = { 42 }; // error
byte d = byte{ 42 }; // OK as of C++17; same value as b
byte e { -1 }; // error

struct A { byte b; };
A a1 = { { 42 } }; // error
A a2 = { byte{ 42 } }; // OK as of C++17

void f(byte);
f({ 42 }); // error

enum class Handle : std::uint32_t { Invalid = 0 };
Handle h { 42 }; // OK as of C++17
 (since C++17)

Example

Run this code
#include <iostream>

// enum that takes 16 bits
enum smallenum: int16_t
{
    a,
    b,
    c
};


// color may be red (value 0), yellow (value 1), green (value 20), or blue (value 21)
enum color
{
    red,
    yellow,
    green = 20,
    blue
};

// altitude may be altitude::high or altitude::low
enum class altitude: char
{ 
     high='h',
     low='l', // C++11 allows the extra comma
}; 

// the constant d is 0, the constant e is 1, the constant f is 3
enum
{
    d,
    e,
    f = e + 2
};

//enumeration types (both scoped and unscoped) can have overloaded operators
std::ostream& operator<<(std::ostream& os, color c)
{
    switch(c)
    {
        case red   : os << "red";    break;
        case yellow: os << "yellow"; break;
        case green : os << "green";  break;
        case blue  : os << "blue";   break;
        default    : os.setstate(std::ios_base::failbit);
    }
    return os;
}

std::ostream& operator<<(std::ostream& os, altitude al)
{
    return os << static_cast<char>(al);
}

int main()
{
    color col = red;
    altitude a;
    a = altitude::low;

    std::cout << "col = " << col << '\n'
              << "a = "   << a   << '\n'
              << "f = "   << f   << '\n';
}

Output: 

col = red
a = l
f = 3

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
CWG 1638 C++14 grammar of opaque enum declaration prohibited use for template specializations nested-name-specifier permitted

See also

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