Difference between revisions of "cpp/language/enum"

Revision as of 19:35, 27 November 2023

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 has the same size, value representation, and alignment requirements as its underlying type. Furthermore, each value of an enumeration has the same representation as the corresponding value of the underlying type.

An enumeration is (re)declared using the following syntax:

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 enumerations

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

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;

struct S
{
    enum E1 : int {};
    enum E1 : int {}; // error: redeclaration of enumeration,
                      // NOT parsed as a zero-length bit-field of type enum E1
};
 
enum E2 { e1 };
 
void f()
{
    false ? new enum E2 : int(); // OK: 'int' is NOT parsed as the underlying type
}

Enumeration name for linkage purposes

An unnamed enumeration that does not have a typedef name for linkage purposes and that has an enumerator is denoted, for linkage purposes, by its underlying type and its first enumerator; such an enumeration is said to have an enumerator as a name for linkage purposes.

Scoped enumerations

#include <iostream>
 
int main()
{
    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 implicit conversion from scoped enum to int
    int n = static_cast<int>(r); // OK, n = 21
    std::cout << n << '\n'; // prints 21
}

enum byte : unsigned char {}; // byte is a new integer type; see also std::byte (C++17)
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 (copy-list-initialization of a constructor parameter)
A a2 = {byte{42}}; // OK as of C++17
 
void f(byte);
f({42}); // error (copy-list-initialization of a function parameter)
 
enum class Handle : std::uint32_t { Invalid = 0 };
Handle h{42}; // OK as of C++17

enum class fruit { orange, apple };
 
struct S
{
    using enum fruit; // OK: introduces orange and apple into S
};
 
void f()
{
    S s;
    s.orange;  // OK: names fruit::orange
    S::orange; // OK: names fruit::orange
}

enum class fruit { orange, apple };
enum class color { red, orange };
 
void f()
{
    using enum fruit;    // OK
    // using enum color; // error: color::orange and fruit::orange conflict
}

Notes

Values of unscoped enumeration type can be promoted or converted to integral types:

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

Values of integer, floating-point, and enumeration types can be converted to any enumeration type by using static_cast. 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 rwe = static_cast<access_t>(7);
assert((rwe & read) && (rwe & write) && (rwe & exec));
 
access_t x = static_cast<access_t>(8.0); // undefined behavior since CWG 1766
access_t y = static_cast<access_t>(8);   // undefined behavior since CWG 1766
 
enum foo { a = 0, b = UINT_MAX }; // range: [0, UINT_MAX]
foo x = foo(-1); // undefined behavior since CWG 1766,
                 // even if foo's underlying type is unsigned int

Example

#include <cstdint>
#include <iostream>
 
// enum that takes 16 bits
enum smallenum: std::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', // trailing comma only allowed after CWG 518
}; 
 
// 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);
}
 
// The scoped enum (C++11) can be partially emulated in earlier C++ revisions:
 
enum struct E11 { x, y }; // since C++11
 
struct E98 { enum { x, y }; }; // OK in pre-C++11
 
namespace N98 { enum { x, y }; } // OK in pre-C++11
 
struct S98 { static const int x = 0, y = 1; }; // OK in pre-C++11
 
void emu()
{
    std::cout << (static_cast<int>(E11::y) + E98::y + N98::y + S98::y) << '\n'; // 4
}
 
namespace cxx20
{
    enum class long_long_long_name { x, y };
 
    void using_enum_demo()
    {
        std::cout << "C++20 `using enum`: __cpp_using_enum == ";
        switch (auto rnd = []{return long_long_long_name::x;}; rnd())
        {
#if defined(__cpp_using_enum)
            using enum long_long_long_name;
            case x: std::cout << __cpp_using_enum << "; x\n"; break;
            case y: std::cout << __cpp_using_enum << "; y\n"; break;
#else
            case long_long_long_name::x: std::cout << "?; x\n"; break;
            case long_long_long_name::y: std::cout << "?; y\n"; break;
#endif
        }
    }
}
 
int main()
{
    color col = red;
    altitude a;
    a = altitude::low;
 
    std::cout << "col = " << col << '\n'
              << "a = "   << a   << '\n'
              << "f = "   << f   << '\n';
 
    cxx20::using_enum_demo();
}

col = red
a = l
f = 3
C++20 `using enum`: __cpp_using_enum == 201907; x

Defect reports

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

See also