Difference between revisions of "cpp/language/enum"

Revision as of 10:07, 17 June 2022

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 (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

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, extended integer types with higher conversion rank (in rank order, signed given preference over unsigned)(since C++11). If the underlying type is fixed, the values can be converted to their underlying type (preferred in overload resolution), which can then be promoted.

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 by static_cast or explicit cast, to any enumeration type. If the underlying type is not fixed and the source value is out of range, the behavior is undefined. (The source value, as converted to the enumeration's underlying type if floating-point, is in range if it would fit in 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 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 CWG1766
access_t y = static_cast<access_t>(8);   // undefined behavior since CWG1766
 
enum foo { a = 0, b = UINT_MAX }; // range: [0, UINT_MAX]
foo x = foo(-1); // undefined behavior since CWG1766,
                 // 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;

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
}

Scoped enumerations

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

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 (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

Although the conversion from an out-of-range to an enumeration without fixed underlying type is made undefined behavior by the resolution of CWG issue 1766, currently no compiler performs the required diagnostic for it in constant evaluation.

Example

#include <iostream>
#include <cstdint>
 
// 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 CWG518
}; 
 
// 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