Difference between revisions of "cpp/language/enum"

Revision as of 14:54, 4 May 2019

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:

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

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 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;

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

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

Example

#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';
}

col = red
a = l
f = 3

Defect reports

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

See also