CS107: C++ Programming

Enumeration Declaration

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 using the following syntax:

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  enum-name (since C++14)

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

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.  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 result is unspecified (until C++17) undefined (since C++17). (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 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

enumclass 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 :unsignedchar{}; // 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)
enumclass Handle :std::uint32_t{ Invalid =0};
Handle h {42};  // OK as of C++17

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

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

#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', // 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);
}
 
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++20usingenum: x