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C++ Enumeration Declarations

An enumeration is a user-defined type that consists of a set of named integral constants that are known as enumerators.

Note Note

This article covers the ISO Standard C++ Language enum type and the scoped (or strongly-typed) enum class type which is introduced in C++11. For information about the public enum class or private enum class types in C++/CLI and C++/CX, see enum class (C++ Component Extensions).

// unscoped enum:
enum [identifier] [: type]

{enum-list}; 

// scoped enum:
enum [class|struct] 
[identifier] [: type] 
{enum-list};

identifier

The type name given to the enumeration.

type

The underlying type of the enumerators; all enumerators have the same underlying type. May be any integral type.

enum-list

Comma-separated list of the enumerators in the enumeration. Every enumerator or variable name in the scope must be unique. However, the values can be duplicated. In a unscoped enum, the scope is the surrounding scope; in a scoped enum, the scope is the enum-list itself.

class

By using this keyword in the declaration, you specify the enum is scoped, and an identifier must be provided. You can also use the struct keyword in place of class, as they are semantically equivalent in this context.

An enumeration provides context to describe a range of values which are represented as named constants and are also called enumerators. In the original C and C++ enum types, the unqualified enumerators are visible throughout the scope in which the enum is declared. In scoped enums, the enumerator name must be qualified by the enum type name. The following example demonstrates this basic difference between the two kinds of enums:

namespace CardGame_Scoped
{
    enum class Suit { Diamonds, Hearts, Clubs, Spades };

    void PlayCard(Suit suit)
    {
        if (suit == Suit::Clubs) // Enumerator must be qualified by enum type
        { /*...*/}
    }
}

namespace CardGame_NonScoped
{
    enum Suit { Diamonds, Hearts, Clubs, Spades };

    void PlayCard(Suit suit)
    {
        if (suit == Clubs) // Enumerator is visible without qualification
        { /*...*/
        }
    }
}

Every name in an enumeration is assigned an integral value that corresponds to its place in the order of the values in the enumeration. By default, the first value is assigned 0, the next one is assigned 1, and so on, but you can explicitly set the value of an enumerator, as shown here:

enum Suit { Diamonds = 1, Hearts, Clubs, Spades };

The enumerator Diamonds is assigned the value 1. Subsequent enumerators, if they are not given an explicit value, receive the value of the previous enumerator plus one. In the previous example, Hearts would have the value 2, Clubs would have 3, and so on.

Every enumerator is treated as a constant and must have a unique name within the scope where the enum is defined (for unscoped enums) or within the enum itself (for scoped enums). The values given to the names do not have to be unique. For example, if the declaration of a unscoped enum Suit is this:

enum Suit { Diamonds = 5, Hearts, Clubs = 4, Spades };

Then the values of Diamonds, Hearts, Clubs, and Spades are 5, 6, 4, and 5, respectively. Notice that 5 is used more than once; this is allowed even though it may not be intended. These rules are the same for scoped enums.

Casting rules

Unscoped enum constants can be implicitly converted to int, but an int is never implicitly convertible to an enum value. The following example shows what happens if you try to assign hand a value that is not a Suit:

int account_num = 135692;
Suit hand;
hand = account_num; // error C2440: '=' : cannot convert from 'int' to 'Suit'

A cast is required to convert an int to a scoped or unscoped enumerator. However, you can promote a unscoped enumerator to an integer value without a cast.

int account_num = Hearts; //OK if Hearts is in a unscoped enum

Using implicit conversions in this way can lead to unintended side-effects. To help eliminate programming errors associated with unscoped enums, scoped enum values are strongly typed. Scoped enumerators must be qualified by the enum type name (identifier) and cannot be implicitly converted, as shown in the following example:

namespace ScopedEnumConversions
{
    enum class Suit { Diamonds, Hearts, Clubs, Spades };
 
    void AttemptConversions()
    {
        Suit hand; 
        hand = Clubs; // error C2065: 'Clubs' : undeclared identifier
        hand = Suit::Clubs; //Correct.
        int account_num = 135692;
        hand = account_num; // error C2440: '=' : cannot convert from 'int' to 'Suit'
        hand = static_cast<Suit>(account_num); // OK, but probably a bug!!!

        account_num = Suit::Hearts; // error C2440: '=' : cannot convert from 'Suit' to 'int'
        account_num = static_cast<int>(Suit::Hearts); // OK
}

Notice that the line hand = account_num; still causes the error that occurs with unscoped enums, as shown earlier. It is allowed with an explicit cast. However, with scoped enums, the attempted conversion in the next statement, account_num = Suit::Hearts;, is no longer allowed without an explicit cast.

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