Enumerations (C++) | Microsoft Docs

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

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++/CLI and C++/CX).

enum-name:  identifier

enum-specifier:  enum-head { enumerator-listopt }  enum-head { enumerator-list , }

enum-head:  enum-key attribute-specifier-seqopt enum-head-nameopt enum-baseopt

enum-head-name:  nested-name-specifieropt identifier

opaque-enum-declaration:  enum-key attribute-specifier-seqopt enum-head-name enum-baseopt ;

enum-key:  enum  enum class  enum struct

enum-base:  : type-specifier-seq

enumerator-list:  enumerator-definition  enumerator-list , enumerator-definition

enumerator-definition:  enumerator  enumerator = constant-expression

enumerator:  identifier attribute-specifier-seqopt

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 an unscoped enum, the scope is the surrounding scope; in a scoped enum, the scope is the enum-list itself. In a scoped enum, the list may be empty, which in effect defines a new integral type.

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're semantically equivalent in this context.

An enumeration provides context to describe a range of values that are represented as named constants. These named constants 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:

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:

The enumerator Diamonds is assigned the value 1. Subsequent enumerators, if they aren't 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 don't have to be unique. For example, consider this declaration of an unscoped enum Suit:

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

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 isn't a Suit:

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

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 can't be implicitly converted, as shown in the following example:

Notice that the line hand = account_num; still causes the error that occurs with unscoped enums, as shown earlier. It's 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.

Visual Studio 2017 version 15.3 and later (Available with /std:c++17 and later): By defining an enum (regular or scoped) with an explicit underlying type and no enumerators, you can in effect introduce a new integral type that has no implicit conversion to any other type. By using this type instead of its built-in underlying type, you can eliminate the potential for subtle errors caused by inadvertent implicit conversions.

The new type is an exact copy of the underlying type, and therefore has the same calling convention, which means it can be used across ABIs without any performance penalty. No cast is required when variables of the type are initialized by using direct-list initialization. The following example shows how to initialize enums with no enumerators in various contexts:

C Enumeration declarations Keywords

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