Enums

Published Aug 3, 2021Updated Apr 24, 2023
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In C++, an enumeration (enum) is a user defined type where a set of values is specified for a variable and the variable can only take one out of a small set of possible values.

Syntax

The keyword enum is used to define an enumeration.

enum name {const1, const2, ...};

Here’s an example:

enum day {sun, mon, tue, wed, thu, fri, sat};
  • sun would have the value 0
  • mon would have the value 1
  • tue would have the value 2
  • wed would have the value 3
  • thu would have the value 4
  • fri would have the value 5
  • sat would have the value 6

Here’s another example where one of the constants is assigned a value:

enum grade {freshman=9, sophomore, junior, senior};

The enumerator freshman is assigned the value 9. Subsequent enumerators, if they are not given an explicit value, receive the value of the previous enumerator plus one.

So here:

  • freshman would have the value 9
  • sophomore would have the value 10
  • junior would have the value 11
  • senior would have the value 12

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Scoped Enums

Scoped enums are a feature added in C++11.

Scoped enums differ from unscoped enums by:

  • Containing their constants in their namespace.
  • Being strongly-typed.
  • By containing their constants to their namespace, scoped enumerations avoid name conflicts with other enumerations.

Example

enum class WeekDay {sun, mon, tue, wed, thu, fri, sat};
WeekDay day = WeekDay::sun; // Notice that "sun" is prefaced with "Weekday::"
int friday = WeekDay::fri; // error, must cast to an int

Here’s an example where scoped enumerations avoid name collisions:

enum class LogResult {Success, InvalidFileName, WriteError};
enum class SocketResult {Success, InvalidAddrError, TimeoutError};
LogResult logger_result = LogResult::Success;
if (logger_result == LogResult::Success) {} // Because Success is scoped to LogResult, it doesn't collide with SocketResult::Success

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