abstract struct Enum

• Enum
• Value
• Object

Overview

Enum is the base type of all enums.

An enum is a set of integer values, where each value has an associated name. For example:

enum Color
  Red   # 0
  Green # 1
  Blue  # 2
end

Values start with the value 0 and are incremented by one, but can be overwritten.

To get the underlying value you invoke value on it:

Color::Green.value # => 1

Each constant (member) in the enum has the type of the enum:

typeof(Color::Red) # => Color

Flags enum

An enum can be marked with the @[Flags] annotation. This changes the default values:

@[Flags]
enum IOMode
  Read  # 1
  Write # 2
  Async # 4
end

Additionally, some methods change their behaviour.

Enums from integers

An enum can be created from an integer:

Color.new(1).to_s # => "Green"

Values that don't correspond to enum's constants are allowed: the value will still be of type Color, but when printed you will get the underlying value:

Color.new(10).to_s # => "10"

This method is mainly intended to convert integers from C to enums in Crystal.

Question methods

An enum automatically defines question methods for each member, using String#underscore for the method name.

• In the case of regular enums, this compares by equality (==).
• In the case of flags enums, this invokes includes?.

For example:

color = Color::Blue
color.red?  # => false
color.blue? # => true

mode = IOMode::Read | IOMode::Async
mode.read?  # => true
mode.write? # => false
mode.async? # => true

This is very convenient in case expressions:

case color
when .red?
  puts "Got red"
when .blue?
  puts "Got blue"
end

Included Modules

• Comparable(Enum)

Defined in:

lib/gi-crystal/src/bindings/g_object/enum.cr

Class Method Summary

• .g_type : UInt64

Class Method Detail