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Manual Reference Pages  -  ENUM (3)

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enum - C style enumerated types and bitmask flags in Perl



  use enum qw(Sun Mon Tue Wed Thu Fri Sat);
  # Sun == 0, Mon == 1, etc

  use enum qw(Forty=40 FortyOne Five=5 Six Seven);
  # Yes, you can change the start indexs at any time as in C

  use enum qw(:Prefix_ One Two Three);
  ## Creates Prefix_One, Prefix_Two, Prefix_Three

  use enum qw(:Letters_ A..Z);
  ## Creates Letters_A, Letters_B, Letters_C, ...

  use enum qw(
      :Months_=0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
      :Days_=0   Sun Mon Tue Wed Thu Fri Sat
      :Letters_=20 A..Z
  ## Prefixes can be changed mid list and can have index changes too

  use enum qw(BITMASK:LOCK_ SH EX NB UN);
  ## Creates bitmask constants for LOCK_SH == 1, LOCK_EX == 2,
  ## LOCK_NB == 4, and LOCK_UN == 8.
  ## NOTE: This example is only valid on FreeBSD-2.2.5 however, so dont
  ## actually do this.  Import from Fnctl instead.


The <B>enumB> module is used to define a set of symbolic constants that will typically have ordered numeric values; they are similar to the enum type in the C programming language. The module can also be used to define bitmask constants, which are described in BITMASKS below.

What are enumerations good for? Typical uses would be for giving mnemonic names to indexes of arrays. Such arrays might be a list of months, days, or a return value index from a function such as localtime():

  use enum qw(
      :Months_=0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
      :Days_=0   Sun Mon Tue Wed Thu Fri Sat
      :LC_=0     Sec Min Hour MDay Mon Year WDay YDay Isdst

  if ((localtime)[LC_Mon] == Months_Jan) {
      print "Its January!\n";
  if ((localtime)[LC_WDay] == Days_Fri) {
      print "Its Friday!\n";

This not only reads easier, but can also be typo-checked at compile time when run under <B>use strictB>. That is, if you misspell <B>Days_FriB> as <B>Days_FryB>, you’ll generate a compile error.


The <B>BITMASKB> option allows the easy creation of bitmask constants such as functions like flock() and sysopen() use. These are also very useful for your own code as they allow you to efficiently store many true/false options within a single integer.

    use enum qw(BITMASK: MY_ FOO BAR CAT DOG);

    my $foo = 0;
    $foo |= MY_FOO;
    $foo |= MY_DOG;
    if ($foo & MY_DOG) {
        print "foo has the MY_DOG option set\n";
    if ($foo & (MY_BAR | MY_DOG)) {
        print "foo has either the MY_BAR or MY_DOG option set\n"

    $foo ^= MY_DOG;  ## Turn MY_DOG option off (set its bit to false)

When using bitmasks, remember that you must use the bitwise operators, <B>|B>, <B>&B>, <B>^B>, and <B>~B>. If you try to do an operation like $foo += MY_DOG; and the <B>MY_DOGB> bit has already been set, you’ll end up setting other bits you probably didn’t want to set. You’ll find the documentation for these operators in the <B>perlopB> manpage.

You can set a starting index for bitmasks just as you can for normal <B>enumB> values, but if the given index isn’t a power of 2 it won’t resolve to a single bit and therefor will generate a compile error. Because of this, whenever you set the <B>BITFIELD:B> directive, the index is automatically set to 1. If you wish to go back to normal <B>enumB> mode, use the <B>ENUM:B> directive. Similarly to the <B>BITFIELDB> directive, the <B>ENUM:B> directive resets the index to 0. Here’s an example:

  use enum qw(
      ENUM: NO YES

In this case, <B>BITS_FOO, BITS_BAR, BITS_CAT, and BITS_DOGB> equal 1, 2, 4 and 8 respectively. <B>FALSE and TRUEB> equal 0 and 1. <B>NO and YESB> also equal 0 and 1. And <B>ONE, TWO, FOUR, EIGHT, and SIX_TEENB> equal, you guessed it, 1, 2, 4, 8, and 16.


Enum names can not be the same as method, function, or constant names. This is probably a Good Thing[tm].

No way (that I know of) to cause compile time errors when one of these enum names get redefined. IMHO, there is absolutely no time when redefining a sub is a Good Thing[tm], and should be taken out of the language, or at least have a pragma that can cause it to be a compile time error.

Enumerated types are package scoped just like constants, not block scoped as some other pragma modules are.

It supports A..Z nonsense. Can anyone give me a Real World[tm] reason why anyone would ever use this feature...?


There are a number of modules that can be used to define enumerations: Class::Enum, enum::fields, enum::hash, Readonly::Enum, Object::Enum, Enumeration.

If you’re using Moose, then MooseX::Enumeration may be of interest. Type::Tiny::Enum is part of the Type-Tiny <> distribution.

There are many CPAN modules related to defining constants in Perl; here are some of the best ones: constant, Const::Fast, constant::lexical, constant::our.

Neil Bowers has written a review of CPAN modules for definining constants <>, which covers all such modules.




Originally written by Byron Brummer (ZENIN), now maintained by Neil Bowers <>.

Based on early versions of the <B>constantB> module by Tom Phoenix.

Original implementation of an interface of Tom Phoenix’s design by Benjamin Holzman, for which we borrow the basic parse algorithm layout.


Copyright 1998 (c) Byron Brummer. Copyright 1998 (c) OMIX, Inc.

Permission to use, modify, and redistribute this module granted under the same terms as Perl itself.

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