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attributes(3perl)                 Perl Programmers Reference Guide                 attributes(3perl)

NAME
       attributes - get/set subroutine or variable attributes

SYNOPSIS
         sub foo : method ;
         my ($x,@y,%z) : Bent = 1;
         my $s = sub : method { ... };

         use attributes ();    # optional, to get subroutine declarations
         my @attrlist = attributes::get(\&foo);

         use attributes 'get'; # import the attributes::get subroutine
         my @attrlist = get \&foo;

DESCRIPTION
       Subroutine declarations and definitions may optionally have attribute lists associated with
       them.  (Variable "my" declarations also may, but see the warning below.)  Perl handles these
       declarations by passing some information about the call site and the thing being declared
       along with the attribute list to this module.  In particular, the first example above is
       equivalent to the following:

           use attributes __PACKAGE__, \&foo, 'method';

       The second example in the synopsis does something equivalent to this:

           use attributes ();
           my ($x,@y,%z);
           attributes::->import(__PACKAGE__, \$x, 'Bent');
           attributes::->import(__PACKAGE__, \@y, 'Bent');
           attributes::->import(__PACKAGE__, \%z, 'Bent');
           ($x,@y,%z) = 1;

       Yes, that's a lot of expansion.

       WARNING: attribute declarations for variables are still evolving.  The semantics and
       interfaces of such declarations could change in future versions.  They are present for
       purposes of experimentation with what the semantics ought to be.  Do not rely on the current
       implementation of this feature.

       There are only a few attributes currently handled by Perl itself (or directly by this module,
       depending on how you look at it.)  However, package-specific attributes are allowed by an
       extension mechanism.  (See "Package-specific Attribute Handling" below.)

       The setting of subroutine attributes happens at compile time.  Variable attributes in "our"
       declarations are also applied at compile time.  However, "my" variables get their attributes
       applied at run-time.  This means that you have to reach the run-time component of the "my"
       before those attributes will get applied.  For example:

           my $x : Bent = 42 if 0;

       will neither assign 42 to $x nor will it apply the "Bent" attribute to the variable.

       An attempt to set an unrecognized attribute is a fatal error.  (The error is trappable, but
       it still stops the compilation within that "eval".)  Setting an attribute with a name that's
       all lowercase letters that's not a built-in attribute (such as "foo") will result in a
       warning with -w or "use warnings 'reserved'".

   What "import" does
       In the description it is mentioned that

         sub foo : method;

       is equivalent to

         use attributes __PACKAGE__, \&foo, 'method';

       As you might know this calls the "import" function of "attributes" at compile time with these
       parameters: 'attributes', the caller's package name, the reference to the code and 'method'.

         attributes->import( __PACKAGE__, \&foo, 'method' );

       So you want to know what "import" actually does?

       First of all "import" gets the type of the third parameter ('CODE' in this case).
       "attributes.pm" checks if there is a subroutine called "MODIFY_<reftype>_ATTRIBUTES" in the
       caller's namespace (here: 'main').  In this case a subroutine "MODIFY_CODE_ATTRIBUTES" is
       required.  Then this method is called to check if you have used a "bad attribute".  The
       subroutine call in this example would look like

         MODIFY_CODE_ATTRIBUTES( 'main', \&foo, 'method' );

       "MODIFY_<reftype>_ATTRIBUTES" has to return a list of all "bad attributes".  If there are any
       bad attributes "import" croaks.

       (See "Package-specific Attribute Handling" below.)

   Built-in Attributes
       The following are the built-in attributes for subroutines:

       lvalue
           Indicates  that  the referenced subroutine is a valid lvalue and can be assigned to.  The
           subroutine must return a modifiable value such as a  scalar  variable,  as  described  in
           perlsub.

           This  module  allows  one  to set this attribute on a subroutine that is already defined.
           For Perl subroutines (XSUBs are fine), it may or may not do what you want,  depending  on
           the  code  inside the subroutine, with details subject to change in future Perl versions.
           You may run into problems with lvalue context not  being  propagated  properly  into  the
           subroutine,  or  maybe even assertion failures.  For this reason, a warning is emitted if
           warnings are enabled.  In other words, you should only do this if you  really  know  what
           you are doing.  You have been warned.

       method
           Indicates  that  the  referenced subroutine is a method.  A subroutine so marked will not
           trigger the "Ambiguous call resolved as CORE::%s" warning.

       prototype(..)
           The "prototype" attribute is an alternate means of specifying a prototype on a sub.   The
           desired prototype is within the parens.

           The  prototype  from the attribute is assigned to the sub immediately after the prototype
           from the sub, which means that if both are declared at the same time,  the  traditionally
           defined  prototype  is  ignored.   In  other  words,  "sub  foo($$) : prototype(@) {}" is
           indistinguishable from "sub foo(@){}".

           If illegalproto warnings are enabled, the prototype declared inside this  attribute  will
           be sanity checked at compile time.

       const
           This  experimental  attribute,  introduced  in  Perl  5.22,  only  applies  to  anonymous
           subroutines.  It causes the subroutine to be called as soon as the  "sub"  expression  is
           evaluated.  The return value is captured and turned into a constant subroutine.

       The following are the built-in attributes for variables:

       shared
           Indicates  that  the referenced variable can be shared across different threads when used
           in conjunction with the threads and threads::shared modules.

   Available Subroutines
       The following subroutines are available for general use once this module has been loaded:

       get This routine expects a single parameter--a reference to a  subroutine  or  variable.   It
           returns  a  list of attributes, which may be empty.  If passed invalid arguments, it uses
           die() (via Carp::croak) to raise a fatal  exception.   If  it  can  find  an  appropriate
           package   name   for  a  class  method  lookup,  it  will  include  the  results  from  a
           "FETCH_type_ATTRIBUTES" call in  its  return  list,  as  described  in  "Package-specific
           Attribute Handling" below.  Otherwise, only built-in attributes will be returned.

       reftype
           This  routine  expects  a  single parameter--a reference to a subroutine or variable.  It
           returns the built-in type of the referenced variable, ignoring any package into which  it
           might  have  been blessed.  This can be useful for determining the type value which forms
           part of the method names described in "Package-specific Attribute Handling" below.

       Note that these routines are not exported by default.

   Package-specific Attribute Handling
       WARNING: the mechanisms described here are still experimental.  Do not rely  on  the  current
       implementation.   In  particular,  there  is  no provision for applying package attributes to
       'cloned' copies of subroutines used as closures.  (See "Making  References"  in  perlref  for
       information  on  closures.)  Package-specific attribute handling may change incompatibly in a
       future release.

       When an attribute list is present in a declaration,  a  check  is  made  to  see  whether  an
       attribute  'modify'  handler  is  present in the appropriate package (or its @ISA inheritance
       tree).  Similarly, when "attributes::get" is called on a valid reference, a check is made for
       an appropriate attribute 'fetch'  handler.   See  "EXAMPLES"  to  see  how  the  "appropriate
       package" determination works.

       The  handler  names are based on the underlying type of the variable being declared or of the
       reference passed.  Because these  attributes  are  associated  with  subroutine  or  variable
       declarations,  this  deliberately ignores any possibility of being blessed into some package.
       Thus, a subroutine declaration uses "CODE" as its type, and even  a  blessed  hash  reference
       uses "HASH" as its type.

       The class methods invoked for modifying and fetching are these:

       FETCH_type_ATTRIBUTES
           This  method is called with two arguments:  the relevant package name, and a reference to
           a variable or subroutine for which package-defined attributes are desired.  The  expected
           return value is a list of associated attributes.  This list may be empty.

       MODIFY_type_ATTRIBUTES
           This  method  is called with two fixed arguments, followed by the list of attributes from
           the relevant declaration.  The two fixed arguments are the relevant package  name  and  a
           reference to the declared subroutine or variable.  The expected return value is a list of
           attributes  which  were  not  recognized  by  this  handler.  Note that this allows for a
           derived class to delegate a call to its base class, and then only examine the  attributes
           which the base class didn't already handle for it.

           The  call  to this method is currently made during the processing of the declaration.  In
           particular, this means that a subroutine reference will  probably  be  for  an  undefined
           subroutine, even if this declaration is actually part of the definition.

       Calling attributes::get() from within the scope of a null package declaration "package ;" for
       an  unblessed  variable  reference will not provide any starting package name for the 'fetch'
       method lookup.  Thus, this circumstance will not result in a method call for  package-defined
       attributes.   A  named subroutine knows to which symbol table entry it belongs (or originally
       belonged), and it will use the corresponding package.   An  anonymous  subroutine  knows  the
       package  name  into  which  it  was compiled (unless it was also compiled with a null package
       declaration), and so it will use that package name.

   Syntax of Attribute Lists
       An attribute list is a sequence of attribute specifications, separated  by  whitespace  or  a
       colon  (with optional whitespace).  Each attribute specification is a simple name, optionally
       followed by a parenthesised parameter list.  If such a  parameter  list  is  present,  it  is
       scanned past as for the rules for the q() operator.  (See "Quote and Quote-like Operators" in
       perlop.)  The parameter list is passed as it was found, however, and not as per q().

       Some examples of syntactically valid attribute lists:

           switch(10,foo(7,3))  :  expensive
           Ugly('\(") :Bad
           _5x5
           lvalue method

       Some examples of syntactically invalid attribute lists (with annotation):

           switch(10,foo()             # ()-string not balanced
           Ugly('(')                   # ()-string not balanced
           5x5                         # "5x5" not a valid identifier
           Y2::north                   # "Y2::north" not a simple identifier
           foo + bar                   # "+" neither a colon nor whitespace

EXPORTS
   Default exports
       None.

   Available exports
       The routines "get" and "reftype" are exportable.

   Export tags defined
       The ":ALL" tag will get all of the above exports.

EXAMPLES
       Here  are  some  samples  of syntactically valid declarations, with annotation as to how they
       resolve internally into "use attributes" invocations by perl.  These examples  are  primarily
       useful  to  see  how  the  "appropriate package" is found for the possible method lookups for
       package-defined attributes.

       1.  Code:

               package Canine;
               package Dog;
               my Canine $spot : Watchful ;

           Effect:

               use attributes ();
               attributes::->import(Canine => \$spot, "Watchful");

       2.  Code:

               package Felis;
               my $cat : Nervous;

           Effect:

               use attributes ();
               attributes::->import(Felis => \$cat, "Nervous");

       3.  Code:

               package X;
               sub foo : lvalue ;

           Effect:

               use attributes X => \&foo, "lvalue";

       4.  Code:

               package X;
               sub Y::x : lvalue { 1 }

           Effect:

               use attributes Y => \&Y::x, "lvalue";

       5.  Code:

               package X;
               sub foo { 1 }

               package Y;
               BEGIN { *bar = \&X::foo; }

               package Z;
               sub Y::bar : lvalue ;

           Effect:

               use attributes X => \&X::foo, "lvalue";

       This last example is purely for purposes of completeness.  You should not be trying  to  mess
       with the attributes of something in a package that's not your own.

MORE EXAMPLES
       1.
               sub MODIFY_CODE_ATTRIBUTES {
                  my ($class,$code,@attrs) = @_;

                  my $allowed = 'MyAttribute';
                  my @bad = grep { $_ ne $allowed } @attrs;

                  return @bad;
               }

               sub foo : MyAttribute {
                  print "foo\n";
               }

           This  example  runs.   At  compile  time  "MODIFY_CODE_ATTRIBUTES"  is  called.   In that
           subroutine, we check if any attribute is disallowed and we return a list  of  these  "bad
           attributes".

           As we return an empty list, everything is fine.

       2.
             sub MODIFY_CODE_ATTRIBUTES {
                my ($class,$code,@attrs) = @_;

                my $allowed = 'MyAttribute';
                my @bad = grep{ $_ ne $allowed }@attrs;

                return @bad;
             }

             sub foo : MyAttribute Test {
                print "foo\n";
             }

           This  example  is  aborted  at  compile  time  as we use the attribute "Test" which isn't
           allowed.   "MODIFY_CODE_ATTRIBUTES"  returns  a  list  that  contains  a  single  element
           ('Test').

SEE ALSO
       "Private Variables via my()" in perlsub and "Subroutine Attributes" in perlsub for details on
       the basic declarations; "use" in perlfunc for details on the normal invocation mechanism.

perl v5.38.2                                 2026-08-18                            attributes(3perl)