MirBSD manpage: attributes(3p)

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


     attributes - get/set subroutine or variable attributes


       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;


     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 pur-
     poses of experimentation with what the semantics ought to
     be.  Do not rely on the current implementation of this

     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

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     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 attri-
     butes 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 com-
     pilation 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'".

     Built-in Attributes

     The following are the built-in attributes for subroutines:

         5.005 threads only!  The use of the "locked" attribute
         currently only makes sense if you are using the depre-
         cated "Perl 5.005 threads" implementation of threads.

         Setting this attribute is only meaningful when the sub-
         routine or method is to be called by multiple threads.
         When set on a method subroutine (i.e., one marked with
         the method attribute below), Perl ensures that any invo-
         cation of it implicitly locks its first argument before
         execution.  When set on a non-method subroutine, Perl
         ensures that a lock is taken on the subroutine itself
         before execution.  The semantics of the lock are exactly
         those of one explicitly taken with the "lock" operator
         immediately after the subroutine is entered.

         Indicates that the referenced subroutine is a method.
         This has a meaning when taken together with the locked
         attribute, as described there.  It also means that a
         subroutine so marked will not trigger the "Ambiguous
         call resolved as CORE::%s" warning.

         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.

     For global variables there is "unique" attribute: see "our"
     in perlfunc.

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     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 attri-
         butes, which may be empty.  If passed invalid arguments,
         it uses die() (via Carp::croak) to raise a fatal excep-
         tion.  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

         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 experimen-
     tal.  Do not rely on the current implementation.  In partic-
     ular, 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 attri-
     bute '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.

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     The class methods invoked for modifying and fetching are

         This method receives a single argument, which is a
         reference to the 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.

         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 whi-
     tespace). 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:

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

     Some examples of syntactically invalid attribute lists (with

         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


     Default exports


     Available exports

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

     Export tags defined

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


     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 pri-
     marily useful to see how the "appropriate package" is found
     for the possible method lookups for package-defined attri-

     1.  Code:

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


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

     2.  Code:

             package Felis;
             my $cat : Nervous;


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

     3.  Code:

             package X;
             sub foo : locked ;


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

     4.  Code:

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


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

     5.  Code:

             package X;
             sub foo { 1 }

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

             package Z;
             sub Y::bar : locked ;


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

     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.


     "Private Variables via my()" in perlsub and "Subroutine
     Attributes" in perlsub for details on the basic declara-
     tions; attrs for the obsolescent form of subroutine attri-
     bute specification which this module replaces; "use" in
     perlfunc for details on the normal invocation mechanism.

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