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



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 infor-
       mation about the call site and the thing being declared along with the attribute list to this module.  In par-
       ticular, 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 decla-
       rations could change in future versions.  They are present for purposes of experimentation with what the seman-
       tics 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 "Pack-
       age-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 exam-
       ple:

           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'".

       Built-in Attributes

       The following are the built-in attributes for subroutines:

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

           Setting this attribute is only meaningful when the subroutine 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 invocation 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 subrou-
           tine is entered.

       method
           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.

       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.

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

       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 han-
       dling 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' han-
       dler 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 receives a single argument, which is a reference to the variable or subroutine for which pack-
           age-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 decla-
           ration.  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 cir-
       cumstance 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 anony-
       mous 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
           locked 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 : locked ;

           Effect:

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

       4.  Code:

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

           Effect:

               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 ;

           Effect:

               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.

SEE ALSO
       "Private Variables via my()" in perlsub and "Subroutine Attributes" in perlsub for details on the basic decla-
       rations; attrs for the obsolescent form of subroutine attribute specification which this module replaces; "use"
       in perlfunc for details on the normal invocation mechanism.



perl v5.8.8                       2001-09-21                     attributes(3)