


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


NAME
       attributes - get/set subroutine or variable attributes

SYNOPSIS
         sub foo : method ;
         my ($x,@y,%z) : Bent ;
         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 equiva-
       lent to this:

           use attributes __PACKAGE__, \$x, 'Bent';
           use attributes __PACKAGE__, \@y, 'Bent';
           use attributes __PACKAGE__, \%z, 'Bent';

       Yes, that's three invocations.

       WARNING: attribute declarations for variables are an
       experimental feature.  The semantics of such declarations
       could change or be removed in future versions.  They are
       present for purposes of experimentation with what the
       semantics ought to be.  Do not rely on the current imple-
       mentation 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 the section on
       "Package-specific Attribute Handling" below.)

       The setting of attributes happens at compile time.  An
       attempt to set an unrecognized attribute is a fatal error.
       (The error is trappable, but it still stops the compila-
       tion 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



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


       or "use warnings 'reserved'".

       Built-in Attributes

       The following are the built-in attributes for subroutines:

       locked
           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 the perlsub manpage.

       There are no built-in attributes for anything other than
       subroutines.

       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 pack-
           age name for a class method lookup, it will include
           the results from a "FETCH_type_ATTRIBUTES" call in its
           return list, as described in the section on "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



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           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 the section on "Pack-
           age-specific Attribute Handling" below.

       Note that these routines are not exported by default.

       Package-specific Attribute Handling

       WARNING: the mechanisms described here are still experi-
       mental.  Do not rely on the current implementation.  In
       particular, there is no provision for applying package
       attributes to 'cloned' copies of subroutines used as clo-
       sures.  (See the Making References entry in the perlref
       manpage 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 inheri-
       tance tree).  Similarly, when "attributes::get" is called
       on a valid reference, a check is made for an appropriate
       attribute 'fetch' handler.  See the section on "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 pos-
       sibility of being blessed into some package.  Thus, a sub-
       routine 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
           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, fol-
           lowed by the list of attributes from the relevant dec-
           laration.  The two fixed arguments are the relevant
           package name and a reference to the declared subrou-
           tine or variable.  The expected return value as a list
           of attributes which were not recognized by this han-
           dler.  Note that this allows for a derived class to
           delegate a call to its base class, and then only



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           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 circum-
       stance 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 com-
       piled (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 specifica-
       tions, 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 the
       Quote and Quote-like Operators entry in the perlop man-
       page.)  The parameter list is passed as it was found, how-
       ever, 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






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


       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 Canine => \$spot, "Watchful";


       2.  Code:

               package Felis;
               my $cat : Nervous;

           Effect:

               use attributes Felis => \$cat, "Nervous";


       3.  Code:

               package X;
               sub foo : locked ;

           Effect:

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


       4.  Code:





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


               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
       the Private Variables via my() entry in the perlsub man-
       page and the Subroutine Attributes entry in the perlsub
       manpage for details on the basic declarations; the attrs
       manpage for the obsolescent form of subroutine attribute
       specification which this module replaces; the use entry in
       the perlfunc manpage for details on the normal invocation
       mechanism.





















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