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



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.

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

       locked
           The "locked" attribute has no effect in 5.10.0 and later. It was used as part of the now-removed
           "Perl 5.005 threads".

   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 v5.12.5                                     2012-11-03                                  attributes(3pm)