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RedHat 9 (Linux i386) - man page for attributes (redhat section 3pm)

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 han-
       dles 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 cur-
       rent implementation of this feature.

       There are only a few attributes currently handled by Perl itself (or directly by this mod-
       ule, 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'".

       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 immedi-
	   ately after the subroutine 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 subrou-
	   tine 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 appropri-
	   ate 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 pack-
       age.  Thus, a subroutine declaration uses "CODE" as its type, and even a blessed hash ref-
       erence 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 sub-
	   routine 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 as
	   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 unde-
	   fined 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 pack-
       age-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 subrou-
       tine 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, option-
       ally 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 Oper-
       ators" 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 primar-
       ily 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 declarations; attrs for the obsolescent form of subroutine attribute specifi-
       cation which this module replaces; "use" in perlfunc for details on the normal invocation
       mechanism.

perl v5.8.0				    2002-06-01				  attributes(3pm)


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