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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 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'".

       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 deprecated "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
	   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 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 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
	   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 declarations; attrs for the
       obsolescent form of subroutine attribute specification which this
       module replaces; "use" in perlfunc for details on the normal invocation
       mechanism.

perl v5.10.0			  2007-12-18			 attributes(3)
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