attributes(3) Perl Programmers Reference Guide attributes(3)NAMEattributes - get/set subroutine or variable attributesSYNOPSIS
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
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
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 exam
ine 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
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:
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.
2001-02-22 perl v5.6.1 attributes(3)
