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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 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, 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.
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