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, depend-ing depending
ing on how you look at it.) However, package-specific attributes are allowed by an extension mecha-nism. mechanism.
nism. (See "Package-specific Attribute Handling" below.)
The setting of subroutine attributes happens at compile time. Variable attributes in "our" declara-tions declarations
tions 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 multi-ple multiple
ple 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 implemen-tation. implementation.
tation. 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 'mod-ify' 'modify'
ify' 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 declara-tion declaration
tion 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 rele-vant relevant
vant 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 particu-lar, particular,
lar, 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 com-piled compiled
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 specifications, separated by whitespace or a colon (with
optional whitespace). Each attribute specification is a simple name, optionally followed by a paren-thesised parenthesised
thesised 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 v5.8.8 2001-09-21 attributes(3pm)
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