Index of Section 3 Manual Pages
| Interix / SUA | attributes.3 | Interix / SUA |
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 $s = sub : method { ... };
use attributes (); # optional, to get subroutine declarations
my @attrlist = attributes::get(\&foo);
use attributes 'get'; # import the attributes::get subroutine
my @attrlist = get \&foo;
DESCRIPTION
Subroutine declarations and definitions may optionally
have attribute lists associated with them. (Variable "my"
declarations also may, but see the warning below.) Perl
handles these declarations by passing some information
about the call site and the thing being declared along
with the attribute list to this module. In particular,
the first example above is equivalent to the following:
use attributes __PACKAGE__, \&foo, 'method';
The second example in the synopsis does something equiva-
lent to this:
use attributes ();
my ($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 declara-
tions 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 warn-
ing 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 depre-
cated "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 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 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 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 "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 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 "Making References" in perlref for informa-
tion 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 "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 is 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 "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 declara-
tions; attrs for the obsolescent form of subroutine
attribute specification which this module replaces; "use"
in perlfunc for details on the normal invocation mecha-
nism.
perl v5.8.8 2001-09-21 attributes(3)