attributes
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 $s = sub : method { ... };
use attributes (); # optional, to get subroutine declarations
my @attrlist = attributes::get(\&foo);
use attributes 'get'; # import the attributes::get subroutine
my @attrlist = get \&foo;
DESCRIPTION
Subroutine declarations and definitions may optionally have attribute
lists associated with them. (Variable "my" declarations also may, but
see the warning below.) Perl handles these declarations by passing
some information about the call site and the thing being declared along
with the attribute list to this module. In particular, the first exam-
ple above is equivalent to the following:
use attributes __PACKAGE__, \&foo, 'method';
The second example in the synopsis does something equivalent to this:
use attributes ();
my ($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 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 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 argu-
ment before execution. When set on a non-method subroutine, Perl
ensures that a lock is taken on the subroutine itself before execu-
tion. The semantics of the lock are exactly those of one explic-
itly 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 perl-
func.
Available Subroutines
The following subroutines are available for general use once this mod-
ule has been loaded:
get This routine expects a single parameter--a reference to a subrou-
tine 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 appro-
priate 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. Other-
wise, only built-in attributes will be returned.
reftype
This routine expects a single parameter--a reference to a subrou-
tine 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 provi-
sion 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 incompat-
ibly 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 deliber-
ately 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 ref-
erence 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 declara-
tion), 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 annota-
tion as to how they resolve internally into "use attributes" invoca-
tions 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 obsoles-
cent form of subroutine attribute specification which this module
replaces; "use" in perlfunc for details on the normal invocation mecha-
nism.
perl v5.8.6 2001-09-21 attributes(3)
