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attributes (3)
  • >> attributes (3) ( Solaris man: Библиотечные вызовы )
  • attributes (3) ( Linux man: Библиотечные вызовы )
  • attributes (5) ( Solaris man: Форматы файлов )


         attributes - get/set subroutine or variable attributes


           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;


         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 __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
         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 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 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:
             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
             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.
             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
         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 the section on "Package-
             specific Attribute Handling" below.  Otherwise, only
             built-in attributes will be returned.
             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 "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 the Making References entry in the perlref
         manpage for information on closures.)  Package-specific
         attribute handling may change incompatibly in a future
         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 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 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
             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.
             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 as 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 the Quote and
         Quote-like Operators entry in the perlop manpage.)  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
             locked method
         Some examples of syntactically invalid attribute lists (with
             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


         Default exports
         Available exports
         The routines `get' and `reftype' are exportable.
         Export tags defined
         The `:ALL' tag will get all of the above exports.


         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
         1.  Code:
                 package Canine;
                 package Dog;
                 my Canine $spot : Watchful ;
                 use attributes Canine => \$spot, "Watchful";
         2.  Code:
                 package Felis;
                 my $cat : Nervous;
                 use attributes Felis => \$cat, "Nervous";
         3.  Code:
                 package X;
                 sub foo : locked ;
                 use attributes X => \&foo, "locked";
         4.  Code:
                 package X;
                 sub Y::x : locked { 1 }
                 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 ;
                 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.


         the Private Variables via my() entry in the perlsub manpage
         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.

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