Scalar::Util(3)       User Contributed Perl Documentation      Scalar::Util(3)

       Scalar::Util - A selection of general-utility scalar subroutines

           use Scalar::Util qw(blessed dualvar isweak readonly refaddr reftype tainted
                               weaken isvstring looks_like_number set_prototype);

       "Scalar::Util" contains a selection of subroutines that people have
       expressed would be nice to have in the perl core, but the usage would
       not really be high enough to warrant the use of a keyword, and the size
       so small such that being individual extensions would be wasteful.

       By default "Scalar::Util" does not export any subroutines. The subrou-
       tines defined are

       blessed EXPR
           If EXPR evaluates to a blessed reference the name of the package
           that it is blessed into is returned. Otherwise "undef" is returned.

              $scalar = "foo";
              $class  = blessed $scalar;           # undef

              $ref    = [];
              $class  = blessed $ref;              # undef

              $obj    = bless [], "Foo";
              $class  = blessed $obj;              # "Foo"

       dualvar NUM, STRING
           Returns a scalar that has the value NUM in a numeric context and
           the value STRING in a string context.

               $foo = dualvar 10, "Hello";
               $num = $foo + 2;                    # 12
               $str = $foo . " world";             # Hello world

       isvstring EXPR
           If EXPR is a scalar which was coded as a vstring the result is

               $vs   = v49.46.48;
               $fmt  = isvstring($vs) ? "%vd" : "%s"; #true

       isweak EXPR
           If EXPR is a scalar which is a weak reference the result is true.

               $ref  = \$foo;
               $weak = isweak($ref);               # false
               $weak = isweak($ref);               # true

           NOTE: Copying a weak reference creates a normal, strong, reference.

               $copy = $ref;
               $weak = isweak($ref);               # false

       looks_like_number EXPR
           Returns true if perl thinks EXPR is a number. See "looks_like_num-
           ber" in perlapi.

       openhandle FH
           Returns FH if FH may be used as a filehandle and is open, or FH is
           a tied handle. Otherwise "undef" is returned.

               $fh = openhandle(*STDIN);           # \*STDIN
               $fh = openhandle(\*STDIN);          # \*STDIN
               $fh = openhandle(*NOTOPEN);         # undef
               $fh = openhandle("scalar");         # undef

       readonly SCALAR
           Returns true if SCALAR is readonly.

               sub foo { readonly($_[0]) }

               $readonly = foo($bar);              # false
               $readonly = foo(0);                 # true

       refaddr EXPR
           If EXPR evaluates to a reference the internal memory address of the
           referenced value is returned. Otherwise "undef" is returned.

               $addr = refaddr "string";           # undef
               $addr = refaddr \$var;              # eg 12345678
               $addr = refaddr [];                 # eg 23456784

               $obj  = bless {}, "Foo";
               $addr = refaddr $obj;               # eg 88123488

       reftype EXPR
           If EXPR evaluates to a reference the type of the variable refer-
           enced is returned. Otherwise "undef" is returned.

               $type = reftype "string";           # undef
               $type = reftype \$var;              # SCALAR
               $type = reftype [];                 # ARRAY

               $obj  = bless {}, "Foo";
               $type = reftype $obj;               # HASH

       set_prototype CODEREF, PROTOTYPE
           Sets the prototype of the given function, or deletes it if PROTO-
           TYPE is undef. Returns the CODEREF.

               set_prototype \&foo, '$$';

       tainted EXPR
           Return true if the result of EXPR is tainted

               $taint = tainted("constant");       # false
               $taint = tainted($ENV{PWD});        # true if running under -T

       weaken REF
           REF will be turned into a weak reference. This means that it will
           not hold a reference count on the object it references. Also when
           the reference count on that object reaches zero, REF will be set to

           This is useful for keeping copies of references , but you don't
           want to prevent the object being DESTROY-ed at its usual time.

                 my $var;
                 $ref = \$var;
                 weaken($ref);                     # Make $ref a weak reference
               # $ref is now undef

           Note that if you take a copy of a scalar with a weakened reference,
           the copy will be a strong reference.

               my $var;
               my $foo = \$var;
               weaken($foo);                       # Make $foo a weak reference
               my $bar = $foo;                     # $bar is now a strong reference

           This may be less obvious in other situations, such as "grep()", for
           instance when grepping through a list of weakened references to
           objects that may have been destroyed already:

               @object = grep { defined } @object;

           This will indeed remove all references to destroyed objects, but
           the remaining references to objects will be strong, causing the
           remaining objects to never be destroyed because there is now always
           a strong reference to them in the @object array.

       There is a bug in perl5.6.0 with UV's that are >= 1<<31. This will show
       up as tests 8 and 9 of dualvar.t failing


       Copyright (c) 1997-2006 Graham Barr <>. All rights
       reserved.  This program is free software; you can redistribute it
       and/or modify it under the same terms as Perl itself.

       Except weaken and isweak which are

       Copyright (c) 1999 Tuomas J. Lukka <>. All rights reserved.
       This program is free software; you can redistribute it and/or modify it
       under the same terms as perl itself.

       The weaken and isweak subroutines in this module and the patch to the
       core Perl were written in connection  with the APress book `Tuomas J.
       Lukka's Definitive Guide to Object-Oriented Programming in Perl', to
       avoid explaining why certain things would have to be done in cumbersome

perl v5.8.6                       2006-12-10                   Scalar::Util(3)