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Scalar::Util(3)
NAME
  Scalar::Util - A selection of general-utility scalar subroutines

SYNOPSIS
      use Scalar::Util qw(blessed dualvar isweak readonly reftype tainted weaken);

DESCRIPTION
  "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 subroutines
  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

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

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

  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

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

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

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

  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 undef.

      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

KNOWN BUGS
  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
  Copyright (c) 1997-2001 Graham Barr <gbarr@pobox.com>. 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 <lukka@iki.fi>. All rights reserved.
  This program is free software; you can redistribute it and/or modify it
  under the same terms as perl itself.

BLATANT PLUG
  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 ways.

Index for Section 3

Alphabetical listing for S

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