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

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

SYNOPSIS
	   use Scalar::Util qw(blessed dualvar isdual readonly refaddr reftype
			       tainted weaken isweak isvstring looks_like_number
			       set_prototype);
			       # and other useful utils appearing below

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

       isdual EXPR
	   If EXPR is a scalar that is a dualvar, the result is true.

	       $foo = dualvar 86, "Nix";
	       $dual = isdual($foo);		   # true

	   Note that a scalar can be made to have both string and numeric content through numeric
	   operations:

	       $foo = "10";
	       $dual = isdual($foo);		   # false
	       $bar = $foo + 0;
	       $dual = isdual($foo);		   # true

	   Note that although $! appears to be dual-valued variable, it is actually implemented
	   using a tied scalar:

	       $! = 1;
	       print("$!\n");			   # "Operation not permitted"
	       $dual = isdual($!);		   # false

	   You can capture its numeric and string content using:

	       $err = dualvar $!, $!;
	       $dual = isdual($err);		   # true

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

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

       looks_like_number EXPR
	   Returns true if perl thinks EXPR is a number. See "looks_like_number" 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 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

       set_prototype CODEREF, PROTOTYPE
	   Sets the prototype of the given function, or deletes it if PROTOTYPE 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 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

	   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.

       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

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

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

DIAGNOSTICS
       Module use may give one of the following errors during import.

       Weak references are not implemented in the version of perl
	   The version of perl that you are using does not implement weak references, to use
	   "isweak" or "weaken" you will need to use a newer release of perl.

       Vstrings are not implemented in the version of perl
	   The version of perl that you are using does not implement Vstrings, to use "isvstring"
	   you will need to use a newer release of perl.

       "NAME" is only available with the XS version of Scalar::Util
	   "Scalar::Util" contains both perl and C implementations of many of its functions so
	   that those without access to a C compiler may still use it. However some of the
	   functions are only available when a C compiler was available to compile the XS version
	   of the extension.

	   At present that list is: weaken, isweak, dualvar, isvstring, set_prototype

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

SEE ALSO
       List::Util

COPYRIGHT
       Copyright (c) 1997-2007 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.

perl v5.16.3				    2012-12-27				  Scalar::Util(3)
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