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Scalar::Util(3)	       Perl Programmers	Reference Guide	       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
       would be	so small that being individual extensions would	be wasteful.

       By default "Scalar::Util" does not export any subroutines.

FUNCTIONS FOR REFERENCES
       The following functions all perform some	useful activity	on reference
       values.

   blessed
	   my $pkg = blessed( $ref );

       If $ref is 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"

       Take care when using this function simply as a truth test (such as in
       "if(blessed $ref)...") because the package name "0" is defined yet
       false.

   refaddr
	   my $addr = refaddr( $ref );

       If $ref is reference, the internal memory address of the	referenced
       value is	returned as a plain integer. 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
	   my $type = reftype( $ref );

       If $ref is a reference, the basic Perl type of the variable referenced
       is returned as a	plain string (such as "ARRAY" or "HASH"). Otherwise
       "undef" is returned.

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

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

   weaken
	   weaken( $ref	);

       The lvalue $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, the reference
       will be set to undef. This function mutates the lvalue passed as	its
       argument	and returns no value.

       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.

   unweaken
	   unweaken( $ref );

       Since version 1.36.

       The lvalue "REF"	will be	turned from a weak reference back into a
       normal (strong) reference again.	This function mutates the lvalue
       passed as its argument and returns no value. This undoes	the action
       performed by "weaken".

       This function is	slightly neater	and more convenient than the
       otherwise-equivalent code

	   my $tmp = $REF;
	   undef $REF;
	   $REF	= $tmp;

       (because	in particular, simply assigning	a weak reference back to
       itself does not work to unweaken	it; "$REF = $REF" does not work).

   isweak
	   my $weak = isweak( $ref );

       Returns true if $ref is a weak reference.

	   $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

OTHER FUNCTIONS
   dualvar
	   my $var = 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
	   my $dual = isdual( $var );

       Since version 1.26.

       If $var is a scalar that	has both numeric and string values, 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 a dual-valued variable, it is
       actually	implemented as a magical variable inside the interpreter:

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

       You can capture its numeric and string content using:

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

   isvstring
	   my $vstring = isvstring( $var );

       If $var 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
	   my $isnum = looks_like_number( $var );

       Returns true if perl thinks $var	is a number. See "looks_like_number"
       in perlapi.

   openhandle
	   my $fh = openhandle(	$fh );

       Returns $fh itself if $fh may be	used as	a filehandle and is open, or
       is 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
	   my $ro = readonly( $var );

       Returns true if $var is readonly.

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

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

   set_prototype
	   my $code = set_prototype( $code, $prototype );

       Sets the	prototype of the function given	by the $code reference,	or
       deletes it if $prototype	is "undef". Returns the	$code reference
       itself.

	   set_prototype \&foo,	'$$';

   tainted
	   my $t = tainted( $var );

       Return true if $var is tainted.

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

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.

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.

       Additionally "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.

       Copyright (C) 2004, 2008	 Matthijs van Duin.  All rights	reserved.
       Copyright (C) 2014 cPanel Inc.  All rights reserved.  This program is
       free software; you can redistribute it and/or modify it under the same
       terms as	Perl itself.

perl v5.26.0			  2017-04-19		       Scalar::Util(3)

NAME | SYNOPSIS | DESCRIPTION | FUNCTIONS FOR REFERENCES | OTHER FUNCTIONS | DIAGNOSTICS | KNOWN BUGS | SEE ALSO | COPYRIGHT

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