ext::List::Util::Perl:Prograemxmte:r:sLiRsetf:e:Util::lib::Scalar::Util(3p)
Scalar::Util - A selection of general-utility scalar subrou- tines
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 peo- ple 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 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); isweak EXPR If EXPR is a scalar which is a weak reference the result is true. perl v5.8.8 2005-02-05 1 ext::List::Util::Perl:Prograemxmte:r:sLiRsetf:e:Util::lib::Scalar::Util(3p) $ref = \$foo; $weak = isweak($ref); # false weaken($ref); $weak = isweak($ref); # true NOTE: Copying a weak reference creates a normal, strong, reference. $copy = $ref; $weak = isweak($ref); # false 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 vari- able referenced is returned. Otherwise "undef" is returned. perl v5.8.8 2005-02-05 2 ext::List::Util::Perl:Prograemxmte:r:sLiRsetf:e:Util::lib::Scalar::Util(3p) $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 des- troyed already: @object = grep { defined } @object; This will indeed remove all references to destroyed perl v5.8.8 2005-02-05 3 ext::List::Util::Perl:Prograemxmte:r:sLiRsetf:e:Util::lib::Scalar::Util(3p) objects, but the remaining references to objects will be strong, causing the remaining objects to never be des- troyed 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) 1997-2005 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.
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 cer- tain things would have to be done in cumbersome ways. perl v5.8.8 2005-02-05 4