Scalar::Util - A selection of general-utility scalar subroutines
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
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.
Scalar::Util
By default Scalar::Util does not export any subroutines.
The following functions all perform some useful activity on reference values.
If $ref is a blessed reference the name of the package that it is blessed into is returned. Otherwise undef is returned.
$ref
undef
$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.
if(blessed $ref)...
"0"
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
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.
ARRAY
HASH
$type = reftype "string"; # undef $type = reftype \$var; # SCALAR $type = reftype []; # ARRAY $obj = bless {}, "Foo"; $type = reftype $obj; # HASH
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.
REF
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:
grep()
@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.
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().
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).
$REF = $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
Returns a scalar that has the value $num in a numeric context and the value $string in a string context.
$num
$string
$foo = dualvar 10, "Hello"; $num = $foo + 2; # 12 $str = $foo . " world"; # Hello world
If $var is a scalar that has both numeric and string values, the result is true.
$var
$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
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);
Returns true if perl thinks $var is a number. See "looks_like_number" in perlapi.
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
$fh = openhandle(*STDIN); # \*STDIN $fh = openhandle(\*STDIN); # \*STDIN $fh = openhandle(*NOTOPEN); # undef $fh = openhandle("scalar"); # undef
Returns true if $var is readonly.
sub foo { readonly($_[0]) } $readonly = foo($bar); # false $readonly = foo(0); # true
Sets the prototype of the function given by the $code reference, or deletes it if $prototype is undef. Returns the $code reference itself.
$code
$prototype
set_prototype \&foo, '$$';
Return true if $var is tainted.
$taint = tainted("constant"); # false $taint = tainted($ENV{PWD}); # true if running under -T
Module use may give one of the following errors during import.
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.
isweak
weaken
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.
isvstring
NAME
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
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
List::Util
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.
To install List::Util, copy and paste the appropriate command in to your terminal.
cpanm
cpanm List::Util
CPAN shell
perl -MCPAN -e shell install List::Util
For more information on module installation, please visit the detailed CPAN module installation guide.