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Scalar::Util(3pm) User Contributed Perl Documentation Scalar::Util(3pm)
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
Note that for internal reasons, all precompiled regexps ("qr/.../") are blessed
references; thus "ref()" returns the package name string "Regexp" on these but "reftype()"
will return the underlying C structure type of "REGEXP" in all capitals.
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 standard
operations:
$foo = "10";
$dual = isdual($foo); # false
$bar = $foo + 0;
$dual = isdual($foo); # true
The $! variable is commonly dual-valued, though it is also magical in other ways:
$! = 1;
$dual = isdual($!); # true
print("$!\n"); # "Operation not permitted"
CAUTION: This function is not as useful as it may seem. Dualvars are not a distinct
concept in Perl, but a standard internal construct of all scalar values. Almost any value
could be considered as a dualvar by this function through the course of normal operations.
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 if it 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.
Vstrings are not implemented in this 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 AT 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 AT 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.
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