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
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 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.
perl v5.26.3 2017-09-08 Scalar::Util(3)
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