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Scalar::Util(3perl) Perl Programmers Reference Guide Scalar::Util(3perl)
NAME
Scalar::Util - A selection of general-utility scalar subroutines
SYNOPSIS
use Scalar::Util qw(blessed dualvar isweak readonly refaddr reftype tainted
weaken isvstring looks_like_number set_prototype);
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 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.
$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 variable referenced is returned.
Otherwise "undef" is returned.
$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 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.
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) 1997-2006 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.
Except 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.
BLATANT PLUG
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 certain things would have to be
done in cumbersome ways.
perl v5.10.0 2011-06-30 Scalar::Util(3perl)
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