ext::List::Util::Perl:Progrext::List::Util::lib::Scalar::Util(3p)NAMEScalar::Util - A selection of general-utility scalar subrou-
tines
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 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.
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$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 $obj; # eg 88123488
reftype EXPR
If EXPR evaluates to a reference the type of the vari-
able referenced is returned. Otherwise "undef" is
returned.
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$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
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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.
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
COPYRIGHT
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.
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 cer-
tain things would have to be done in cumbersome ways.
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