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Scalar::Util - A selection of general-utility scalar subroutines
use Scalar::Util qw(blessed dualvar isweak readonly refaddr reftype tainted
weaken isvstring looks_like_number set_prototype);
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
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"
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
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);
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
Returns true if perl thinks EXPR is a number. See looks_like_number in the perlapi manpage.
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
$fh = openhandle("scalar"); # undef
=item readonly SCALAR
Returns true if SCALAR is readonly.
sub foo { readonly($_[0]) }
$readonly = foo($bar); # false
$readonly = foo(0); # true
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
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
Sets the prototype of the given function, or deletes it if PROTOTYPE is undef. Returns the CODEREF.
set_prototype \&foo, '$$';
Return true if the result of EXPR is tainted
$taint = tainted("constant"); # false
$taint = tainted($ENV{PWD}); # true if running under -T
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.
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 (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.
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.