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
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
isdual EXPR
If EXPR is a scalar that is a dualvar, 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 dual-valued variable, it is
actually implemented using a tied scalar:
$! = 1;
print("$!\n"); # "Operation not permitted"
$dual = isdual($!); # false
You can capture its numeric and string content using:
$err = dualvar $!, $!;
$dual = isdual($err); # true
isvstring 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);
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.
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;
$weak = isweak($copy); # false
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.
"NAME" is only available with the XS version of Scalar::Util
"Scalar::Util" contains both perl and C implementations of many of
its functions so that those without access to a C compiler may
still use it. However some of the functions are only available when
a C compiler was available to compile the XS version of the
extension.
At present that list is: weaken, isweak, dualvar, isvstring,
set_prototype
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.
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.
perl v5.16.3 2012-12-27 Scalar::Util(3)
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