Scalar::Util(3)

NAME
Scalar::Util - A selection of general-utility scalar subroutines

SYNOPSIS
    use Scalar::Util qw(blessed dualvar isweak readonly reftype tainted weaken);

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

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

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

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

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

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) 1997-2001 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 certain things would have to be done in cumbersome ways.