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Section: Perl Programmers Reference Guide (3)
Updated: 2001-09-21
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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

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

isweak EXPR
If EXPR is a scalar which is a weak reference the result is true.

    $ref  = \$foo;
    $weak = isweak($ref);               # false
    $weak = isweak($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.



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 <>. 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 <>. 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.




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