• NAME
• SYNOPSIS
• DESCRIPTION
• KNOWN BUGS
• COPYRIGHT
• BLATANT PLUG

NAME

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

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 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
      printf($fmt,$vs);

• 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

• 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
      
  =item 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

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) 1997-2004 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.