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# Scalar::Util

Perl 5 version 18.2 documentation
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# Scalar::Util

- NAME
- SYNOPSIS
- DESCRIPTION
- DIAGNOSTICS
- KNOWN BUGS
- SEE ALSO
- COPYRIGHT

# 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@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.

Page index

- NAME
- SYNOPSIS
- DESCRIPTION
- DIAGNOSTICS
- KNOWN BUGS
- SEE ALSO
- COPYRIGHT