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"

Take care when using this function simply as a truth test (such as in "if(blessed $ref)...") because the package name "0" is defined yet false.

  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.