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.

FUNCTIONS FOR REFERENCES
The following functions all perform some useful activity on reference values.

  $pkg = blessed( $ref )
If $ref is 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.

  $addr = refaddr( $ref )
If $ref is reference the internal memory address of the referenced value is returned as a plain integer. 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

  $type = reftype( $ref )
If $ref is a reference the basic Perl type of the variable referenced is returned as a plain string (such as "ARRAY" or "HASH"). Otherwise "undef" is returned.

    $type = reftype "string";           # undef
    $type = reftype \$var;              # SCALAR
    $type = reftype [];                 # ARRAY

    $obj  = bless {}, "Foo";
    $type = reftype $obj;               # HASH

  weaken( REF )
The lvalue "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, the reference will be set to undef. This function mutates the lvalue passed as its argument and returns no value.

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.

  unweaken( REF )
The lvalue "REF" will be turned from a weak reference back into a normal (strong) reference again. This function mutates the lvalue passed as its argument and returns no value. This undoes the action performed by "weaken()".

This function is slightly neater and more convenient than the otherwise-equivalent code

    my $tmp = $REF;
    undef $REF;
    $REF = $tmp;

(because in particular, simply assigning a weak reference back to itself does not work to unweaken it; "$REF = $REF" does not work).

  $weak = isweak( $ref )
Returns true if $ref is a weak reference.

    $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

OTHER FUNCTIONS

  $var = 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

  $dual = isdual( $var )
If $var is a scalar that has both numeric and string values, 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

  $vstring = isvstring( $var )
If $var 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);

  $isnum = looks_like_number( $var )
Returns true if perl thinks $var is a number. See "looks_like_number" in perlapi.

  $fh = openhandle( $fh )
Returns $fh itself if $fh may be used as a filehandle and is open, or is is a tied handle. Otherwise "undef" is returned.

    $fh = openhandle(*STDIN);           # \*STDIN
    $fh = openhandle(\*STDIN);          # \*STDIN
    $fh = openhandle(*NOTOPEN);         # undef
    $fh = openhandle("scalar");         # undef

  $ro = readonly( $var )
Returns true if $var is readonly.

    sub foo { readonly($_[0]) }

    $readonly = foo($bar);              # false
    $readonly = foo(0);                 # true

  $code = set_prototype( $code, $prototype )
Sets the prototype of the function given by the $code reference, or deletes it if $prototype is "undef". Returns the $code reference itself.

    set_prototype \&foo, '$$';

  $t = tainted( $var )
Return true if $var is tainted.

    $taint = tainted("constant");       # false
    $taint = tainted($ENV{PWD});        # true if running under -T

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.