Scalar::Util(3pm)

📦
version 5.42.3 · 16 versions
, in alpine 3.24

🌸 NAME 🌸

Scalar::Util — a selection of general-utility scalar subroutines. yep, this is the Scalar::Util(3pm) page from the Perl Programmers Reference Guide, and we're gonna make it make sense 💖

✨ 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 🦋

okay so Scalar::Util is a little stash of subroutines that people said would be nice to have in the perl core. But the usage wouldn't really be high enough to justify a keyword, and each one is so small that making them individual extensions would be wasteful. So here we are, bestie.

By default, Scalar::Util does not export any subroutines. You gotta ask for them.

🌈 Core Perl builtin Functions 🌈

Many functions in this module have served as the inspiration for a new experimental facility in recent versions of Perl. From various development versions, starting at 5.35.7, equivalent functions to many of these utilities are available in the builtin:: package.

    use Scalar::Util qw(blessed);

    $class = blessed $obj;

    $class = builtin::blessed $obj;  # equivalent

For more information, see the documentation on builtin.

🔥 FUNCTIONS FOR REFERENCES 🔥

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

🎀 blessed

    my $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. lowkey sneaky.

Since Perl version 5.35.7 an equivalent function is available as builtin::blessed.

💎 refaddr

    my $addr = refaddr( $ref );

If $ref is a 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

Since Perl version 5.35.7 an equivalent function is available as builtin::refaddr.

🦋 reftype

    my $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

Note that for internal reasons, all precompiled regexps ("qr/.../") are blessed references; thus ref() returns the package name string "Regexp" on these but reftype() will return the underlying C structure type of "REGEXP" in all capitals. iykyk ✨

Since Perl version 5.35.7 an equivalent function is available as builtin::reftype.

🌸 weaken

    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. no cap, that's important.

Since Perl version 5.35.7 an equivalent function is available as builtin::weaken.

🎀 unweaken

    unweaken( $ref );

Since version 1.36.

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).

Since Perl version 5.35.7 an equivalent function is available as builtin::unweaken.

💜 isweak

    my $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

Since Perl version 5.35.7 an equivalent function is available as builtin::is_weak.

🌟 OTHER FUNCTIONS 🌟

🍓 dualvar

    my $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

✨ isdual

    my $dual = isdual( $var );

Since version 1.26.

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 standard operations:

    $foo = "10";
    $dual = isdual($foo);               # false
    $bar = $foo + 0;
    $dual = isdual($foo);               # true

The $! variable is commonly dual-valued, though it is also magical in other ways:

    $! = 1;
    $dual = isdual($!);                 # true
    print("$!\n");                      # "Operation not permitted"
CAUTION: This function is not as useful as it may seem. Dualvars are not a distinct concept in Perl, but a standard internal construct of all scalar values. Almost any value could be considered as a dualvar by this function through the course of normal operations.

🧚 isvstring

    my $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);

🌈 looks_like_number

    my $isnum = looks_like_number( $var );

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

💅 openhandle

    my $fh = openhandle( $fh );

Returns $fh itself, if $fh may be used as a filehandle and is open, or if it 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

    my $ro = readonly( $var );

Returns true if $var is readonly.

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

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

🎀 set_prototype

    my $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, '$$';

🫶 tainted

    my $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.

💀 Vstrings are not implemented in this 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.

🐛 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 (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.

Additionally 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.

Copyright (C) 2004, 2008 Matthijs van Duin. All rights reserved.

Copyright (C) 2014 cPanel Inc. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

💖 That's the whole Scalar::Util(3pm) page, from perl v5.42.3, dated 2026-11-03. Bye bestie, go weaken some references responsibly! 💅✨

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