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

- NAME
- SYNOPSIS
- DESCRIPTION
- FUNCTIONS FOR REFERENCES
  - blessed
  - refaddr
  - reftype
  - weaken
  - unweaken
  - isweak
- OTHER FUNCTIONS
  - dualvar
  - isdual
  - isvstring
  - looks_like_number
  - openhandle
  - readonly
  - set_prototype
  - tainted
- 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 would be so small 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.

## 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.

## refaddr

        my $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

## 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

## 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.

## 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).

## 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

# 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 numeric operations:

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

Note that although `$!` appears to be a dual-valued variable, it is actually implemented as a magical variable inside the interpreter:

        $! = 1;
        print("$!\n");                      # "Operation not permitted"
        $dual = isdual($!);                 # false

You can capture its numeric and string content using:

        $err = dualvar $!, $!;
        $dual = isdual($err);               # true

## isvstring

        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 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

## 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.

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.

# 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.

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.

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