man Scalar::Util (3): 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.

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 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

    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.
"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

COPYRIGHT

Additionally ``weaken'' and ``isweak'' which are

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.