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Scalar::Util(3pm)      Perl Programmers Reference Guide      Scalar::Util(3pm)




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(3)


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.



perl v5.26.1                      2017-07-18                 Scalar::Util(3pm)

perl 5.26.1 - Generated Mon Nov 6 18:35:12 CST 2017
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