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

Scalar::Util(3p) Perl Programmers Reference Guide Scalar::Util(3p) #

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

NNAAMMEE #

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

SSYYNNOOPPSSIISS #

     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

DDEESSCCRRIIPPTTIIOONN #

 "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.

CCoorree PPeerrll “"bbuuiillttiinn"” FFuunnccttiioonnss 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.

FFUUNNCCTTIIOONNSS FFOORR RREEFFEERREENNCCEESS #

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

bblleesssseedd 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.

 _S_i_n_c_e _P_e_r_l _v_e_r_s_i_o_n _5_._3_5_._7 an equivalent function is available as
 "builtin::blessed".

rreeffaaddddrr 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

 _S_i_n_c_e _P_e_r_l _v_e_r_s_i_o_n _5_._3_5_._7 an equivalent function is available as
 "builtin::refaddr".

rreeffttyyppee 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.

 _S_i_n_c_e _P_e_r_l _v_e_r_s_i_o_n _5_._3_5_._7 an equivalent function is available as
 "builtin::refaddr".

wweeaakkeenn 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.

 _S_i_n_c_e _P_e_r_l _v_e_r_s_i_o_n _5_._3_5_._7 an equivalent function is available as
 "builtin::weaken".

uunnwweeaakkeenn unweaken( $ref );

 _S_i_n_c_e _v_e_r_s_i_o_n _1_._3_6_.

 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).

 _S_i_n_c_e _P_e_r_l _v_e_r_s_i_o_n _5_._3_5_._7 an equivalent function is available as
 "builtin::unweaken".

iisswweeaakk my $weak = isweak( $ref );

 Returns true if $ref is a weak reference.

     $ref  = \$foo;
     $weak = isweak($ref);               # false
     weaken($ref);
     $weak = isweak($ref);               # true

 NNOOTTEE: Copying a weak reference creates a normal, strong, reference.

     $copy = $ref;
     $weak = isweak($copy);              # false

 _S_i_n_c_e _P_e_r_l _v_e_r_s_i_o_n _5_._3_5_._7 an equivalent function is available as
 "builtin::isweak".

OOTTHHEERR FFUUNNCCTTIIOONNSS #

dduuaallvvaarr 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

iissdduuaall my $dual = isdual( $var );

 _S_i_n_c_e _v_e_r_s_i_o_n _1_._2_6_.

 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"

 CCAAUUTTIIOONN: 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.

iissvvssttrriinngg 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);

llooookkss__lliikkee__nnuummbbeerr my $isnum = looks_like_number( $var );

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

ooppeennhhaannddllee 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

rreeaaddoonnllyy my $ro = readonly( $var );

 Returns true if $var is readonly.

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

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

sseett__pprroottoottyyppee 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, '$$';

ttaaiinntteedd my $t = tainted( $var );

 Return true if $var is tainted.

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

DDIIAAGGNNOOSSTTIICCSS #

 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.

KKNNOOWWNN BBUUGGSS #

 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

SSEEEE AALLSSOO #

 List::Util

CCOOPPYYRRIIGGHHTT #

 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 v5.36.3 2023-02-15 Scalar::Util(3p)