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)