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.