Scalar::Util - A selection of general-utility scalar subroutines
- 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
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.
The following functions all perform some useful activity on reference values.
- 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.
- 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
- 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( $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.
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:
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( $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
(because in particular, simply assigning a weak reference back to itself does
not work to unweaken it; $REF = $REF
does not work).
- 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
- 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
- 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
- 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);
- my $isnum = looks_like_number( $var );
Returns true if perl thinks $var
is a number. See
looks_like_number in perlapi.
- 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
- my $ro = readonly( $var );
Returns true if $var
is readonly.
- 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, '$$';
- my $t = tainted( $var );
Return true if $var
is tainted.
- $taint = tainted("constant"); # false
- $taint = tainted($ENV{PWD}); # true if running under -T
Module use may give one of the following errors during import.
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.
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
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 (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.