3 # Copyright (c) 1997-2007 Graham Barr <gbarr@pobox.com>. All rights reserved.
4 # This program is free software; you can redistribute it and/or
5 # modify it under the same terms as Perl itself.
11 require List::Util; # List::Util loads the XS
13 our @ISA = qw(Exporter);
29 our $VERSION = "1.27";
30 $VERSION = eval $VERSION;
34 unless (defined &weaken) {
35 push @EXPORT_FAIL, qw(weaken);
37 unless (defined &isweak) {
38 push @EXPORT_FAIL, qw(isweak isvstring);
40 unless (defined &isvstring) {
41 push @EXPORT_FAIL, qw(isvstring);
45 if (grep { /^(?:weaken|isweak)$/ } @_ ) {
47 Carp::croak("Weak references are not implemented in the version of perl");
50 if (grep { /^isvstring$/ } @_ ) {
52 Carp::croak("Vstrings are not implemented in the version of perl");
64 Scalar::Util - A selection of general-utility scalar subroutines
68 use Scalar::Util qw(blessed dualvar isdual readonly refaddr reftype
69 tainted weaken isweak isvstring looks_like_number
71 # and other useful utils appearing below
75 C<Scalar::Util> contains a selection of subroutines that people have
76 expressed would be nice to have in the perl core, but the usage would
77 not really be high enough to warrant the use of a keyword, and the size
78 so small such that being individual extensions would be wasteful.
80 By default C<Scalar::Util> does not export any subroutines. The
81 subroutines defined are
87 If EXPR evaluates to a blessed reference the name of the package
88 that it is blessed into is returned. Otherwise C<undef> is returned.
91 $class = blessed $scalar; # undef
94 $class = blessed $ref; # undef
96 $obj = bless [], "Foo";
97 $class = blessed $obj; # "Foo"
99 =item dualvar NUM, STRING
101 Returns a scalar that has the value NUM in a numeric context and the
102 value STRING in a string context.
104 $foo = dualvar 10, "Hello";
105 $num = $foo + 2; # 12
106 $str = $foo . " world"; # Hello world
110 If EXPR is a scalar that is a dualvar, the result is true.
112 $foo = dualvar 86, "Nix";
113 $dual = isdual($foo); # true
115 Note that a scalar can be made to have both string and numeric content
116 through numeric operations:
119 $dual = isdual($foo); # false
121 $dual = isdual($foo); # true
123 Note that although C<$!> appears to be dual-valued variable, it is
124 actually implemented using a tied scalar:
127 print("$!\n"); # "Operation not permitted"
128 $dual = isdual($!); # false
130 You can capture its numeric and string content using:
132 $err = dualvar $!, $!;
133 $dual = isdual($err); # true
137 If EXPR is a scalar which was coded as a vstring the result is true.
140 $fmt = isvstring($vs) ? "%vd" : "%s"; #true
143 =item looks_like_number EXPR
145 Returns true if perl thinks EXPR is a number. See
146 L<perlapi/looks_like_number>.
150 Returns FH if FH may be used as a filehandle and is open, or FH is a tied
151 handle. Otherwise C<undef> is returned.
153 $fh = openhandle(*STDIN); # \*STDIN
154 $fh = openhandle(\*STDIN); # \*STDIN
155 $fh = openhandle(*NOTOPEN); # undef
156 $fh = openhandle("scalar"); # undef
158 =item readonly SCALAR
160 Returns true if SCALAR is readonly.
162 sub foo { readonly($_[0]) }
164 $readonly = foo($bar); # false
165 $readonly = foo(0); # true
169 If EXPR evaluates to a reference the internal memory address of
170 the referenced value is returned. Otherwise C<undef> is returned.
172 $addr = refaddr "string"; # undef
173 $addr = refaddr \$var; # eg 12345678
174 $addr = refaddr []; # eg 23456784
176 $obj = bless {}, "Foo";
177 $addr = refaddr $obj; # eg 88123488
181 If EXPR evaluates to a reference the type of the variable referenced
182 is returned. Otherwise C<undef> is returned.
184 $type = reftype "string"; # undef
185 $type = reftype \$var; # SCALAR
186 $type = reftype []; # ARRAY
188 $obj = bless {}, "Foo";
189 $type = reftype $obj; # HASH
191 =item set_prototype CODEREF, PROTOTYPE
193 Sets the prototype of the given function, or deletes it if PROTOTYPE is
194 undef. Returns the CODEREF.
196 set_prototype \&foo, '$$';
200 Return true if the result of EXPR is tainted
202 $taint = tainted("constant"); # false
203 $taint = tainted($ENV{PWD}); # true if running under -T
207 REF will be turned into a weak reference. This means that it will not
208 hold a reference count on the object it references. Also when the reference
209 count on that object reaches zero, REF will be set to undef.
211 This is useful for keeping copies of references , but you don't want to
212 prevent the object being DESTROY-ed at its usual time.
217 weaken($ref); # Make $ref a weak reference
221 Note that if you take a copy of a scalar with a weakened reference,
222 the copy will be a strong reference.
226 weaken($foo); # Make $foo a weak reference
227 my $bar = $foo; # $bar is now a strong reference
229 This may be less obvious in other situations, such as C<grep()>, for instance
230 when grepping through a list of weakened references to objects that may have
231 been destroyed already:
233 @object = grep { defined } @object;
235 This will indeed remove all references to destroyed objects, but the remaining
236 references to objects will be strong, causing the remaining objects to never
237 be destroyed because there is now always a strong reference to them in the
242 If EXPR is a scalar which is a weak reference the result is true.
245 $weak = isweak($ref); # false
247 $weak = isweak($ref); # true
249 B<NOTE>: Copying a weak reference creates a normal, strong, reference.
252 $weak = isweak($copy); # false
258 Module use may give one of the following errors during import.
262 =item Weak references are not implemented in the version of perl
264 The version of perl that you are using does not implement weak references, to use
265 C<isweak> or C<weaken> you will need to use a newer release of perl.
267 =item Vstrings are not implemented in the version of perl
269 The version of perl that you are using does not implement Vstrings, to use
270 C<isvstring> you will need to use a newer release of perl.
272 =item C<NAME> is only available with the XS version of Scalar::Util
274 C<Scalar::Util> contains both perl and C implementations of many of its functions
275 so that those without access to a C compiler may still use it. However some of the functions
276 are only available when a C compiler was available to compile the XS version of the extension.
278 At present that list is: weaken, isweak, dualvar, isvstring, set_prototype
284 There is a bug in perl5.6.0 with UV's that are >= 1<<31. This will
285 show up as tests 8 and 9 of dualvar.t failing
293 Copyright (c) 1997-2007 Graham Barr <gbarr@pobox.com>. All rights reserved.
294 This program is free software; you can redistribute it and/or modify it
295 under the same terms as Perl itself.
297 Except weaken and isweak which are
299 Copyright (c) 1999 Tuomas J. Lukka <lukka@iki.fi>. All rights reserved.
300 This program is free software; you can redistribute it and/or modify it
301 under the same terms as perl itself.