1Scalar::Util(3pm) Perl Programmers Reference Guide Scalar::Util(3pm)
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6 Scalar::Util - A selection of general-utility scalar subroutines
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9 use Scalar::Util qw(blessed dualvar isweak readonly refaddr reftype tainted
10 weaken isvstring looks_like_number set_prototype);
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13 "Scalar::Util" contains a selection of subroutines that people have
14 expressed would be nice to have in the perl core, but the usage would
15 not really be high enough to warrant the use of a keyword, and the size
16 so small such that being individual extensions would be wasteful.
17 By default "Scalar::Util" does not export any subroutines. The subrou‐
19 tines defined are
20 blessed EXPR
22 If EXPR evaluates to a blessed reference the name of the package
23 that it is blessed into is returned. Otherwise "undef" is returned.
24 $scalar = "foo";
26 $class = blessed $scalar; # undef
27 $ref = [];
29 $class = blessed $ref; # undef
30 $obj = bless [], "Foo";
32 $class = blessed $obj; # "Foo"
33 dualvar NUM, STRING
35 Returns a scalar that has the value NUM in a numeric context and
36 the value STRING in a string context.
37 $foo = dualvar 10, "Hello";
39 $num = $foo + 2; # 12
40 $str = $foo . " world"; # Hello world
41 isvstring EXPR
43 If EXPR is a scalar which was coded as a vstring the result is
44 true.
45 $vs = v49.46.48;
47 $fmt = isvstring($vs) ? "%vd" : "%s"; #true
48 printf($fmt,$vs);
49 isweak EXPR
51 If EXPR is a scalar which is a weak reference the result is true.
52 $ref = \$foo;
54 $weak = isweak($ref); # false
55 weaken($ref);
56 $weak = isweak($ref); # true
57 NOTE: Copying a weak reference creates a normal, strong, reference.
59 $copy = $ref;
61 $weak = isweak($ref); # false
62 looks_like_number EXPR
64 Returns true if perl thinks EXPR is a number. See "looks_like_num‐
65 ber" in perlapi.
66 openhandle FH
68 Returns FH if FH may be used as a filehandle and is open, or FH is
69 a tied handle. Otherwise "undef" is returned.
70 $fh = openhandle(*STDIN); # \*STDIN
72 $fh = openhandle(\*STDIN); # \*STDIN
73 $fh = openhandle(*NOTOPEN); # undef
74 $fh = openhandle("scalar"); # undef
75 readonly SCALAR
77 Returns true if SCALAR is readonly.
78 sub foo { readonly($_[0]) }
80 $readonly = foo($bar); # false
82 $readonly = foo(0); # true
83 refaddr EXPR
85 If EXPR evaluates to a reference the internal memory address of the
86 referenced value is returned. Otherwise "undef" is returned.
87 $addr = refaddr "string"; # undef
89 $addr = refaddr \$var; # eg 12345678
90 $addr = refaddr []; # eg 23456784
91 $obj = bless {}, "Foo";
93 $addr = refaddr $obj; # eg 88123488
94 reftype EXPR
96 If EXPR evaluates to a reference the type of the variable refer‐
97 enced is returned. Otherwise "undef" is returned.
98 $type = reftype "string"; # undef
100 $type = reftype \$var; # SCALAR
101 $type = reftype []; # ARRAY
102 $obj = bless {}, "Foo";
104 $type = reftype $obj; # HASH
105 set_prototype CODEREF, PROTOTYPE
107 Sets the prototype of the given function, or deletes it if PROTO‐
108 TYPE is undef. Returns the CODEREF.
109 set_prototype \&foo, '$$';
111 tainted EXPR
113 Return true if the result of EXPR is tainted
114 $taint = tainted("constant"); # false
116 $taint = tainted($ENV{PWD}); # true if running under -T
117 weaken REF
119 REF will be turned into a weak reference. This means that it will
120 not hold a reference count on the object it references. Also when
121 the reference count on that object reaches zero, REF will be set to
122 undef.
123 This is useful for keeping copies of references , but you don't
125 want to prevent the object being DESTROY-ed at its usual time.
126 {
128 my $var;
129 $ref = \$var;
130 weaken($ref); # Make $ref a weak reference
131 }
132 # $ref is now undef
133 Note that if you take a copy of a scalar with a weakened reference,
135 the copy will be a strong reference.
136 my $var;
138 my $foo = \$var;
139 weaken($foo); # Make $foo a weak reference
140 my $bar = $foo; # $bar is now a strong reference
141 This may be less obvious in other situations, such as "grep()", for
143 instance when grepping through a list of weakened references to
144 objects that may have been destroyed already:
145 @object = grep { defined } @object;
147 This will indeed remove all references to destroyed objects, but
149 the remaining references to objects will be strong, causing the
150 remaining objects to never be destroyed because there is now always
151 a strong reference to them in the @object array.
152
154 There is a bug in perl5.6.0 with UV's that are >= 1<<31. This will show
155 up as tests 8 and 9 of dualvar.t failing
156
158 Copyright (c) 1997-2005 Graham Barr <gbarr@pobox.com>. All rights
159 reserved. This program is free software; you can redistribute it
160 and/or modify it under the same terms as Perl itself.
161 Except weaken and isweak which are
163 Copyright (c) 1999 Tuomas J. Lukka <lukka@iki.fi>. All rights reserved.
165 This program is free software; you can redistribute it and/or modify it
166 under the same terms as perl itself.
167
169 The weaken and isweak subroutines in this module and the patch to the
170 core Perl were written in connection with the APress book `Tuomas J.
171 Lukka's Definitive Guide to Object-Oriented Programming in Perl', to
172 avoid explaining why certain things would have to be done in cumbersome
173 ways.
174perl v5.8.8 2001-09-21 Scalar::Util(3pm)