1 | =head1 NAME
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2 | X<reference> X<pointer> X<data structure> X<structure> X<struct>
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3 |
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4 | perlref - Perl references and nested data structures
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5 |
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6 | =head1 NOTE
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7 |
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8 | This is complete documentation about all aspects of references.
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9 | For a shorter, tutorial introduction to just the essential features,
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10 | see L<perlreftut>.
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11 |
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12 | =head1 DESCRIPTION
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13 |
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14 | Before release 5 of Perl it was difficult to represent complex data
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15 | structures, because all references had to be symbolic--and even then
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16 | it was difficult to refer to a variable instead of a symbol table entry.
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17 | Perl now not only makes it easier to use symbolic references to variables,
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18 | but also lets you have "hard" references to any piece of data or code.
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19 | Any scalar may hold a hard reference. Because arrays and hashes contain
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20 | scalars, you can now easily build arrays of arrays, arrays of hashes,
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21 | hashes of arrays, arrays of hashes of functions, and so on.
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22 |
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23 | Hard references are smart--they keep track of reference counts for you,
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24 | automatically freeing the thing referred to when its reference count goes
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25 | to zero. (Reference counts for values in self-referential or
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26 | cyclic data structures may not go to zero without a little help; see
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27 | L<perlobj/"Two-Phased Garbage Collection"> for a detailed explanation.)
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28 | If that thing happens to be an object, the object is destructed. See
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29 | L<perlobj> for more about objects. (In a sense, everything in Perl is an
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30 | object, but we usually reserve the word for references to objects that
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31 | have been officially "blessed" into a class package.)
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32 |
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33 | Symbolic references are names of variables or other objects, just as a
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34 | symbolic link in a Unix filesystem contains merely the name of a file.
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35 | The C<*glob> notation is something of a symbolic reference. (Symbolic
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36 | references are sometimes called "soft references", but please don't call
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37 | them that; references are confusing enough without useless synonyms.)
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38 | X<reference, symbolic> X<reference, soft>
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39 | X<symbolic reference> X<soft reference>
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40 |
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41 | In contrast, hard references are more like hard links in a Unix file
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42 | system: They are used to access an underlying object without concern for
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43 | what its (other) name is. When the word "reference" is used without an
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44 | adjective, as in the following paragraph, it is usually talking about a
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45 | hard reference.
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46 | X<reference, hard> X<hard reference>
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47 |
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48 | References are easy to use in Perl. There is just one overriding
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49 | principle: Perl does no implicit referencing or dereferencing. When a
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50 | scalar is holding a reference, it always behaves as a simple scalar. It
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51 | doesn't magically start being an array or hash or subroutine; you have to
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52 | tell it explicitly to do so, by dereferencing it.
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53 |
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54 | =head2 Making References
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55 | X<reference, creation> X<referencing>
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56 |
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57 | References can be created in several ways.
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58 |
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59 | =over 4
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60 |
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61 | =item 1.
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62 | X<\> X<backslash>
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63 |
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64 | By using the backslash operator on a variable, subroutine, or value.
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65 | (This works much like the & (address-of) operator in C.)
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66 | This typically creates I<another> reference to a variable, because
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67 | there's already a reference to the variable in the symbol table. But
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68 | the symbol table reference might go away, and you'll still have the
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69 | reference that the backslash returned. Here are some examples:
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70 |
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71 | $scalarref = \$foo;
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72 | $arrayref = \@ARGV;
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73 | $hashref = \%ENV;
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74 | $coderef = \&handler;
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75 | $globref = \*foo;
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76 |
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77 | It isn't possible to create a true reference to an IO handle (filehandle
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78 | or dirhandle) using the backslash operator. The most you can get is a
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79 | reference to a typeglob, which is actually a complete symbol table entry.
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80 | But see the explanation of the C<*foo{THING}> syntax below. However,
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81 | you can still use type globs and globrefs as though they were IO handles.
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82 |
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83 | =item 2.
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84 | X<array, anonymous> X<[> X<[]> X<square bracket>
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85 | X<bracket, square> X<arrayref> X<array reference> X<reference, array>
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86 |
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87 | A reference to an anonymous array can be created using square
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88 | brackets:
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89 |
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90 | $arrayref = [1, 2, ['a', 'b', 'c']];
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91 |
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92 | Here we've created a reference to an anonymous array of three elements
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93 | whose final element is itself a reference to another anonymous array of three
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94 | elements. (The multidimensional syntax described later can be used to
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95 | access this. For example, after the above, C<< $arrayref->[2][1] >> would have
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96 | the value "b".)
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97 |
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98 | Taking a reference to an enumerated list is not the same
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99 | as using square brackets--instead it's the same as creating
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100 | a list of references!
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101 |
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102 | @list = (\$a, \@b, \%c);
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103 | @list = \($a, @b, %c); # same thing!
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104 |
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105 | As a special case, C<\(@foo)> returns a list of references to the contents
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106 | of C<@foo>, not a reference to C<@foo> itself. Likewise for C<%foo>,
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107 | except that the key references are to copies (since the keys are just
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108 | strings rather than full-fledged scalars).
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109 |
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110 | =item 3.
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111 | X<hash, anonymous> X<{> X<{}> X<curly bracket>
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112 | X<bracket, curly> X<brace> X<hashref> X<hash reference> X<reference, hash>
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113 |
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114 | A reference to an anonymous hash can be created using curly
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115 | brackets:
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116 |
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117 | $hashref = {
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118 | 'Adam' => 'Eve',
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119 | 'Clyde' => 'Bonnie',
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120 | };
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121 |
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122 | Anonymous hash and array composers like these can be intermixed freely to
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123 | produce as complicated a structure as you want. The multidimensional
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124 | syntax described below works for these too. The values above are
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125 | literals, but variables and expressions would work just as well, because
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126 | assignment operators in Perl (even within local() or my()) are executable
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127 | statements, not compile-time declarations.
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128 |
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129 | Because curly brackets (braces) are used for several other things
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130 | including BLOCKs, you may occasionally have to disambiguate braces at the
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131 | beginning of a statement by putting a C<+> or a C<return> in front so
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132 | that Perl realizes the opening brace isn't starting a BLOCK. The economy and
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133 | mnemonic value of using curlies is deemed worth this occasional extra
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134 | hassle.
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135 |
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136 | For example, if you wanted a function to make a new hash and return a
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137 | reference to it, you have these options:
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138 |
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139 | sub hashem { { @_ } } # silently wrong
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140 | sub hashem { +{ @_ } } # ok
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141 | sub hashem { return { @_ } } # ok
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142 |
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143 | On the other hand, if you want the other meaning, you can do this:
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144 |
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145 | sub showem { { @_ } } # ambiguous (currently ok, but may change)
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146 | sub showem { {; @_ } } # ok
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147 | sub showem { { return @_ } } # ok
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148 |
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149 | The leading C<+{> and C<{;> always serve to disambiguate
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150 | the expression to mean either the HASH reference, or the BLOCK.
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151 |
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152 | =item 4.
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153 | X<subroutine, anonymous> X<subroutine, reference> X<reference, subroutine>
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154 | X<scope, lexical> X<closure> X<lexical> X<lexical scope>
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155 |
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156 | A reference to an anonymous subroutine can be created by using
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157 | C<sub> without a subname:
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158 |
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159 | $coderef = sub { print "Boink!\n" };
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160 |
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161 | Note the semicolon. Except for the code
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162 | inside not being immediately executed, a C<sub {}> is not so much a
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163 | declaration as it is an operator, like C<do{}> or C<eval{}>. (However, no
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164 | matter how many times you execute that particular line (unless you're in an
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165 | C<eval("...")>), $coderef will still have a reference to the I<same>
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166 | anonymous subroutine.)
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167 |
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168 | Anonymous subroutines act as closures with respect to my() variables,
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169 | that is, variables lexically visible within the current scope. Closure
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170 | is a notion out of the Lisp world that says if you define an anonymous
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171 | function in a particular lexical context, it pretends to run in that
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172 | context even when it's called outside the context.
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173 |
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174 | In human terms, it's a funny way of passing arguments to a subroutine when
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175 | you define it as well as when you call it. It's useful for setting up
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176 | little bits of code to run later, such as callbacks. You can even
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177 | do object-oriented stuff with it, though Perl already provides a different
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178 | mechanism to do that--see L<perlobj>.
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179 |
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180 | You might also think of closure as a way to write a subroutine
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181 | template without using eval(). Here's a small example of how
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182 | closures work:
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183 |
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184 | sub newprint {
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185 | my $x = shift;
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186 | return sub { my $y = shift; print "$x, $y!\n"; };
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187 | }
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188 | $h = newprint("Howdy");
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189 | $g = newprint("Greetings");
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190 |
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191 | # Time passes...
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192 |
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193 | &$h("world");
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194 | &$g("earthlings");
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195 |
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196 | This prints
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197 |
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198 | Howdy, world!
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199 | Greetings, earthlings!
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200 |
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201 | Note particularly that $x continues to refer to the value passed
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202 | into newprint() I<despite> "my $x" having gone out of scope by the
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203 | time the anonymous subroutine runs. That's what a closure is all
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204 | about.
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205 |
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206 | This applies only to lexical variables, by the way. Dynamic variables
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207 | continue to work as they have always worked. Closure is not something
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208 | that most Perl programmers need trouble themselves about to begin with.
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209 |
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210 | =item 5.
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211 | X<constructor> X<new>
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212 |
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213 | References are often returned by special subroutines called constructors.
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214 | Perl objects are just references to a special type of object that happens to know
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215 | which package it's associated with. Constructors are just special
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216 | subroutines that know how to create that association. They do so by
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217 | starting with an ordinary reference, and it remains an ordinary reference
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218 | even while it's also being an object. Constructors are often
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219 | named new() and called indirectly:
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220 |
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221 | $objref = new Doggie (Tail => 'short', Ears => 'long');
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222 |
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223 | But don't have to be:
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224 |
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225 | $objref = Doggie->new(Tail => 'short', Ears => 'long');
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226 |
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227 | use Term::Cap;
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228 | $terminal = Term::Cap->Tgetent( { OSPEED => 9600 });
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229 |
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230 | use Tk;
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231 | $main = MainWindow->new();
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232 | $menubar = $main->Frame(-relief => "raised",
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233 | -borderwidth => 2)
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234 |
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235 | =item 6.
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236 | X<autovivification>
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237 |
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238 | References of the appropriate type can spring into existence if you
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239 | dereference them in a context that assumes they exist. Because we haven't
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240 | talked about dereferencing yet, we can't show you any examples yet.
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241 |
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242 | =item 7.
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243 | X<*foo{THING}> X<*>
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244 |
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245 | A reference can be created by using a special syntax, lovingly known as
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246 | the *foo{THING} syntax. *foo{THING} returns a reference to the THING
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247 | slot in *foo (which is the symbol table entry which holds everything
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248 | known as foo).
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249 |
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250 | $scalarref = *foo{SCALAR};
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251 | $arrayref = *ARGV{ARRAY};
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252 | $hashref = *ENV{HASH};
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253 | $coderef = *handler{CODE};
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254 | $ioref = *STDIN{IO};
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255 | $globref = *foo{GLOB};
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256 | $formatref = *foo{FORMAT};
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257 |
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258 | All of these are self-explanatory except for C<*foo{IO}>. It returns
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259 | the IO handle, used for file handles (L<perlfunc/open>), sockets
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260 | (L<perlfunc/socket> and L<perlfunc/socketpair>), and directory
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261 | handles (L<perlfunc/opendir>). For compatibility with previous
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262 | versions of Perl, C<*foo{FILEHANDLE}> is a synonym for C<*foo{IO}>, though it
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263 | is deprecated as of 5.8.0. If deprecation warnings are in effect, it will warn
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264 | of its use.
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265 |
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266 | C<*foo{THING}> returns undef if that particular THING hasn't been used yet,
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267 | except in the case of scalars. C<*foo{SCALAR}> returns a reference to an
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268 | anonymous scalar if $foo hasn't been used yet. This might change in a
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269 | future release.
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270 |
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271 | C<*foo{IO}> is an alternative to the C<*HANDLE> mechanism given in
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272 | L<perldata/"Typeglobs and Filehandles"> for passing filehandles
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273 | into or out of subroutines, or storing into larger data structures.
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274 | Its disadvantage is that it won't create a new filehandle for you.
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275 | Its advantage is that you have less risk of clobbering more than
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276 | you want to with a typeglob assignment. (It still conflates file
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277 | and directory handles, though.) However, if you assign the incoming
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278 | value to a scalar instead of a typeglob as we do in the examples
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279 | below, there's no risk of that happening.
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280 |
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281 | splutter(*STDOUT); # pass the whole glob
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282 | splutter(*STDOUT{IO}); # pass both file and dir handles
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283 |
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284 | sub splutter {
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285 | my $fh = shift;
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286 | print $fh "her um well a hmmm\n";
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287 | }
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288 |
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289 | $rec = get_rec(*STDIN); # pass the whole glob
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290 | $rec = get_rec(*STDIN{IO}); # pass both file and dir handles
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291 |
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292 | sub get_rec {
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293 | my $fh = shift;
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294 | return scalar <$fh>;
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295 | }
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296 |
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297 | =back
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298 |
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299 | =head2 Using References
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300 | X<reference, use> X<dereferencing> X<dereference>
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301 |
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302 | That's it for creating references. By now you're probably dying to
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303 | know how to use references to get back to your long-lost data. There
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304 | are several basic methods.
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305 |
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306 | =over 4
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307 |
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308 | =item 1.
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309 |
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310 | Anywhere you'd put an identifier (or chain of identifiers) as part
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311 | of a variable or subroutine name, you can replace the identifier with
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312 | a simple scalar variable containing a reference of the correct type:
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313 |
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314 | $bar = $$scalarref;
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315 | push(@$arrayref, $filename);
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316 | $$arrayref[0] = "January";
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317 | $$hashref{"KEY"} = "VALUE";
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318 | &$coderef(1,2,3);
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319 | print $globref "output\n";
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320 |
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321 | It's important to understand that we are specifically I<not> dereferencing
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322 | C<$arrayref[0]> or C<$hashref{"KEY"}> there. The dereference of the
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323 | scalar variable happens I<before> it does any key lookups. Anything more
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324 | complicated than a simple scalar variable must use methods 2 or 3 below.
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325 | However, a "simple scalar" includes an identifier that itself uses method
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326 | 1 recursively. Therefore, the following prints "howdy".
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327 |
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328 | $refrefref = \\\"howdy";
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329 | print $$$$refrefref;
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330 |
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331 | =item 2.
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332 | X<${}> X<@{}> X<%{}>
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333 |
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334 | Anywhere you'd put an identifier (or chain of identifiers) as part of a
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335 | variable or subroutine name, you can replace the identifier with a
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336 | BLOCK returning a reference of the correct type. In other words, the
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337 | previous examples could be written like this:
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338 |
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339 | $bar = ${$scalarref};
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340 | push(@{$arrayref}, $filename);
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341 | ${$arrayref}[0] = "January";
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342 | ${$hashref}{"KEY"} = "VALUE";
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343 | &{$coderef}(1,2,3);
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344 | $globref->print("output\n"); # iff IO::Handle is loaded
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345 |
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346 | Admittedly, it's a little silly to use the curlies in this case, but
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347 | the BLOCK can contain any arbitrary expression, in particular,
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348 | subscripted expressions:
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349 |
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350 | &{ $dispatch{$index} }(1,2,3); # call correct routine
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351 |
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352 | Because of being able to omit the curlies for the simple case of C<$$x>,
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353 | people often make the mistake of viewing the dereferencing symbols as
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354 | proper operators, and wonder about their precedence. If they were,
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355 | though, you could use parentheses instead of braces. That's not the case.
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356 | Consider the difference below; case 0 is a short-hand version of case 1,
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357 | I<not> case 2:
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358 |
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359 | $$hashref{"KEY"} = "VALUE"; # CASE 0
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360 | ${$hashref}{"KEY"} = "VALUE"; # CASE 1
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361 | ${$hashref{"KEY"}} = "VALUE"; # CASE 2
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362 | ${$hashref->{"KEY"}} = "VALUE"; # CASE 3
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363 |
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364 | Case 2 is also deceptive in that you're accessing a variable
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365 | called %hashref, not dereferencing through $hashref to the hash
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366 | it's presumably referencing. That would be case 3.
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367 |
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368 | =item 3.
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369 | X<autovivification> X<< -> >> X<arrow>
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370 |
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371 | Subroutine calls and lookups of individual array elements arise often
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372 | enough that it gets cumbersome to use method 2. As a form of
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373 | syntactic sugar, the examples for method 2 may be written:
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374 |
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375 | $arrayref->[0] = "January"; # Array element
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376 | $hashref->{"KEY"} = "VALUE"; # Hash element
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377 | $coderef->(1,2,3); # Subroutine call
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378 |
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379 | The left side of the arrow can be any expression returning a reference,
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380 | including a previous dereference. Note that C<$array[$x]> is I<not> the
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381 | same thing as C<< $array->[$x] >> here:
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382 |
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383 | $array[$x]->{"foo"}->[0] = "January";
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384 |
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385 | This is one of the cases we mentioned earlier in which references could
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386 | spring into existence when in an lvalue context. Before this
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387 | statement, C<$array[$x]> may have been undefined. If so, it's
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388 | automatically defined with a hash reference so that we can look up
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389 | C<{"foo"}> in it. Likewise C<< $array[$x]->{"foo"} >> will automatically get
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390 | defined with an array reference so that we can look up C<[0]> in it.
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391 | This process is called I<autovivification>.
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392 |
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393 | One more thing here. The arrow is optional I<between> brackets
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394 | subscripts, so you can shrink the above down to
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395 |
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396 | $array[$x]{"foo"}[0] = "January";
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397 |
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398 | Which, in the degenerate case of using only ordinary arrays, gives you
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399 | multidimensional arrays just like C's:
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400 |
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401 | $score[$x][$y][$z] += 42;
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402 |
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403 | Well, okay, not entirely like C's arrays, actually. C doesn't know how
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404 | to grow its arrays on demand. Perl does.
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405 |
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406 | =item 4.
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407 | X<encapsulation>
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408 |
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409 | If a reference happens to be a reference to an object, then there are
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410 | probably methods to access the things referred to, and you should probably
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411 | stick to those methods unless you're in the class package that defines the
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412 | object's methods. In other words, be nice, and don't violate the object's
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413 | encapsulation without a very good reason. Perl does not enforce
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414 | encapsulation. We are not totalitarians here. We do expect some basic
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415 | civility though.
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416 |
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417 | =back
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418 |
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419 | Using a string or number as a reference produces a symbolic reference,
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420 | as explained above. Using a reference as a number produces an
|
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421 | integer representing its storage location in memory. The only
|
---|
422 | useful thing to be done with this is to compare two references
|
---|
423 | numerically to see whether they refer to the same location.
|
---|
424 | X<reference, numeric context>
|
---|
425 |
|
---|
426 | if ($ref1 == $ref2) { # cheap numeric compare of references
|
---|
427 | print "refs 1 and 2 refer to the same thing\n";
|
---|
428 | }
|
---|
429 |
|
---|
430 | Using a reference as a string produces both its referent's type,
|
---|
431 | including any package blessing as described in L<perlobj>, as well
|
---|
432 | as the numeric address expressed in hex. The ref() operator returns
|
---|
433 | just the type of thing the reference is pointing to, without the
|
---|
434 | address. See L<perlfunc/ref> for details and examples of its use.
|
---|
435 | X<reference, string context>
|
---|
436 |
|
---|
437 | The bless() operator may be used to associate the object a reference
|
---|
438 | points to with a package functioning as an object class. See L<perlobj>.
|
---|
439 |
|
---|
440 | A typeglob may be dereferenced the same way a reference can, because
|
---|
441 | the dereference syntax always indicates the type of reference desired.
|
---|
442 | So C<${*foo}> and C<${\$foo}> both indicate the same scalar variable.
|
---|
443 |
|
---|
444 | Here's a trick for interpolating a subroutine call into a string:
|
---|
445 |
|
---|
446 | print "My sub returned @{[mysub(1,2,3)]} that time.\n";
|
---|
447 |
|
---|
448 | The way it works is that when the C<@{...}> is seen in the double-quoted
|
---|
449 | string, it's evaluated as a block. The block creates a reference to an
|
---|
450 | anonymous array containing the results of the call to C<mysub(1,2,3)>. So
|
---|
451 | the whole block returns a reference to an array, which is then
|
---|
452 | dereferenced by C<@{...}> and stuck into the double-quoted string. This
|
---|
453 | chicanery is also useful for arbitrary expressions:
|
---|
454 |
|
---|
455 | print "That yields @{[$n + 5]} widgets\n";
|
---|
456 |
|
---|
457 | =head2 Symbolic references
|
---|
458 | X<reference, symbolic> X<reference, soft>
|
---|
459 | X<symbolic reference> X<soft reference>
|
---|
460 |
|
---|
461 | We said that references spring into existence as necessary if they are
|
---|
462 | undefined, but we didn't say what happens if a value used as a
|
---|
463 | reference is already defined, but I<isn't> a hard reference. If you
|
---|
464 | use it as a reference, it'll be treated as a symbolic
|
---|
465 | reference. That is, the value of the scalar is taken to be the I<name>
|
---|
466 | of a variable, rather than a direct link to a (possibly) anonymous
|
---|
467 | value.
|
---|
468 |
|
---|
469 | People frequently expect it to work like this. So it does.
|
---|
470 |
|
---|
471 | $name = "foo";
|
---|
472 | $$name = 1; # Sets $foo
|
---|
473 | ${$name} = 2; # Sets $foo
|
---|
474 | ${$name x 2} = 3; # Sets $foofoo
|
---|
475 | $name->[0] = 4; # Sets $foo[0]
|
---|
476 | @$name = (); # Clears @foo
|
---|
477 | &$name(); # Calls &foo() (as in Perl 4)
|
---|
478 | $pack = "THAT";
|
---|
479 | ${"${pack}::$name"} = 5; # Sets $THAT::foo without eval
|
---|
480 |
|
---|
481 | This is powerful, and slightly dangerous, in that it's possible
|
---|
482 | to intend (with the utmost sincerity) to use a hard reference, and
|
---|
483 | accidentally use a symbolic reference instead. To protect against
|
---|
484 | that, you can say
|
---|
485 |
|
---|
486 | use strict 'refs';
|
---|
487 |
|
---|
488 | and then only hard references will be allowed for the rest of the enclosing
|
---|
489 | block. An inner block may countermand that with
|
---|
490 |
|
---|
491 | no strict 'refs';
|
---|
492 |
|
---|
493 | Only package variables (globals, even if localized) are visible to
|
---|
494 | symbolic references. Lexical variables (declared with my()) aren't in
|
---|
495 | a symbol table, and thus are invisible to this mechanism. For example:
|
---|
496 |
|
---|
497 | local $value = 10;
|
---|
498 | $ref = "value";
|
---|
499 | {
|
---|
500 | my $value = 20;
|
---|
501 | print $$ref;
|
---|
502 | }
|
---|
503 |
|
---|
504 | This will still print 10, not 20. Remember that local() affects package
|
---|
505 | variables, which are all "global" to the package.
|
---|
506 |
|
---|
507 | =head2 Not-so-symbolic references
|
---|
508 |
|
---|
509 | A new feature contributing to readability in perl version 5.001 is that the
|
---|
510 | brackets around a symbolic reference behave more like quotes, just as they
|
---|
511 | always have within a string. That is,
|
---|
512 |
|
---|
513 | $push = "pop on ";
|
---|
514 | print "${push}over";
|
---|
515 |
|
---|
516 | has always meant to print "pop on over", even though push is
|
---|
517 | a reserved word. This has been generalized to work the same outside
|
---|
518 | of quotes, so that
|
---|
519 |
|
---|
520 | print ${push} . "over";
|
---|
521 |
|
---|
522 | and even
|
---|
523 |
|
---|
524 | print ${ push } . "over";
|
---|
525 |
|
---|
526 | will have the same effect. (This would have been a syntax error in
|
---|
527 | Perl 5.000, though Perl 4 allowed it in the spaceless form.) This
|
---|
528 | construct is I<not> considered to be a symbolic reference when you're
|
---|
529 | using strict refs:
|
---|
530 |
|
---|
531 | use strict 'refs';
|
---|
532 | ${ bareword }; # Okay, means $bareword.
|
---|
533 | ${ "bareword" }; # Error, symbolic reference.
|
---|
534 |
|
---|
535 | Similarly, because of all the subscripting that is done using single
|
---|
536 | words, we've applied the same rule to any bareword that is used for
|
---|
537 | subscripting a hash. So now, instead of writing
|
---|
538 |
|
---|
539 | $array{ "aaa" }{ "bbb" }{ "ccc" }
|
---|
540 |
|
---|
541 | you can write just
|
---|
542 |
|
---|
543 | $array{ aaa }{ bbb }{ ccc }
|
---|
544 |
|
---|
545 | and not worry about whether the subscripts are reserved words. In the
|
---|
546 | rare event that you do wish to do something like
|
---|
547 |
|
---|
548 | $array{ shift }
|
---|
549 |
|
---|
550 | you can force interpretation as a reserved word by adding anything that
|
---|
551 | makes it more than a bareword:
|
---|
552 |
|
---|
553 | $array{ shift() }
|
---|
554 | $array{ +shift }
|
---|
555 | $array{ shift @_ }
|
---|
556 |
|
---|
557 | The C<use warnings> pragma or the B<-w> switch will warn you if it
|
---|
558 | interprets a reserved word as a string.
|
---|
559 | But it will no longer warn you about using lowercase words, because the
|
---|
560 | string is effectively quoted.
|
---|
561 |
|
---|
562 | =head2 Pseudo-hashes: Using an array as a hash
|
---|
563 | X<pseudo-hash> X<pseudo hash> X<pseudohash>
|
---|
564 |
|
---|
565 | B<WARNING>: This section describes an experimental feature. Details may
|
---|
566 | change without notice in future versions.
|
---|
567 |
|
---|
568 | B<NOTE>: The current user-visible implementation of pseudo-hashes
|
---|
569 | (the weird use of the first array element) is deprecated starting from
|
---|
570 | Perl 5.8.0 and will be removed in Perl 5.10.0, and the feature will be
|
---|
571 | implemented differently. Not only is the current interface rather ugly,
|
---|
572 | but the current implementation slows down normal array and hash use quite
|
---|
573 | noticeably. The 'fields' pragma interface will remain available.
|
---|
574 |
|
---|
575 | Beginning with release 5.005 of Perl, you may use an array reference
|
---|
576 | in some contexts that would normally require a hash reference. This
|
---|
577 | allows you to access array elements using symbolic names, as if they
|
---|
578 | were fields in a structure.
|
---|
579 |
|
---|
580 | For this to work, the array must contain extra information. The first
|
---|
581 | element of the array has to be a hash reference that maps field names
|
---|
582 | to array indices. Here is an example:
|
---|
583 |
|
---|
584 | $struct = [{foo => 1, bar => 2}, "FOO", "BAR"];
|
---|
585 |
|
---|
586 | $struct->{foo}; # same as $struct->[1], i.e. "FOO"
|
---|
587 | $struct->{bar}; # same as $struct->[2], i.e. "BAR"
|
---|
588 |
|
---|
589 | keys %$struct; # will return ("foo", "bar") in some order
|
---|
590 | values %$struct; # will return ("FOO", "BAR") in same some order
|
---|
591 |
|
---|
592 | while (my($k,$v) = each %$struct) {
|
---|
593 | print "$k => $v\n";
|
---|
594 | }
|
---|
595 |
|
---|
596 | Perl will raise an exception if you try to access nonexistent fields.
|
---|
597 | To avoid inconsistencies, always use the fields::phash() function
|
---|
598 | provided by the C<fields> pragma.
|
---|
599 |
|
---|
600 | use fields;
|
---|
601 | $pseudohash = fields::phash(foo => "FOO", bar => "BAR");
|
---|
602 |
|
---|
603 | For better performance, Perl can also do the translation from field
|
---|
604 | names to array indices at compile time for typed object references.
|
---|
605 | See L<fields>.
|
---|
606 |
|
---|
607 | There are two ways to check for the existence of a key in a
|
---|
608 | pseudo-hash. The first is to use exists(). This checks to see if the
|
---|
609 | given field has ever been set. It acts this way to match the behavior
|
---|
610 | of a regular hash. For instance:
|
---|
611 |
|
---|
612 | use fields;
|
---|
613 | $phash = fields::phash([qw(foo bar pants)], ['FOO']);
|
---|
614 | $phash->{pants} = undef;
|
---|
615 |
|
---|
616 | print exists $phash->{foo}; # true, 'foo' was set in the declaration
|
---|
617 | print exists $phash->{bar}; # false, 'bar' has not been used.
|
---|
618 | print exists $phash->{pants}; # true, your 'pants' have been touched
|
---|
619 |
|
---|
620 | The second is to use exists() on the hash reference sitting in the
|
---|
621 | first array element. This checks to see if the given key is a valid
|
---|
622 | field in the pseudo-hash.
|
---|
623 |
|
---|
624 | print exists $phash->[0]{bar}; # true, 'bar' is a valid field
|
---|
625 | print exists $phash->[0]{shoes};# false, 'shoes' can't be used
|
---|
626 |
|
---|
627 | delete() on a pseudo-hash element only deletes the value corresponding
|
---|
628 | to the key, not the key itself. To delete the key, you'll have to
|
---|
629 | explicitly delete it from the first hash element.
|
---|
630 |
|
---|
631 | print delete $phash->{foo}; # prints $phash->[1], "FOO"
|
---|
632 | print exists $phash->{foo}; # false
|
---|
633 | print exists $phash->[0]{foo}; # true, key still exists
|
---|
634 | print delete $phash->[0]{foo}; # now key is gone
|
---|
635 | print $phash->{foo}; # runtime exception
|
---|
636 |
|
---|
637 | =head2 Function Templates
|
---|
638 | X<scope, lexical> X<closure> X<lexical> X<lexical scope>
|
---|
639 | X<subroutine, nested> X<sub, nested> X<subroutine, local> X<sub, local>
|
---|
640 |
|
---|
641 | As explained above, an anonymous function with access to the lexical
|
---|
642 | variables visible when that function was compiled, creates a closure. It
|
---|
643 | retains access to those variables even though it doesn't get run until
|
---|
644 | later, such as in a signal handler or a Tk callback.
|
---|
645 |
|
---|
646 | Using a closure as a function template allows us to generate many functions
|
---|
647 | that act similarly. Suppose you wanted functions named after the colors
|
---|
648 | that generated HTML font changes for the various colors:
|
---|
649 |
|
---|
650 | print "Be ", red("careful"), "with that ", green("light");
|
---|
651 |
|
---|
652 | The red() and green() functions would be similar. To create these,
|
---|
653 | we'll assign a closure to a typeglob of the name of the function we're
|
---|
654 | trying to build.
|
---|
655 |
|
---|
656 | @colors = qw(red blue green yellow orange purple violet);
|
---|
657 | for my $name (@colors) {
|
---|
658 | no strict 'refs'; # allow symbol table manipulation
|
---|
659 | *$name = *{uc $name} = sub { "<FONT COLOR='$name'>@_</FONT>" };
|
---|
660 | }
|
---|
661 |
|
---|
662 | Now all those different functions appear to exist independently. You can
|
---|
663 | call red(), RED(), blue(), BLUE(), green(), etc. This technique saves on
|
---|
664 | both compile time and memory use, and is less error-prone as well, since
|
---|
665 | syntax checks happen at compile time. It's critical that any variables in
|
---|
666 | the anonymous subroutine be lexicals in order to create a proper closure.
|
---|
667 | That's the reasons for the C<my> on the loop iteration variable.
|
---|
668 |
|
---|
669 | This is one of the only places where giving a prototype to a closure makes
|
---|
670 | much sense. If you wanted to impose scalar context on the arguments of
|
---|
671 | these functions (probably not a wise idea for this particular example),
|
---|
672 | you could have written it this way instead:
|
---|
673 |
|
---|
674 | *$name = sub ($) { "<FONT COLOR='$name'>$_[0]</FONT>" };
|
---|
675 |
|
---|
676 | However, since prototype checking happens at compile time, the assignment
|
---|
677 | above happens too late to be of much use. You could address this by
|
---|
678 | putting the whole loop of assignments within a BEGIN block, forcing it
|
---|
679 | to occur during compilation.
|
---|
680 |
|
---|
681 | Access to lexicals that change over type--like those in the C<for> loop
|
---|
682 | above--only works with closures, not general subroutines. In the general
|
---|
683 | case, then, named subroutines do not nest properly, although anonymous
|
---|
684 | ones do. Thus is because named subroutines are created (and capture any
|
---|
685 | outer lexicals) only once at compile time, whereas anonymous subroutines
|
---|
686 | get to capture each time you execute the 'sub' operator. If you are
|
---|
687 | accustomed to using nested subroutines in other programming languages with
|
---|
688 | their own private variables, you'll have to work at it a bit in Perl. The
|
---|
689 | intuitive coding of this type of thing incurs mysterious warnings about
|
---|
690 | "will not stay shared". For example, this won't work:
|
---|
691 |
|
---|
692 | sub outer {
|
---|
693 | my $x = $_[0] + 35;
|
---|
694 | sub inner { return $x * 19 } # WRONG
|
---|
695 | return $x + inner();
|
---|
696 | }
|
---|
697 |
|
---|
698 | A work-around is the following:
|
---|
699 |
|
---|
700 | sub outer {
|
---|
701 | my $x = $_[0] + 35;
|
---|
702 | local *inner = sub { return $x * 19 };
|
---|
703 | return $x + inner();
|
---|
704 | }
|
---|
705 |
|
---|
706 | Now inner() can only be called from within outer(), because of the
|
---|
707 | temporary assignments of the closure (anonymous subroutine). But when
|
---|
708 | it does, it has normal access to the lexical variable $x from the scope
|
---|
709 | of outer().
|
---|
710 |
|
---|
711 | This has the interesting effect of creating a function local to another
|
---|
712 | function, something not normally supported in Perl.
|
---|
713 |
|
---|
714 | =head1 WARNING
|
---|
715 | X<reference, string context> X<reference, use as hash key>
|
---|
716 |
|
---|
717 | You may not (usefully) use a reference as the key to a hash. It will be
|
---|
718 | converted into a string:
|
---|
719 |
|
---|
720 | $x{ \$a } = $a;
|
---|
721 |
|
---|
722 | If you try to dereference the key, it won't do a hard dereference, and
|
---|
723 | you won't accomplish what you're attempting. You might want to do something
|
---|
724 | more like
|
---|
725 |
|
---|
726 | $r = \@a;
|
---|
727 | $x{ $r } = $r;
|
---|
728 |
|
---|
729 | And then at least you can use the values(), which will be
|
---|
730 | real refs, instead of the keys(), which won't.
|
---|
731 |
|
---|
732 | The standard Tie::RefHash module provides a convenient workaround to this.
|
---|
733 |
|
---|
734 | =head1 SEE ALSO
|
---|
735 |
|
---|
736 | Besides the obvious documents, source code can be instructive.
|
---|
737 | Some pathological examples of the use of references can be found
|
---|
738 | in the F<t/op/ref.t> regression test in the Perl source directory.
|
---|
739 |
|
---|
740 | See also L<perldsc> and L<perllol> for how to use references to create
|
---|
741 | complex data structures, and L<perltoot>, L<perlobj>, and L<perlbot>
|
---|
742 | for how to use them to create objects.
|
---|