1 | =head1 NAME
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2 |
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3 | perlbot - Bag'o Object Tricks (the BOT)
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4 |
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5 | =head1 DESCRIPTION
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6 |
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7 | The following collection of tricks and hints is intended to whet curious
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8 | appetites about such things as the use of instance variables and the
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9 | mechanics of object and class relationships. The reader is encouraged to
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10 | consult relevant textbooks for discussion of Object Oriented definitions and
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11 | methodology. This is not intended as a tutorial for object-oriented
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12 | programming or as a comprehensive guide to Perl's object oriented features,
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13 | nor should it be construed as a style guide. If you're looking for tutorials,
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14 | be sure to read L<perlboot>, L<perltoot>, and L<perltooc>.
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15 |
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16 | The Perl motto still holds: There's more than one way to do it.
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17 |
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18 | =head1 OO SCALING TIPS
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19 |
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20 | =over 5
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21 |
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22 | =item 1
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23 |
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24 | Do not attempt to verify the type of $self. That'll break if the class is
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25 | inherited, when the type of $self is valid but its package isn't what you
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26 | expect. See rule 5.
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27 |
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28 | =item 2
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29 |
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30 | If an object-oriented (OO) or indirect-object (IO) syntax was used, then the
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31 | object is probably the correct type and there's no need to become paranoid
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32 | about it. Perl isn't a paranoid language anyway. If people subvert the OO
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33 | or IO syntax then they probably know what they're doing and you should let
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34 | them do it. See rule 1.
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35 |
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36 | =item 3
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37 |
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38 | Use the two-argument form of bless(). Let a subclass use your constructor.
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39 | See L<INHERITING A CONSTRUCTOR>.
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40 |
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41 | =item 4
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42 |
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43 | The subclass is allowed to know things about its immediate superclass, the
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44 | superclass is allowed to know nothing about a subclass.
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45 |
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46 | =item 5
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47 |
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48 | Don't be trigger happy with inheritance. A "using", "containing", or
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49 | "delegation" relationship (some sort of aggregation, at least) is often more
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50 | appropriate. See L<OBJECT RELATIONSHIPS>, L<USING RELATIONSHIP WITH SDBM>,
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51 | and L<"DELEGATION">.
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52 |
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53 | =item 6
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54 |
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55 | The object is the namespace. Make package globals accessible via the
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56 | object. This will remove the guess work about the symbol's home package.
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57 | See L<CLASS CONTEXT AND THE OBJECT>.
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58 |
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59 | =item 7
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60 |
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61 | IO syntax is certainly less noisy, but it is also prone to ambiguities that
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62 | can cause difficult-to-find bugs. Allow people to use the sure-thing OO
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63 | syntax, even if you don't like it.
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64 |
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65 | =item 8
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66 |
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67 | Do not use function-call syntax on a method. You're going to be bitten
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68 | someday. Someone might move that method into a superclass and your code
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69 | will be broken. On top of that you're feeding the paranoia in rule 2.
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70 |
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71 | =item 9
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72 |
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73 | Don't assume you know the home package of a method. You're making it
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74 | difficult for someone to override that method. See L<THINKING OF CODE REUSE>.
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75 |
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76 | =back
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77 |
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78 | =head1 INSTANCE VARIABLES
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79 |
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80 | An anonymous array or anonymous hash can be used to hold instance
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81 | variables. Named parameters are also demonstrated.
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82 |
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83 | package Foo;
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84 |
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85 | sub new {
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86 | my $type = shift;
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87 | my %params = @_;
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88 | my $self = {};
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89 | $self->{'High'} = $params{'High'};
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90 | $self->{'Low'} = $params{'Low'};
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91 | bless $self, $type;
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92 | }
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93 |
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94 |
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95 | package Bar;
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96 |
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97 | sub new {
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98 | my $type = shift;
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99 | my %params = @_;
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100 | my $self = [];
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101 | $self->[0] = $params{'Left'};
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102 | $self->[1] = $params{'Right'};
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103 | bless $self, $type;
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104 | }
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105 |
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106 | package main;
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107 |
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108 | $a = Foo->new( 'High' => 42, 'Low' => 11 );
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109 | print "High=$a->{'High'}\n";
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110 | print "Low=$a->{'Low'}\n";
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111 |
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112 | $b = Bar->new( 'Left' => 78, 'Right' => 40 );
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113 | print "Left=$b->[0]\n";
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114 | print "Right=$b->[1]\n";
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115 |
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116 | =head1 SCALAR INSTANCE VARIABLES
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117 |
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118 | An anonymous scalar can be used when only one instance variable is needed.
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119 |
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120 | package Foo;
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121 |
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122 | sub new {
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123 | my $type = shift;
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124 | my $self;
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125 | $self = shift;
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126 | bless \$self, $type;
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127 | }
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128 |
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129 | package main;
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130 |
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131 | $a = Foo->new( 42 );
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132 | print "a=$$a\n";
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133 |
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134 |
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135 | =head1 INSTANCE VARIABLE INHERITANCE
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136 |
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137 | This example demonstrates how one might inherit instance variables from a
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138 | superclass for inclusion in the new class. This requires calling the
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139 | superclass's constructor and adding one's own instance variables to the new
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140 | object.
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141 |
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142 | package Bar;
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143 |
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144 | sub new {
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145 | my $type = shift;
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146 | my $self = {};
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147 | $self->{'buz'} = 42;
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148 | bless $self, $type;
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149 | }
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150 |
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151 | package Foo;
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152 | @ISA = qw( Bar );
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153 |
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154 | sub new {
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155 | my $type = shift;
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156 | my $self = Bar->new;
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157 | $self->{'biz'} = 11;
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158 | bless $self, $type;
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159 | }
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160 |
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161 | package main;
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162 |
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163 | $a = Foo->new;
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164 | print "buz = ", $a->{'buz'}, "\n";
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165 | print "biz = ", $a->{'biz'}, "\n";
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166 |
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167 |
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168 |
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169 | =head1 OBJECT RELATIONSHIPS
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170 |
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171 | The following demonstrates how one might implement "containing" and "using"
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172 | relationships between objects.
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173 |
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174 | package Bar;
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175 |
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176 | sub new {
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177 | my $type = shift;
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178 | my $self = {};
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179 | $self->{'buz'} = 42;
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180 | bless $self, $type;
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181 | }
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182 |
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183 | package Foo;
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184 |
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185 | sub new {
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186 | my $type = shift;
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187 | my $self = {};
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188 | $self->{'Bar'} = Bar->new;
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189 | $self->{'biz'} = 11;
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190 | bless $self, $type;
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191 | }
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192 |
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193 | package main;
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194 |
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195 | $a = Foo->new;
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196 | print "buz = ", $a->{'Bar'}->{'buz'}, "\n";
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197 | print "biz = ", $a->{'biz'}, "\n";
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198 |
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199 |
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200 |
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201 | =head1 OVERRIDING SUPERCLASS METHODS
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202 |
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203 | The following example demonstrates how to override a superclass method and
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204 | then call the overridden method. The B<SUPER> pseudo-class allows the
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205 | programmer to call an overridden superclass method without actually knowing
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206 | where that method is defined.
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207 |
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208 | package Buz;
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209 | sub goo { print "here's the goo\n" }
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210 |
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211 | package Bar; @ISA = qw( Buz );
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212 | sub google { print "google here\n" }
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213 |
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214 | package Baz;
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215 | sub mumble { print "mumbling\n" }
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216 |
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217 | package Foo;
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218 | @ISA = qw( Bar Baz );
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219 |
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220 | sub new {
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221 | my $type = shift;
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222 | bless [], $type;
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223 | }
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224 | sub grr { print "grumble\n" }
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225 | sub goo {
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226 | my $self = shift;
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227 | $self->SUPER::goo();
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228 | }
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229 | sub mumble {
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230 | my $self = shift;
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231 | $self->SUPER::mumble();
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232 | }
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233 | sub google {
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234 | my $self = shift;
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235 | $self->SUPER::google();
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236 | }
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237 |
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238 | package main;
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239 |
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240 | $foo = Foo->new;
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241 | $foo->mumble;
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242 | $foo->grr;
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243 | $foo->goo;
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244 | $foo->google;
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245 |
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246 | Note that C<SUPER> refers to the superclasses of the current package
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247 | (C<Foo>), not to the superclasses of C<$self>.
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248 |
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249 |
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250 | =head1 USING RELATIONSHIP WITH SDBM
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251 |
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252 | This example demonstrates an interface for the SDBM class. This creates a
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253 | "using" relationship between the SDBM class and the new class Mydbm.
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254 |
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255 | package Mydbm;
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256 |
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257 | require SDBM_File;
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258 | require Tie::Hash;
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259 | @ISA = qw( Tie::Hash );
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260 |
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261 | sub TIEHASH {
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262 | my $type = shift;
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263 | my $ref = SDBM_File->new(@_);
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264 | bless {'dbm' => $ref}, $type;
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265 | }
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266 | sub FETCH {
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267 | my $self = shift;
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268 | my $ref = $self->{'dbm'};
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269 | $ref->FETCH(@_);
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270 | }
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271 | sub STORE {
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272 | my $self = shift;
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273 | if (defined $_[0]){
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274 | my $ref = $self->{'dbm'};
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275 | $ref->STORE(@_);
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276 | } else {
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277 | die "Cannot STORE an undefined key in Mydbm\n";
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278 | }
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279 | }
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280 |
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281 | package main;
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282 | use Fcntl qw( O_RDWR O_CREAT );
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283 |
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284 | tie %foo, "Mydbm", "Sdbm", O_RDWR|O_CREAT, 0640;
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285 | $foo{'bar'} = 123;
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286 | print "foo-bar = $foo{'bar'}\n";
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287 |
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288 | tie %bar, "Mydbm", "Sdbm2", O_RDWR|O_CREAT, 0640;
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289 | $bar{'Cathy'} = 456;
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290 | print "bar-Cathy = $bar{'Cathy'}\n";
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291 |
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292 | =head1 THINKING OF CODE REUSE
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293 |
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294 | One strength of Object-Oriented languages is the ease with which old code
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295 | can use new code. The following examples will demonstrate first how one can
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296 | hinder code reuse and then how one can promote code reuse.
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297 |
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298 | This first example illustrates a class which uses a fully-qualified method
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299 | call to access the "private" method BAZ(). The second example will show
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300 | that it is impossible to override the BAZ() method.
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301 |
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302 | package FOO;
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303 |
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304 | sub new {
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305 | my $type = shift;
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306 | bless {}, $type;
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307 | }
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308 | sub bar {
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309 | my $self = shift;
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310 | $self->FOO::private::BAZ;
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311 | }
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312 |
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313 | package FOO::private;
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314 |
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315 | sub BAZ {
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316 | print "in BAZ\n";
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317 | }
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318 |
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319 | package main;
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320 |
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321 | $a = FOO->new;
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322 | $a->bar;
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323 |
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324 | Now we try to override the BAZ() method. We would like FOO::bar() to call
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325 | GOOP::BAZ(), but this cannot happen because FOO::bar() explicitly calls
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326 | FOO::private::BAZ().
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327 |
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328 | package FOO;
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329 |
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330 | sub new {
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331 | my $type = shift;
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332 | bless {}, $type;
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333 | }
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334 | sub bar {
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335 | my $self = shift;
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336 | $self->FOO::private::BAZ;
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337 | }
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338 |
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339 | package FOO::private;
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340 |
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341 | sub BAZ {
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342 | print "in BAZ\n";
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343 | }
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344 |
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345 | package GOOP;
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346 | @ISA = qw( FOO );
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347 | sub new {
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348 | my $type = shift;
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349 | bless {}, $type;
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350 | }
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351 |
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352 | sub BAZ {
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353 | print "in GOOP::BAZ\n";
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354 | }
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355 |
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356 | package main;
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357 |
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358 | $a = GOOP->new;
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359 | $a->bar;
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360 |
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361 | To create reusable code we must modify class FOO, flattening class
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362 | FOO::private. The next example shows a reusable class FOO which allows the
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363 | method GOOP::BAZ() to be used in place of FOO::BAZ().
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364 |
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365 | package FOO;
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366 |
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367 | sub new {
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368 | my $type = shift;
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369 | bless {}, $type;
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370 | }
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371 | sub bar {
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372 | my $self = shift;
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373 | $self->BAZ;
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374 | }
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375 |
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376 | sub BAZ {
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377 | print "in BAZ\n";
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378 | }
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379 |
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380 | package GOOP;
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381 | @ISA = qw( FOO );
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382 |
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383 | sub new {
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384 | my $type = shift;
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385 | bless {}, $type;
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386 | }
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387 | sub BAZ {
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388 | print "in GOOP::BAZ\n";
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389 | }
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390 |
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391 | package main;
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392 |
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393 | $a = GOOP->new;
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394 | $a->bar;
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395 |
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396 | =head1 CLASS CONTEXT AND THE OBJECT
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397 |
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398 | Use the object to solve package and class context problems. Everything a
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399 | method needs should be available via the object or should be passed as a
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400 | parameter to the method.
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401 |
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402 | A class will sometimes have static or global data to be used by the
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403 | methods. A subclass may want to override that data and replace it with new
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404 | data. When this happens the superclass may not know how to find the new
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405 | copy of the data.
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406 |
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407 | This problem can be solved by using the object to define the context of the
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408 | method. Let the method look in the object for a reference to the data. The
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409 | alternative is to force the method to go hunting for the data ("Is it in my
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410 | class, or in a subclass? Which subclass?"), and this can be inconvenient
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411 | and will lead to hackery. It is better just to let the object tell the
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412 | method where that data is located.
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413 |
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414 | package Bar;
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415 |
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416 | %fizzle = ( 'Password' => 'XYZZY' );
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417 |
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418 | sub new {
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419 | my $type = shift;
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420 | my $self = {};
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421 | $self->{'fizzle'} = \%fizzle;
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422 | bless $self, $type;
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423 | }
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424 |
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425 | sub enter {
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426 | my $self = shift;
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427 |
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428 | # Don't try to guess if we should use %Bar::fizzle
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429 | # or %Foo::fizzle. The object already knows which
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430 | # we should use, so just ask it.
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431 | #
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432 | my $fizzle = $self->{'fizzle'};
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433 |
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434 | print "The word is ", $fizzle->{'Password'}, "\n";
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435 | }
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436 |
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437 | package Foo;
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438 | @ISA = qw( Bar );
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439 |
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440 | %fizzle = ( 'Password' => 'Rumple' );
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441 |
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442 | sub new {
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443 | my $type = shift;
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444 | my $self = Bar->new;
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445 | $self->{'fizzle'} = \%fizzle;
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446 | bless $self, $type;
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447 | }
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448 |
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449 | package main;
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450 |
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451 | $a = Bar->new;
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452 | $b = Foo->new;
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453 | $a->enter;
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454 | $b->enter;
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455 |
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456 | =head1 INHERITING A CONSTRUCTOR
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457 |
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458 | An inheritable constructor should use the second form of bless() which allows
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459 | blessing directly into a specified class. Notice in this example that the
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460 | object will be a BAR not a FOO, even though the constructor is in class FOO.
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461 |
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462 | package FOO;
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463 |
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464 | sub new {
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465 | my $type = shift;
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466 | my $self = {};
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467 | bless $self, $type;
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468 | }
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469 |
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470 | sub baz {
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471 | print "in FOO::baz()\n";
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472 | }
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473 |
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474 | package BAR;
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475 | @ISA = qw(FOO);
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476 |
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477 | sub baz {
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478 | print "in BAR::baz()\n";
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479 | }
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480 |
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481 | package main;
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482 |
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483 | $a = BAR->new;
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484 | $a->baz;
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485 |
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486 | =head1 DELEGATION
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487 |
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488 | Some classes, such as SDBM_File, cannot be effectively subclassed because
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489 | they create foreign objects. Such a class can be extended with some sort of
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490 | aggregation technique such as the "using" relationship mentioned earlier or
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491 | by delegation.
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492 |
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493 | The following example demonstrates delegation using an AUTOLOAD() function to
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494 | perform message-forwarding. This will allow the Mydbm object to behave
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495 | exactly like an SDBM_File object. The Mydbm class could now extend the
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496 | behavior by adding custom FETCH() and STORE() methods, if this is desired.
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497 |
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498 | package Mydbm;
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499 |
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500 | require SDBM_File;
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501 | require Tie::Hash;
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502 | @ISA = qw(Tie::Hash);
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503 |
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504 | sub TIEHASH {
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505 | my $type = shift;
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506 | my $ref = SDBM_File->new(@_);
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507 | bless {'delegate' => $ref};
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508 | }
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509 |
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510 | sub AUTOLOAD {
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511 | my $self = shift;
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512 |
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513 | # The Perl interpreter places the name of the
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514 | # message in a variable called $AUTOLOAD.
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515 |
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516 | # DESTROY messages should never be propagated.
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517 | return if $AUTOLOAD =~ /::DESTROY$/;
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518 |
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519 | # Remove the package name.
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520 | $AUTOLOAD =~ s/^Mydbm:://;
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521 |
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522 | # Pass the message to the delegate.
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523 | $self->{'delegate'}->$AUTOLOAD(@_);
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524 | }
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525 |
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526 | package main;
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527 | use Fcntl qw( O_RDWR O_CREAT );
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528 |
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529 | tie %foo, "Mydbm", "adbm", O_RDWR|O_CREAT, 0640;
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530 | $foo{'bar'} = 123;
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---|
531 | print "foo-bar = $foo{'bar'}\n";
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532 |
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533 | =head1 SEE ALSO
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534 |
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535 | L<perlboot>, L<perltoot>, L<perltooc>.
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