1 | package overload; |
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2 | |
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3 | $overload::hint_bits = 0x20000; |
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4 | |
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5 | sub nil {} |
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6 | |
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7 | sub OVERLOAD { |
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8 | $package = shift; |
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9 | my %arg = @_; |
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10 | my ($sub, $fb); |
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11 | $ {$package . "::OVERLOAD"}{dummy}++; # Register with magic by touching. |
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12 | *{$package . "::()"} = \&nil; # Make it findable via fetchmethod. |
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13 | for (keys %arg) { |
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14 | if ($_ eq 'fallback') { |
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15 | $fb = $arg{$_}; |
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16 | } else { |
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17 | $sub = $arg{$_}; |
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18 | if (not ref $sub and $sub !~ /::/) { |
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19 | $ {$package . "::(" . $_} = $sub; |
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20 | $sub = \&nil; |
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21 | } |
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22 | #print STDERR "Setting `$ {'package'}::\cO$_' to \\&`$sub'.\n"; |
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23 | *{$package . "::(" . $_} = \&{ $sub }; |
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24 | } |
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25 | } |
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26 | ${$package . "::()"} = $fb; # Make it findable too (fallback only). |
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27 | } |
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28 | |
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29 | sub import { |
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30 | $package = (caller())[0]; |
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31 | # *{$package . "::OVERLOAD"} = \&OVERLOAD; |
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32 | shift; |
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33 | $package->overload::OVERLOAD(@_); |
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34 | } |
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35 | |
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36 | sub unimport { |
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37 | $package = (caller())[0]; |
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38 | ${$package . "::OVERLOAD"}{dummy}++; # Upgrade the table |
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39 | shift; |
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40 | for (@_) { |
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41 | if ($_ eq 'fallback') { |
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42 | undef $ {$package . "::()"}; |
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43 | } else { |
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44 | delete $ {$package . "::"}{"(" . $_}; |
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45 | } |
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46 | } |
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47 | } |
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48 | |
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49 | sub Overloaded { |
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50 | my $package = shift; |
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51 | $package = ref $package if ref $package; |
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52 | $package->can('()'); |
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53 | } |
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54 | |
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55 | sub ov_method { |
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56 | my $globref = shift; |
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57 | return undef unless $globref; |
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58 | my $sub = \&{*$globref}; |
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59 | return $sub if $sub ne \&nil; |
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60 | return shift->can($ {*$globref}); |
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61 | } |
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62 | |
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63 | sub OverloadedStringify { |
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64 | my $package = shift; |
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65 | $package = ref $package if ref $package; |
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66 | #$package->can('(""') |
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67 | ov_method mycan($package, '(""'), $package |
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68 | or ov_method mycan($package, '(0+'), $package |
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69 | or ov_method mycan($package, '(bool'), $package |
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70 | or ov_method mycan($package, '(nomethod'), $package; |
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71 | } |
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72 | |
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73 | sub Method { |
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74 | my $package = shift; |
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75 | $package = ref $package if ref $package; |
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76 | #my $meth = $package->can('(' . shift); |
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77 | ov_method mycan($package, '(' . shift), $package; |
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78 | #return $meth if $meth ne \&nil; |
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79 | #return $ {*{$meth}}; |
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80 | } |
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81 | |
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82 | sub AddrRef { |
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83 | my $package = ref $_[0]; |
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84 | return "$_[0]" unless $package; |
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85 | bless $_[0], overload::Fake; # Non-overloaded package |
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86 | my $str = "$_[0]"; |
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87 | bless $_[0], $package; # Back |
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88 | $package . substr $str, index $str, '='; |
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89 | } |
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90 | |
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91 | sub StrVal { |
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92 | (OverloadedStringify($_[0]) or ref($_[0]) eq 'Regexp') ? |
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93 | (AddrRef(shift)) : |
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94 | "$_[0]"; |
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95 | } |
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96 | |
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97 | sub mycan { # Real can would leave stubs. |
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98 | my ($package, $meth) = @_; |
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99 | return \*{$package . "::$meth"} if defined &{$package . "::$meth"}; |
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100 | my $p; |
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101 | foreach $p (@{$package . "::ISA"}) { |
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102 | my $out = mycan($p, $meth); |
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103 | return $out if $out; |
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104 | } |
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105 | return undef; |
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106 | } |
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107 | |
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108 | %constants = ( |
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109 | 'integer' => 0x1000, |
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110 | 'float' => 0x2000, |
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111 | 'binary' => 0x4000, |
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112 | 'q' => 0x8000, |
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113 | 'qr' => 0x10000, |
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114 | ); |
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115 | |
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116 | %ops = ( with_assign => "+ - * / % ** << >> x .", |
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117 | assign => "+= -= *= /= %= **= <<= >>= x= .=", |
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118 | num_comparison => "< <= > >= == !=", |
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119 | '3way_comparison'=> "<=> cmp", |
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120 | str_comparison => "lt le gt ge eq ne", |
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121 | binary => "& | ^", |
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122 | unary => "neg ! ~", |
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123 | mutators => '++ --', |
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124 | func => "atan2 cos sin exp abs log sqrt", |
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125 | conversion => 'bool "" 0+', |
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126 | iterators => '<>', |
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127 | dereferencing => '${} @{} %{} &{} *{}', |
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128 | special => 'nomethod fallback ='); |
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129 | |
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130 | sub constant { |
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131 | # Arguments: what, sub |
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132 | while (@_) { |
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133 | $^H{$_[0]} = $_[1]; |
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134 | $^H |= $constants{$_[0]} | $overload::hint_bits; |
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135 | shift, shift; |
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136 | } |
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137 | } |
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138 | |
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139 | sub remove_constant { |
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140 | # Arguments: what, sub |
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141 | while (@_) { |
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142 | delete $^H{$_[0]}; |
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143 | $^H &= ~ $constants{$_[0]}; |
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144 | shift, shift; |
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145 | } |
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146 | } |
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147 | |
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148 | 1; |
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149 | |
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150 | __END__ |
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151 | |
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152 | =head1 NAME |
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153 | |
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154 | overload - Package for overloading perl operations |
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155 | |
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156 | =head1 SYNOPSIS |
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157 | |
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158 | package SomeThing; |
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159 | |
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160 | use overload |
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161 | '+' => \&myadd, |
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162 | '-' => \&mysub; |
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163 | # etc |
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164 | ... |
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165 | |
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166 | package main; |
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167 | $a = new SomeThing 57; |
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168 | $b=5+$a; |
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169 | ... |
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170 | if (overload::Overloaded $b) {...} |
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171 | ... |
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172 | $strval = overload::StrVal $b; |
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173 | |
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174 | =head1 DESCRIPTION |
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175 | |
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176 | =head2 Declaration of overloaded functions |
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177 | |
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178 | The compilation directive |
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179 | |
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180 | package Number; |
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181 | use overload |
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182 | "+" => \&add, |
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183 | "*=" => "muas"; |
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184 | |
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185 | declares function Number::add() for addition, and method muas() in |
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186 | the "class" C<Number> (or one of its base classes) |
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187 | for the assignment form C<*=> of multiplication. |
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188 | |
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189 | Arguments of this directive come in (key, value) pairs. Legal values |
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190 | are values legal inside a C<&{ ... }> call, so the name of a |
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191 | subroutine, a reference to a subroutine, or an anonymous subroutine |
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192 | will all work. Note that values specified as strings are |
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193 | interpreted as methods, not subroutines. Legal keys are listed below. |
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194 | |
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195 | The subroutine C<add> will be called to execute C<$a+$b> if $a |
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196 | is a reference to an object blessed into the package C<Number>, or if $a is |
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197 | not an object from a package with defined mathemagic addition, but $b is a |
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198 | reference to a C<Number>. It can also be called in other situations, like |
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199 | C<$a+=7>, or C<$a++>. See L<MAGIC AUTOGENERATION>. (Mathemagical |
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200 | methods refer to methods triggered by an overloaded mathematical |
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201 | operator.) |
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202 | |
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203 | Since overloading respects inheritance via the @ISA hierarchy, the |
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204 | above declaration would also trigger overloading of C<+> and C<*=> in |
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205 | all the packages which inherit from C<Number>. |
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206 | |
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207 | =head2 Calling Conventions for Binary Operations |
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208 | |
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209 | The functions specified in the C<use overload ...> directive are called |
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210 | with three (in one particular case with four, see L<Last Resort>) |
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211 | arguments. If the corresponding operation is binary, then the first |
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212 | two arguments are the two arguments of the operation. However, due to |
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213 | general object calling conventions, the first argument should always be |
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214 | an object in the package, so in the situation of C<7+$a>, the |
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215 | order of the arguments is interchanged. It probably does not matter |
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216 | when implementing the addition method, but whether the arguments |
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217 | are reversed is vital to the subtraction method. The method can |
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218 | query this information by examining the third argument, which can take |
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219 | three different values: |
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220 | |
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221 | =over 7 |
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222 | |
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223 | =item FALSE |
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224 | |
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225 | the order of arguments is as in the current operation. |
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226 | |
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227 | =item TRUE |
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228 | |
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229 | the arguments are reversed. |
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230 | |
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231 | =item C<undef> |
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232 | |
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233 | the current operation is an assignment variant (as in |
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234 | C<$a+=7>), but the usual function is called instead. This additional |
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235 | information can be used to generate some optimizations. Compare |
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236 | L<Calling Conventions for Mutators>. |
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237 | |
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238 | =back |
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239 | |
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240 | =head2 Calling Conventions for Unary Operations |
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241 | |
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242 | Unary operation are considered binary operations with the second |
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243 | argument being C<undef>. Thus the functions that overloads C<{"++"}> |
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244 | is called with arguments C<($a,undef,'')> when $a++ is executed. |
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245 | |
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246 | =head2 Calling Conventions for Mutators |
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247 | |
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248 | Two types of mutators have different calling conventions: |
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249 | |
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250 | =over |
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251 | |
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252 | =item C<++> and C<--> |
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253 | |
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254 | The routines which implement these operators are expected to actually |
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255 | I<mutate> their arguments. So, assuming that $obj is a reference to a |
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256 | number, |
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257 | |
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258 | sub incr { my $n = $ {$_[0]}; ++$n; $_[0] = bless \$n} |
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259 | |
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260 | is an appropriate implementation of overloaded C<++>. Note that |
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261 | |
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262 | sub incr { ++$ {$_[0]} ; shift } |
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263 | |
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264 | is OK if used with preincrement and with postincrement. (In the case |
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265 | of postincrement a copying will be performed, see L<Copy Constructor>.) |
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266 | |
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267 | =item C<x=> and other assignment versions |
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268 | |
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269 | There is nothing special about these methods. They may change the |
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270 | value of their arguments, and may leave it as is. The result is going |
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271 | to be assigned to the value in the left-hand-side if different from |
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272 | this value. |
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273 | |
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274 | This allows for the same method to be used as overloaded C<+=> and |
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275 | C<+>. Note that this is I<allowed>, but not recommended, since by the |
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276 | semantic of L<"Fallback"> Perl will call the method for C<+> anyway, |
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277 | if C<+=> is not overloaded. |
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278 | |
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279 | =back |
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280 | |
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281 | B<Warning.> Due to the presense of assignment versions of operations, |
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282 | routines which may be called in assignment context may create |
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283 | self-referential structures. Currently Perl will not free self-referential |
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284 | structures until cycles are C<explicitly> broken. You may get problems |
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285 | when traversing your structures too. |
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286 | |
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287 | Say, |
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288 | |
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289 | use overload '+' => sub { bless [ \$_[0], \$_[1] ] }; |
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290 | |
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291 | is asking for trouble, since for code C<$obj += $foo> the subroutine |
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292 | is called as C<$obj = add($obj, $foo, undef)>, or C<$obj = [\$obj, |
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293 | \$foo]>. If using such a subroutine is an important optimization, one |
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294 | can overload C<+=> explicitly by a non-"optimized" version, or switch |
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295 | to non-optimized version if C<not defined $_[2]> (see |
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296 | L<Calling Conventions for Binary Operations>). |
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297 | |
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298 | Even if no I<explicit> assignment-variants of operators are present in |
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299 | the script, they may be generated by the optimizer. Say, C<",$obj,"> or |
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300 | C<',' . $obj . ','> may be both optimized to |
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301 | |
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302 | my $tmp = ',' . $obj; $tmp .= ','; |
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303 | |
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304 | =head2 Overloadable Operations |
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305 | |
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306 | The following symbols can be specified in C<use overload> directive: |
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307 | |
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308 | =over 5 |
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309 | |
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310 | =item * I<Arithmetic operations> |
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311 | |
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312 | "+", "+=", "-", "-=", "*", "*=", "/", "/=", "%", "%=", |
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313 | "**", "**=", "<<", "<<=", ">>", ">>=", "x", "x=", ".", ".=", |
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314 | |
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315 | For these operations a substituted non-assignment variant can be called if |
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316 | the assignment variant is not available. Methods for operations "C<+>", |
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317 | "C<->", "C<+=>", and "C<-=>" can be called to automatically generate |
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318 | increment and decrement methods. The operation "C<->" can be used to |
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319 | autogenerate missing methods for unary minus or C<abs>. |
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320 | |
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321 | See L<"MAGIC AUTOGENERATION">, L<"Calling Conventions for Mutators"> and |
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322 | L<"Calling Conventions for Binary Operations">) for details of these |
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323 | substitutions. |
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324 | |
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325 | =item * I<Comparison operations> |
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326 | |
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327 | "<", "<=", ">", ">=", "==", "!=", "<=>", |
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328 | "lt", "le", "gt", "ge", "eq", "ne", "cmp", |
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329 | |
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330 | If the corresponding "spaceship" variant is available, it can be |
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331 | used to substitute for the missing operation. During C<sort>ing |
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332 | arrays, C<cmp> is used to compare values subject to C<use overload>. |
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333 | |
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334 | =item * I<Bit operations> |
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335 | |
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336 | "&", "^", "|", "neg", "!", "~", |
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337 | |
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338 | "C<neg>" stands for unary minus. If the method for C<neg> is not |
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339 | specified, it can be autogenerated using the method for |
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340 | subtraction. If the method for "C<!>" is not specified, it can be |
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341 | autogenerated using the methods for "C<bool>", or "C<\"\">", or "C<0+>". |
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342 | |
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343 | =item * I<Increment and decrement> |
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344 | |
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345 | "++", "--", |
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346 | |
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347 | If undefined, addition and subtraction methods can be |
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348 | used instead. These operations are called both in prefix and |
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349 | postfix form. |
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350 | |
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351 | =item * I<Transcendental functions> |
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352 | |
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353 | "atan2", "cos", "sin", "exp", "abs", "log", "sqrt", |
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354 | |
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355 | If C<abs> is unavailable, it can be autogenerated using methods |
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356 | for "E<lt>" or "E<lt>=E<gt>" combined with either unary minus or subtraction. |
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357 | |
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358 | =item * I<Boolean, string and numeric conversion> |
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359 | |
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360 | "bool", "\"\"", "0+", |
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361 | |
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362 | If one or two of these operations are not overloaded, the remaining ones can |
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363 | be used instead. C<bool> is used in the flow control operators |
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364 | (like C<while>) and for the ternary "C<?:>" operation. These functions can |
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365 | return any arbitrary Perl value. If the corresponding operation for this value |
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366 | is overloaded too, that operation will be called again with this value. |
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367 | |
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368 | =item * I<Iteration> |
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369 | |
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370 | "<>" |
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371 | |
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372 | If not overloaded, the argument will be converted to a filehandle or |
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373 | glob (which may require a stringification). The same overloading |
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374 | happens both for the I<read-filehandle> syntax C<E<lt>$varE<gt>> and |
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375 | I<globbing> syntax C<E<lt>${var}E<gt>>. |
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376 | |
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377 | =item * I<Dereferencing> |
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378 | |
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379 | '${}', '@{}', '%{}', '&{}', '*{}'. |
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380 | |
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381 | If not overloaded, the argument will be dereferenced I<as is>, thus |
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382 | should be of correct type. These functions should return a reference |
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383 | of correct type, or another object with overloaded dereferencing. |
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384 | |
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385 | =item * I<Special> |
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386 | |
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387 | "nomethod", "fallback", "=", |
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388 | |
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389 | see L<SPECIAL SYMBOLS FOR C<use overload>>. |
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390 | |
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391 | =back |
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392 | |
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393 | See L<"Fallback"> for an explanation of when a missing method can be |
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394 | autogenerated. |
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395 | |
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396 | A computer-readable form of the above table is available in the hash |
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397 | %overload::ops, with values being space-separated lists of names: |
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398 | |
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399 | with_assign => '+ - * / % ** << >> x .', |
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400 | assign => '+= -= *= /= %= **= <<= >>= x= .=', |
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401 | num_comparison => '< <= > >= == !=', |
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402 | '3way_comparison'=> '<=> cmp', |
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403 | str_comparison => 'lt le gt ge eq ne', |
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404 | binary => '& | ^', |
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405 | unary => 'neg ! ~', |
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406 | mutators => '++ --', |
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407 | func => 'atan2 cos sin exp abs log sqrt', |
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408 | conversion => 'bool "" 0+', |
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409 | iterators => '<>', |
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410 | dereferencing => '${} @{} %{} &{} *{}', |
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411 | special => 'nomethod fallback =' |
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412 | |
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413 | =head2 Inheritance and overloading |
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414 | |
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415 | Inheritance interacts with overloading in two ways. |
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416 | |
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417 | =over |
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418 | |
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419 | =item Strings as values of C<use overload> directive |
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420 | |
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421 | If C<value> in |
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422 | |
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423 | use overload key => value; |
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424 | |
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425 | is a string, it is interpreted as a method name. |
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426 | |
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427 | =item Overloading of an operation is inherited by derived classes |
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428 | |
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429 | Any class derived from an overloaded class is also overloaded. The |
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430 | set of overloaded methods is the union of overloaded methods of all |
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431 | the ancestors. If some method is overloaded in several ancestor, then |
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432 | which description will be used is decided by the usual inheritance |
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433 | rules: |
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434 | |
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435 | If C<A> inherits from C<B> and C<C> (in this order), C<B> overloads |
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436 | C<+> with C<\&D::plus_sub>, and C<C> overloads C<+> by C<"plus_meth">, |
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437 | then the subroutine C<D::plus_sub> will be called to implement |
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438 | operation C<+> for an object in package C<A>. |
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439 | |
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440 | =back |
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441 | |
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442 | Note that since the value of the C<fallback> key is not a subroutine, |
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443 | its inheritance is not governed by the above rules. In the current |
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444 | implementation, the value of C<fallback> in the first overloaded |
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445 | ancestor is used, but this is accidental and subject to change. |
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446 | |
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447 | =head1 SPECIAL SYMBOLS FOR C<use overload> |
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448 | |
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449 | Three keys are recognized by Perl that are not covered by the above |
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450 | description. |
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451 | |
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452 | =head2 Last Resort |
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453 | |
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454 | C<"nomethod"> should be followed by a reference to a function of four |
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455 | parameters. If defined, it is called when the overloading mechanism |
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456 | cannot find a method for some operation. The first three arguments of |
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457 | this function coincide with the arguments for the corresponding method if |
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458 | it were found, the fourth argument is the symbol |
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459 | corresponding to the missing method. If several methods are tried, |
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460 | the last one is used. Say, C<1-$a> can be equivalent to |
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461 | |
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462 | &nomethodMethod($a,1,1,"-") |
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463 | |
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464 | if the pair C<"nomethod" =E<gt> "nomethodMethod"> was specified in the |
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465 | C<use overload> directive. |
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466 | |
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467 | If some operation cannot be resolved, and there is no function |
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468 | assigned to C<"nomethod">, then an exception will be raised via die()-- |
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469 | unless C<"fallback"> was specified as a key in C<use overload> directive. |
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470 | |
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471 | =head2 Fallback |
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472 | |
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473 | The key C<"fallback"> governs what to do if a method for a particular |
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474 | operation is not found. Three different cases are possible depending on |
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475 | the value of C<"fallback">: |
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476 | |
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477 | =over 16 |
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478 | |
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479 | =item * C<undef> |
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480 | |
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481 | Perl tries to use a |
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482 | substituted method (see L<MAGIC AUTOGENERATION>). If this fails, it |
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483 | then tries to calls C<"nomethod"> value; if missing, an exception |
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484 | will be raised. |
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485 | |
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486 | =item * TRUE |
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487 | |
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488 | The same as for the C<undef> value, but no exception is raised. Instead, |
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489 | it silently reverts to what it would have done were there no C<use overload> |
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490 | present. |
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491 | |
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492 | =item * defined, but FALSE |
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493 | |
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494 | No autogeneration is tried. Perl tries to call |
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495 | C<"nomethod"> value, and if this is missing, raises an exception. |
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496 | |
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497 | =back |
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498 | |
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499 | B<Note.> C<"fallback"> inheritance via @ISA is not carved in stone |
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500 | yet, see L<"Inheritance and overloading">. |
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501 | |
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502 | =head2 Copy Constructor |
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503 | |
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504 | The value for C<"="> is a reference to a function with three |
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505 | arguments, i.e., it looks like the other values in C<use |
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506 | overload>. However, it does not overload the Perl assignment |
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507 | operator. This would go against Camel hair. |
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508 | |
---|
509 | This operation is called in the situations when a mutator is applied |
---|
510 | to a reference that shares its object with some other reference, such |
---|
511 | as |
---|
512 | |
---|
513 | $a=$b; |
---|
514 | ++$a; |
---|
515 | |
---|
516 | To make this change $a and not change $b, a copy of C<$$a> is made, |
---|
517 | and $a is assigned a reference to this new object. This operation is |
---|
518 | done during execution of the C<++$a>, and not during the assignment, |
---|
519 | (so before the increment C<$$a> coincides with C<$$b>). This is only |
---|
520 | done if C<++> is expressed via a method for C<'++'> or C<'+='> (or |
---|
521 | C<nomethod>). Note that if this operation is expressed via C<'+'> |
---|
522 | a nonmutator, i.e., as in |
---|
523 | |
---|
524 | $a=$b; |
---|
525 | $a=$a+1; |
---|
526 | |
---|
527 | then C<$a> does not reference a new copy of C<$$a>, since $$a does not |
---|
528 | appear as lvalue when the above code is executed. |
---|
529 | |
---|
530 | If the copy constructor is required during the execution of some mutator, |
---|
531 | but a method for C<'='> was not specified, it can be autogenerated as a |
---|
532 | string copy if the object is a plain scalar. |
---|
533 | |
---|
534 | =over 5 |
---|
535 | |
---|
536 | =item B<Example> |
---|
537 | |
---|
538 | The actually executed code for |
---|
539 | |
---|
540 | $a=$b; |
---|
541 | Something else which does not modify $a or $b.... |
---|
542 | ++$a; |
---|
543 | |
---|
544 | may be |
---|
545 | |
---|
546 | $a=$b; |
---|
547 | Something else which does not modify $a or $b.... |
---|
548 | $a = $a->clone(undef,""); |
---|
549 | $a->incr(undef,""); |
---|
550 | |
---|
551 | if $b was mathemagical, and C<'++'> was overloaded with C<\&incr>, |
---|
552 | C<'='> was overloaded with C<\&clone>. |
---|
553 | |
---|
554 | =back |
---|
555 | |
---|
556 | Same behaviour is triggered by C<$b = $a++>, which is consider a synonym for |
---|
557 | C<$b = $a; ++$a>. |
---|
558 | |
---|
559 | =head1 MAGIC AUTOGENERATION |
---|
560 | |
---|
561 | If a method for an operation is not found, and the value for C<"fallback"> is |
---|
562 | TRUE or undefined, Perl tries to autogenerate a substitute method for |
---|
563 | the missing operation based on the defined operations. Autogenerated method |
---|
564 | substitutions are possible for the following operations: |
---|
565 | |
---|
566 | =over 16 |
---|
567 | |
---|
568 | =item I<Assignment forms of arithmetic operations> |
---|
569 | |
---|
570 | C<$a+=$b> can use the method for C<"+"> if the method for C<"+="> |
---|
571 | is not defined. |
---|
572 | |
---|
573 | =item I<Conversion operations> |
---|
574 | |
---|
575 | String, numeric, and boolean conversion are calculated in terms of one |
---|
576 | another if not all of them are defined. |
---|
577 | |
---|
578 | =item I<Increment and decrement> |
---|
579 | |
---|
580 | The C<++$a> operation can be expressed in terms of C<$a+=1> or C<$a+1>, |
---|
581 | and C<$a--> in terms of C<$a-=1> and C<$a-1>. |
---|
582 | |
---|
583 | =item C<abs($a)> |
---|
584 | |
---|
585 | can be expressed in terms of C<$aE<lt>0> and C<-$a> (or C<0-$a>). |
---|
586 | |
---|
587 | =item I<Unary minus> |
---|
588 | |
---|
589 | can be expressed in terms of subtraction. |
---|
590 | |
---|
591 | =item I<Negation> |
---|
592 | |
---|
593 | C<!> and C<not> can be expressed in terms of boolean conversion, or |
---|
594 | string or numerical conversion. |
---|
595 | |
---|
596 | =item I<Concatenation> |
---|
597 | |
---|
598 | can be expressed in terms of string conversion. |
---|
599 | |
---|
600 | =item I<Comparison operations> |
---|
601 | |
---|
602 | can be expressed in terms of its "spaceship" counterpart: either |
---|
603 | C<E<lt>=E<gt>> or C<cmp>: |
---|
604 | |
---|
605 | <, >, <=, >=, ==, != in terms of <=> |
---|
606 | lt, gt, le, ge, eq, ne in terms of cmp |
---|
607 | |
---|
608 | =item I<Iterator> |
---|
609 | |
---|
610 | <> in terms of builtin operations |
---|
611 | |
---|
612 | =item I<Dereferencing> |
---|
613 | |
---|
614 | ${} @{} %{} &{} *{} in terms of builtin operations |
---|
615 | |
---|
616 | =item I<Copy operator> |
---|
617 | |
---|
618 | can be expressed in terms of an assignment to the dereferenced value, if this |
---|
619 | value is a scalar and not a reference. |
---|
620 | |
---|
621 | =back |
---|
622 | |
---|
623 | =head1 Losing overloading |
---|
624 | |
---|
625 | The restriction for the comparison operation is that even if, for example, |
---|
626 | `C<cmp>' should return a blessed reference, the autogenerated `C<lt>' |
---|
627 | function will produce only a standard logical value based on the |
---|
628 | numerical value of the result of `C<cmp>'. In particular, a working |
---|
629 | numeric conversion is needed in this case (possibly expressed in terms of |
---|
630 | other conversions). |
---|
631 | |
---|
632 | Similarly, C<.=> and C<x=> operators lose their mathemagical properties |
---|
633 | if the string conversion substitution is applied. |
---|
634 | |
---|
635 | When you chop() a mathemagical object it is promoted to a string and its |
---|
636 | mathemagical properties are lost. The same can happen with other |
---|
637 | operations as well. |
---|
638 | |
---|
639 | =head1 Run-time Overloading |
---|
640 | |
---|
641 | Since all C<use> directives are executed at compile-time, the only way to |
---|
642 | change overloading during run-time is to |
---|
643 | |
---|
644 | eval 'use overload "+" => \&addmethod'; |
---|
645 | |
---|
646 | You can also use |
---|
647 | |
---|
648 | eval 'no overload "+", "--", "<="'; |
---|
649 | |
---|
650 | though the use of these constructs during run-time is questionable. |
---|
651 | |
---|
652 | =head1 Public functions |
---|
653 | |
---|
654 | Package C<overload.pm> provides the following public functions: |
---|
655 | |
---|
656 | =over 5 |
---|
657 | |
---|
658 | =item overload::StrVal(arg) |
---|
659 | |
---|
660 | Gives string value of C<arg> as in absence of stringify overloading. |
---|
661 | |
---|
662 | =item overload::Overloaded(arg) |
---|
663 | |
---|
664 | Returns true if C<arg> is subject to overloading of some operations. |
---|
665 | |
---|
666 | =item overload::Method(obj,op) |
---|
667 | |
---|
668 | Returns C<undef> or a reference to the method that implements C<op>. |
---|
669 | |
---|
670 | =back |
---|
671 | |
---|
672 | =head1 Overloading constants |
---|
673 | |
---|
674 | For some application Perl parser mangles constants too much. It is possible |
---|
675 | to hook into this process via overload::constant() and overload::remove_constant() |
---|
676 | functions. |
---|
677 | |
---|
678 | These functions take a hash as an argument. The recognized keys of this hash |
---|
679 | are |
---|
680 | |
---|
681 | =over 8 |
---|
682 | |
---|
683 | =item integer |
---|
684 | |
---|
685 | to overload integer constants, |
---|
686 | |
---|
687 | =item float |
---|
688 | |
---|
689 | to overload floating point constants, |
---|
690 | |
---|
691 | =item binary |
---|
692 | |
---|
693 | to overload octal and hexadecimal constants, |
---|
694 | |
---|
695 | =item q |
---|
696 | |
---|
697 | to overload C<q>-quoted strings, constant pieces of C<qq>- and C<qx>-quoted |
---|
698 | strings and here-documents, |
---|
699 | |
---|
700 | =item qr |
---|
701 | |
---|
702 | to overload constant pieces of regular expressions. |
---|
703 | |
---|
704 | =back |
---|
705 | |
---|
706 | The corresponding values are references to functions which take three arguments: |
---|
707 | the first one is the I<initial> string form of the constant, the second one |
---|
708 | is how Perl interprets this constant, the third one is how the constant is used. |
---|
709 | Note that the initial string form does not |
---|
710 | contain string delimiters, and has backslashes in backslash-delimiter |
---|
711 | combinations stripped (thus the value of delimiter is not relevant for |
---|
712 | processing of this string). The return value of this function is how this |
---|
713 | constant is going to be interpreted by Perl. The third argument is undefined |
---|
714 | unless for overloaded C<q>- and C<qr>- constants, it is C<q> in single-quote |
---|
715 | context (comes from strings, regular expressions, and single-quote HERE |
---|
716 | documents), it is C<tr> for arguments of C<tr>/C<y> operators, |
---|
717 | it is C<s> for right-hand side of C<s>-operator, and it is C<qq> otherwise. |
---|
718 | |
---|
719 | Since an expression C<"ab$cd,,"> is just a shortcut for C<'ab' . $cd . ',,'>, |
---|
720 | it is expected that overloaded constant strings are equipped with reasonable |
---|
721 | overloaded catenation operator, otherwise absurd results will result. |
---|
722 | Similarly, negative numbers are considered as negations of positive constants. |
---|
723 | |
---|
724 | Note that it is probably meaningless to call the functions overload::constant() |
---|
725 | and overload::remove_constant() from anywhere but import() and unimport() methods. |
---|
726 | From these methods they may be called as |
---|
727 | |
---|
728 | sub import { |
---|
729 | shift; |
---|
730 | return unless @_; |
---|
731 | die "unknown import: @_" unless @_ == 1 and $_[0] eq ':constant'; |
---|
732 | overload::constant integer => sub {Math::BigInt->new(shift)}; |
---|
733 | } |
---|
734 | |
---|
735 | B<BUGS> Currently overloaded-ness of constants does not propagate |
---|
736 | into C<eval '...'>. |
---|
737 | |
---|
738 | =head1 IMPLEMENTATION |
---|
739 | |
---|
740 | What follows is subject to change RSN. |
---|
741 | |
---|
742 | The table of methods for all operations is cached in magic for the |
---|
743 | symbol table hash for the package. The cache is invalidated during |
---|
744 | processing of C<use overload>, C<no overload>, new function |
---|
745 | definitions, and changes in @ISA. However, this invalidation remains |
---|
746 | unprocessed until the next C<bless>ing into the package. Hence if you |
---|
747 | want to change overloading structure dynamically, you'll need an |
---|
748 | additional (fake) C<bless>ing to update the table. |
---|
749 | |
---|
750 | (Every SVish thing has a magic queue, and magic is an entry in that |
---|
751 | queue. This is how a single variable may participate in multiple |
---|
752 | forms of magic simultaneously. For instance, environment variables |
---|
753 | regularly have two forms at once: their %ENV magic and their taint |
---|
754 | magic. However, the magic which implements overloading is applied to |
---|
755 | the stashes, which are rarely used directly, thus should not slow down |
---|
756 | Perl.) |
---|
757 | |
---|
758 | If an object belongs to a package using overload, it carries a special |
---|
759 | flag. Thus the only speed penalty during arithmetic operations without |
---|
760 | overloading is the checking of this flag. |
---|
761 | |
---|
762 | In fact, if C<use overload> is not present, there is almost no overhead |
---|
763 | for overloadable operations, so most programs should not suffer |
---|
764 | measurable performance penalties. A considerable effort was made to |
---|
765 | minimize the overhead when overload is used in some package, but the |
---|
766 | arguments in question do not belong to packages using overload. When |
---|
767 | in doubt, test your speed with C<use overload> and without it. So far |
---|
768 | there have been no reports of substantial speed degradation if Perl is |
---|
769 | compiled with optimization turned on. |
---|
770 | |
---|
771 | There is no size penalty for data if overload is not used. The only |
---|
772 | size penalty if overload is used in some package is that I<all> the |
---|
773 | packages acquire a magic during the next C<bless>ing into the |
---|
774 | package. This magic is three-words-long for packages without |
---|
775 | overloading, and carries the cache table if the package is overloaded. |
---|
776 | |
---|
777 | Copying (C<$a=$b>) is shallow; however, a one-level-deep copying is |
---|
778 | carried out before any operation that can imply an assignment to the |
---|
779 | object $a (or $b) refers to, like C<$a++>. You can override this |
---|
780 | behavior by defining your own copy constructor (see L<"Copy Constructor">). |
---|
781 | |
---|
782 | It is expected that arguments to methods that are not explicitly supposed |
---|
783 | to be changed are constant (but this is not enforced). |
---|
784 | |
---|
785 | =head1 Metaphor clash |
---|
786 | |
---|
787 | One may wonder why the semantic of overloaded C<=> is so counter intuitive. |
---|
788 | If it I<looks> counter intuitive to you, you are subject to a metaphor |
---|
789 | clash. |
---|
790 | |
---|
791 | Here is a Perl object metaphor: |
---|
792 | |
---|
793 | I< object is a reference to blessed data> |
---|
794 | |
---|
795 | and an arithmetic metaphor: |
---|
796 | |
---|
797 | I< object is a thing by itself>. |
---|
798 | |
---|
799 | The I<main> problem of overloading C<=> is the fact that these metaphors |
---|
800 | imply different actions on the assignment C<$a = $b> if $a and $b are |
---|
801 | objects. Perl-think implies that $a becomes a reference to whatever |
---|
802 | $b was referencing. Arithmetic-think implies that the value of "object" |
---|
803 | $a is changed to become the value of the object $b, preserving the fact |
---|
804 | that $a and $b are separate entities. |
---|
805 | |
---|
806 | The difference is not relevant in the absence of mutators. After |
---|
807 | a Perl-way assignment an operation which mutates the data referenced by $a |
---|
808 | would change the data referenced by $b too. Effectively, after |
---|
809 | C<$a = $b> values of $a and $b become I<indistinguishable>. |
---|
810 | |
---|
811 | On the other hand, anyone who has used algebraic notation knows the |
---|
812 | expressive power of the arithmetic metaphor. Overloading works hard |
---|
813 | to enable this metaphor while preserving the Perlian way as far as |
---|
814 | possible. Since it is not not possible to freely mix two contradicting |
---|
815 | metaphors, overloading allows the arithmetic way to write things I<as |
---|
816 | far as all the mutators are called via overloaded access only>. The |
---|
817 | way it is done is described in L<Copy Constructor>. |
---|
818 | |
---|
819 | If some mutator methods are directly applied to the overloaded values, |
---|
820 | one may need to I<explicitly unlink> other values which references the |
---|
821 | same value: |
---|
822 | |
---|
823 | $a = new Data 23; |
---|
824 | ... |
---|
825 | $b = $a; # $b is "linked" to $a |
---|
826 | ... |
---|
827 | $a = $a->clone; # Unlink $b from $a |
---|
828 | $a->increment_by(4); |
---|
829 | |
---|
830 | Note that overloaded access makes this transparent: |
---|
831 | |
---|
832 | $a = new Data 23; |
---|
833 | $b = $a; # $b is "linked" to $a |
---|
834 | $a += 4; # would unlink $b automagically |
---|
835 | |
---|
836 | However, it would not make |
---|
837 | |
---|
838 | $a = new Data 23; |
---|
839 | $a = 4; # Now $a is a plain 4, not 'Data' |
---|
840 | |
---|
841 | preserve "objectness" of $a. But Perl I<has> a way to make assignments |
---|
842 | to an object do whatever you want. It is just not the overload, but |
---|
843 | tie()ing interface (see L<perlfunc/tie>). Adding a FETCH() method |
---|
844 | which returns the object itself, and STORE() method which changes the |
---|
845 | value of the object, one can reproduce the arithmetic metaphor in its |
---|
846 | completeness, at least for variables which were tie()d from the start. |
---|
847 | |
---|
848 | (Note that a workaround for a bug may be needed, see L<"BUGS">.) |
---|
849 | |
---|
850 | =head1 Cookbook |
---|
851 | |
---|
852 | Please add examples to what follows! |
---|
853 | |
---|
854 | =head2 Two-face scalars |
---|
855 | |
---|
856 | Put this in F<two_face.pm> in your Perl library directory: |
---|
857 | |
---|
858 | package two_face; # Scalars with separate string and |
---|
859 | # numeric values. |
---|
860 | sub new { my $p = shift; bless [@_], $p } |
---|
861 | use overload '""' => \&str, '0+' => \&num, fallback => 1; |
---|
862 | sub num {shift->[1]} |
---|
863 | sub str {shift->[0]} |
---|
864 | |
---|
865 | Use it as follows: |
---|
866 | |
---|
867 | require two_face; |
---|
868 | my $seven = new two_face ("vii", 7); |
---|
869 | printf "seven=$seven, seven=%d, eight=%d\n", $seven, $seven+1; |
---|
870 | print "seven contains `i'\n" if $seven =~ /i/; |
---|
871 | |
---|
872 | (The second line creates a scalar which has both a string value, and a |
---|
873 | numeric value.) This prints: |
---|
874 | |
---|
875 | seven=vii, seven=7, eight=8 |
---|
876 | seven contains `i' |
---|
877 | |
---|
878 | =head2 Two-face references |
---|
879 | |
---|
880 | Suppose you want to create an object which is accessible as both an |
---|
881 | array reference, and a hash reference, similar to the builtin |
---|
882 | L<array-accessible-as-a-hash|perlref/"Pseudo-hashes: Using an array as |
---|
883 | a hash"> builtin Perl type. Let us make it better than the builtin |
---|
884 | type, there will be no restriction that you cannot use the index 0 of |
---|
885 | your array. |
---|
886 | |
---|
887 | package two_refs; |
---|
888 | use overload '%{}' => \&gethash, '@{}' => sub { $ {shift()} }; |
---|
889 | sub new { |
---|
890 | my $p = shift; |
---|
891 | bless \ [@_], $p; |
---|
892 | } |
---|
893 | sub gethash { |
---|
894 | my %h; |
---|
895 | my $self = shift; |
---|
896 | tie %h, ref $self, $self; |
---|
897 | \%h; |
---|
898 | } |
---|
899 | |
---|
900 | sub TIEHASH { my $p = shift; bless \ shift, $p } |
---|
901 | my %fields; |
---|
902 | my $i = 0; |
---|
903 | $fields{$_} = $i++ foreach qw{zero one two three}; |
---|
904 | sub STORE { |
---|
905 | my $self = ${shift()}; |
---|
906 | my $key = $fields{shift()}; |
---|
907 | defined $key or die "Out of band access"; |
---|
908 | $$self->[$key] = shift; |
---|
909 | } |
---|
910 | sub FETCH { |
---|
911 | my $self = ${shift()}; |
---|
912 | my $key = $fields{shift()}; |
---|
913 | defined $key or die "Out of band access"; |
---|
914 | $$self->[$key]; |
---|
915 | } |
---|
916 | |
---|
917 | Now one can access an object using both the array and hash syntax: |
---|
918 | |
---|
919 | my $bar = new two_refs 3,4,5,6; |
---|
920 | $bar->[2] = 11; |
---|
921 | $bar->{two} == 11 or die 'bad hash fetch'; |
---|
922 | |
---|
923 | Note several important features of this example. First of all, the |
---|
924 | I<actual> type of $bar is a scalar reference, and we do not overload |
---|
925 | the scalar dereference. Thus we can get the I<actual> non-overloaded |
---|
926 | contents of $bar by just using C<$$bar> (what we do in functions which |
---|
927 | overload dereference). Similarly, the object returned by the |
---|
928 | TIEHASH() method is a scalar reference. |
---|
929 | |
---|
930 | Second, we create a new tied hash each time the hash syntax is used. |
---|
931 | This allows us not to worry about a possibility of a reference loop, |
---|
932 | would would lead to a memory leak. |
---|
933 | |
---|
934 | Both these problems can be cured. Say, if we want to overload hash |
---|
935 | dereference on a reference to an object which is I<implemented> as a |
---|
936 | hash itself, the only problem one has to circumvent is how to access |
---|
937 | this I<actual> hash (as opposed to the I<virtual> exhibited by |
---|
938 | overloaded dereference operator). Here is one possible fetching routine: |
---|
939 | |
---|
940 | sub access_hash { |
---|
941 | my ($self, $key) = (shift, shift); |
---|
942 | my $class = ref $self; |
---|
943 | bless $self, 'overload::dummy'; # Disable overloading of %{} |
---|
944 | my $out = $self->{$key}; |
---|
945 | bless $self, $class; # Restore overloading |
---|
946 | $out; |
---|
947 | } |
---|
948 | |
---|
949 | To move creation of the tied hash on each access, one may an extra |
---|
950 | level of indirection which allows a non-circular structure of references: |
---|
951 | |
---|
952 | package two_refs1; |
---|
953 | use overload '%{}' => sub { ${shift()}->[1] }, |
---|
954 | '@{}' => sub { ${shift()}->[0] }; |
---|
955 | sub new { |
---|
956 | my $p = shift; |
---|
957 | my $a = [@_]; |
---|
958 | my %h; |
---|
959 | tie %h, $p, $a; |
---|
960 | bless \ [$a, \%h], $p; |
---|
961 | } |
---|
962 | sub gethash { |
---|
963 | my %h; |
---|
964 | my $self = shift; |
---|
965 | tie %h, ref $self, $self; |
---|
966 | \%h; |
---|
967 | } |
---|
968 | |
---|
969 | sub TIEHASH { my $p = shift; bless \ shift, $p } |
---|
970 | my %fields; |
---|
971 | my $i = 0; |
---|
972 | $fields{$_} = $i++ foreach qw{zero one two three}; |
---|
973 | sub STORE { |
---|
974 | my $a = ${shift()}; |
---|
975 | my $key = $fields{shift()}; |
---|
976 | defined $key or die "Out of band access"; |
---|
977 | $a->[$key] = shift; |
---|
978 | } |
---|
979 | sub FETCH { |
---|
980 | my $a = ${shift()}; |
---|
981 | my $key = $fields{shift()}; |
---|
982 | defined $key or die "Out of band access"; |
---|
983 | $a->[$key]; |
---|
984 | } |
---|
985 | |
---|
986 | Now if $baz is overloaded like this, then C<$bar> is a reference to a |
---|
987 | reference to the intermediate array, which keeps a reference to an |
---|
988 | actual array, and the access hash. The tie()ing object for the access |
---|
989 | hash is also a reference to a reference to the actual array, so |
---|
990 | |
---|
991 | =over |
---|
992 | |
---|
993 | =item * |
---|
994 | |
---|
995 | There are no loops of references. |
---|
996 | |
---|
997 | =item * |
---|
998 | |
---|
999 | Both "objects" which are blessed into the class C<two_refs1> are |
---|
1000 | references to a reference to an array, thus references to a I<scalar>. |
---|
1001 | Thus the accessor expression C<$$foo-E<gt>[$ind]> involves no |
---|
1002 | overloaded operations. |
---|
1003 | |
---|
1004 | =back |
---|
1005 | |
---|
1006 | =head2 Symbolic calculator |
---|
1007 | |
---|
1008 | Put this in F<symbolic.pm> in your Perl library directory: |
---|
1009 | |
---|
1010 | package symbolic; # Primitive symbolic calculator |
---|
1011 | use overload nomethod => \&wrap; |
---|
1012 | |
---|
1013 | sub new { shift; bless ['n', @_] } |
---|
1014 | sub wrap { |
---|
1015 | my ($obj, $other, $inv, $meth) = @_; |
---|
1016 | ($obj, $other) = ($other, $obj) if $inv; |
---|
1017 | bless [$meth, $obj, $other]; |
---|
1018 | } |
---|
1019 | |
---|
1020 | This module is very unusual as overloaded modules go: it does not |
---|
1021 | provide any usual overloaded operators, instead it provides the L<Last |
---|
1022 | Resort> operator C<nomethod>. In this example the corresponding |
---|
1023 | subroutine returns an object which encapsulates operations done over |
---|
1024 | the objects: C<new symbolic 3> contains C<['n', 3]>, C<2 + new |
---|
1025 | symbolic 3> contains C<['+', 2, ['n', 3]]>. |
---|
1026 | |
---|
1027 | Here is an example of the script which "calculates" the side of |
---|
1028 | circumscribed octagon using the above package: |
---|
1029 | |
---|
1030 | require symbolic; |
---|
1031 | my $iter = 1; # 2**($iter+2) = 8 |
---|
1032 | my $side = new symbolic 1; |
---|
1033 | my $cnt = $iter; |
---|
1034 | |
---|
1035 | while ($cnt--) { |
---|
1036 | $side = (sqrt(1 + $side**2) - 1)/$side; |
---|
1037 | } |
---|
1038 | print "OK\n"; |
---|
1039 | |
---|
1040 | The value of $side is |
---|
1041 | |
---|
1042 | ['/', ['-', ['sqrt', ['+', 1, ['**', ['n', 1], 2]], |
---|
1043 | undef], 1], ['n', 1]] |
---|
1044 | |
---|
1045 | Note that while we obtained this value using a nice little script, |
---|
1046 | there is no simple way to I<use> this value. In fact this value may |
---|
1047 | be inspected in debugger (see L<perldebug>), but ony if |
---|
1048 | C<bareStringify> B<O>ption is set, and not via C<p> command. |
---|
1049 | |
---|
1050 | If one attempts to print this value, then the overloaded operator |
---|
1051 | C<""> will be called, which will call C<nomethod> operator. The |
---|
1052 | result of this operator will be stringified again, but this result is |
---|
1053 | again of type C<symbolic>, which will lead to an infinite loop. |
---|
1054 | |
---|
1055 | Add a pretty-printer method to the module F<symbolic.pm>: |
---|
1056 | |
---|
1057 | sub pretty { |
---|
1058 | my ($meth, $a, $b) = @{+shift}; |
---|
1059 | $a = 'u' unless defined $a; |
---|
1060 | $b = 'u' unless defined $b; |
---|
1061 | $a = $a->pretty if ref $a; |
---|
1062 | $b = $b->pretty if ref $b; |
---|
1063 | "[$meth $a $b]"; |
---|
1064 | } |
---|
1065 | |
---|
1066 | Now one can finish the script by |
---|
1067 | |
---|
1068 | print "side = ", $side->pretty, "\n"; |
---|
1069 | |
---|
1070 | The method C<pretty> is doing object-to-string conversion, so it |
---|
1071 | is natural to overload the operator C<""> using this method. However, |
---|
1072 | inside such a method it is not necessary to pretty-print the |
---|
1073 | I<components> $a and $b of an object. In the above subroutine |
---|
1074 | C<"[$meth $a $b]"> is a catenation of some strings and components $a |
---|
1075 | and $b. If these components use overloading, the catenation operator |
---|
1076 | will look for an overloaded operator C<.>, if not present, it will |
---|
1077 | look for an overloaded operator C<"">. Thus it is enough to use |
---|
1078 | |
---|
1079 | use overload nomethod => \&wrap, '""' => \&str; |
---|
1080 | sub str { |
---|
1081 | my ($meth, $a, $b) = @{+shift}; |
---|
1082 | $a = 'u' unless defined $a; |
---|
1083 | $b = 'u' unless defined $b; |
---|
1084 | "[$meth $a $b]"; |
---|
1085 | } |
---|
1086 | |
---|
1087 | Now one can change the last line of the script to |
---|
1088 | |
---|
1089 | print "side = $side\n"; |
---|
1090 | |
---|
1091 | which outputs |
---|
1092 | |
---|
1093 | side = [/ [- [sqrt [+ 1 [** [n 1 u] 2]] u] 1] [n 1 u]] |
---|
1094 | |
---|
1095 | and one can inspect the value in debugger using all the possible |
---|
1096 | methods. |
---|
1097 | |
---|
1098 | Something is is still amiss: consider the loop variable $cnt of the |
---|
1099 | script. It was a number, not an object. We cannot make this value of |
---|
1100 | type C<symbolic>, since then the loop will not terminate. |
---|
1101 | |
---|
1102 | Indeed, to terminate the cycle, the $cnt should become false. |
---|
1103 | However, the operator C<bool> for checking falsity is overloaded (this |
---|
1104 | time via overloaded C<"">), and returns a long string, thus any object |
---|
1105 | of type C<symbolic> is true. To overcome this, we need a way to |
---|
1106 | compare an object to 0. In fact, it is easier to write a numeric |
---|
1107 | conversion routine. |
---|
1108 | |
---|
1109 | Here is the text of F<symbolic.pm> with such a routine added (and |
---|
1110 | slightly modified str()): |
---|
1111 | |
---|
1112 | package symbolic; # Primitive symbolic calculator |
---|
1113 | use overload |
---|
1114 | nomethod => \&wrap, '""' => \&str, '0+' => \# |
---|
1115 | |
---|
1116 | sub new { shift; bless ['n', @_] } |
---|
1117 | sub wrap { |
---|
1118 | my ($obj, $other, $inv, $meth) = @_; |
---|
1119 | ($obj, $other) = ($other, $obj) if $inv; |
---|
1120 | bless [$meth, $obj, $other]; |
---|
1121 | } |
---|
1122 | sub str { |
---|
1123 | my ($meth, $a, $b) = @{+shift}; |
---|
1124 | $a = 'u' unless defined $a; |
---|
1125 | if (defined $b) { |
---|
1126 | "[$meth $a $b]"; |
---|
1127 | } else { |
---|
1128 | "[$meth $a]"; |
---|
1129 | } |
---|
1130 | } |
---|
1131 | my %subr = ( n => sub {$_[0]}, |
---|
1132 | sqrt => sub {sqrt $_[0]}, |
---|
1133 | '-' => sub {shift() - shift()}, |
---|
1134 | '+' => sub {shift() + shift()}, |
---|
1135 | '/' => sub {shift() / shift()}, |
---|
1136 | '*' => sub {shift() * shift()}, |
---|
1137 | '**' => sub {shift() ** shift()}, |
---|
1138 | ); |
---|
1139 | sub num { |
---|
1140 | my ($meth, $a, $b) = @{+shift}; |
---|
1141 | my $subr = $subr{$meth} |
---|
1142 | or die "Do not know how to ($meth) in symbolic"; |
---|
1143 | $a = $a->num if ref $a eq __PACKAGE__; |
---|
1144 | $b = $b->num if ref $b eq __PACKAGE__; |
---|
1145 | $subr->($a,$b); |
---|
1146 | } |
---|
1147 | |
---|
1148 | All the work of numeric conversion is done in %subr and num(). Of |
---|
1149 | course, %subr is not complete, it contains only operators used in the |
---|
1150 | example below. Here is the extra-credit question: why do we need an |
---|
1151 | explicit recursion in num()? (Answer is at the end of this section.) |
---|
1152 | |
---|
1153 | Use this module like this: |
---|
1154 | |
---|
1155 | require symbolic; |
---|
1156 | my $iter = new symbolic 2; # 16-gon |
---|
1157 | my $side = new symbolic 1; |
---|
1158 | my $cnt = $iter; |
---|
1159 | |
---|
1160 | while ($cnt) { |
---|
1161 | $cnt = $cnt - 1; # Mutator `--' not implemented |
---|
1162 | $side = (sqrt(1 + $side**2) - 1)/$side; |
---|
1163 | } |
---|
1164 | printf "%s=%f\n", $side, $side; |
---|
1165 | printf "pi=%f\n", $side*(2**($iter+2)); |
---|
1166 | |
---|
1167 | It prints (without so many line breaks) |
---|
1168 | |
---|
1169 | [/ [- [sqrt [+ 1 [** [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] |
---|
1170 | [n 1]] 2]]] 1] |
---|
1171 | [/ [- [sqrt [+ 1 [** [n 1] 2]]] 1] [n 1]]]=0.198912 |
---|
1172 | pi=3.182598 |
---|
1173 | |
---|
1174 | The above module is very primitive. It does not implement |
---|
1175 | mutator methods (C<++>, C<-=> and so on), does not do deep copying |
---|
1176 | (not required without mutators!), and implements only those arithmetic |
---|
1177 | operations which are used in the example. |
---|
1178 | |
---|
1179 | To implement most arithmetic operations is easy, one should just use |
---|
1180 | the tables of operations, and change the code which fills %subr to |
---|
1181 | |
---|
1182 | my %subr = ( 'n' => sub {$_[0]} ); |
---|
1183 | foreach my $op (split " ", $overload::ops{with_assign}) { |
---|
1184 | $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; |
---|
1185 | } |
---|
1186 | my @bins = qw(binary 3way_comparison num_comparison str_comparison); |
---|
1187 | foreach my $op (split " ", "@overload::ops{ @bins }") { |
---|
1188 | $subr{$op} = eval "sub {shift() $op shift()}"; |
---|
1189 | } |
---|
1190 | foreach my $op (split " ", "@overload::ops{qw(unary func)}") { |
---|
1191 | print "defining `$op'\n"; |
---|
1192 | $subr{$op} = eval "sub {$op shift()}"; |
---|
1193 | } |
---|
1194 | |
---|
1195 | Due to L<Calling Conventions for Mutators>, we do not need anything |
---|
1196 | special to make C<+=> and friends work, except filling C<+=> entry of |
---|
1197 | %subr, and defining a copy constructor (needed since Perl has no |
---|
1198 | way to know that the implementation of C<'+='> does not mutate |
---|
1199 | the argument, compare L<Copy Constructor>). |
---|
1200 | |
---|
1201 | To implement a copy constructor, add C<'=' => \&cpy> to C<use overload> |
---|
1202 | line, and code (this code assumes that mutators change things one level |
---|
1203 | deep only, so recursive copying is not needed): |
---|
1204 | |
---|
1205 | sub cpy { |
---|
1206 | my $self = shift; |
---|
1207 | bless [@$self], ref $self; |
---|
1208 | } |
---|
1209 | |
---|
1210 | To make C<++> and C<--> work, we need to implement actual mutators, |
---|
1211 | either directly, or in C<nomethod>. We continue to do things inside |
---|
1212 | C<nomethod>, thus add |
---|
1213 | |
---|
1214 | if ($meth eq '++' or $meth eq '--') { |
---|
1215 | @$obj = ($meth, (bless [@$obj]), 1); # Avoid circular reference |
---|
1216 | return $obj; |
---|
1217 | } |
---|
1218 | |
---|
1219 | after the first line of wrap(). This is not a most effective |
---|
1220 | implementation, one may consider |
---|
1221 | |
---|
1222 | sub inc { $_[0] = bless ['++', shift, 1]; } |
---|
1223 | |
---|
1224 | instead. |
---|
1225 | |
---|
1226 | As a final remark, note that one can fill %subr by |
---|
1227 | |
---|
1228 | my %subr = ( 'n' => sub {$_[0]} ); |
---|
1229 | foreach my $op (split " ", $overload::ops{with_assign}) { |
---|
1230 | $subr{$op} = $subr{"$op="} = eval "sub {shift() $op shift()}"; |
---|
1231 | } |
---|
1232 | my @bins = qw(binary 3way_comparison num_comparison str_comparison); |
---|
1233 | foreach my $op (split " ", "@overload::ops{ @bins }") { |
---|
1234 | $subr{$op} = eval "sub {shift() $op shift()}"; |
---|
1235 | } |
---|
1236 | foreach my $op (split " ", "@overload::ops{qw(unary func)}") { |
---|
1237 | $subr{$op} = eval "sub {$op shift()}"; |
---|
1238 | } |
---|
1239 | $subr{'++'} = $subr{'+'}; |
---|
1240 | $subr{'--'} = $subr{'-'}; |
---|
1241 | |
---|
1242 | This finishes implementation of a primitive symbolic calculator in |
---|
1243 | 50 lines of Perl code. Since the numeric values of subexpressions |
---|
1244 | are not cached, the calculator is very slow. |
---|
1245 | |
---|
1246 | Here is the answer for the exercise: In the case of str(), we need no |
---|
1247 | explicit recursion since the overloaded C<.>-operator will fall back |
---|
1248 | to an existing overloaded operator C<"">. Overloaded arithmetic |
---|
1249 | operators I<do not> fall back to numeric conversion if C<fallback> is |
---|
1250 | not explicitly requested. Thus without an explicit recursion num() |
---|
1251 | would convert C<['+', $a, $b]> to C<$a + $b>, which would just rebuild |
---|
1252 | the argument of num(). |
---|
1253 | |
---|
1254 | If you wonder why defaults for conversion are different for str() and |
---|
1255 | num(), note how easy it was to write the symbolic calculator. This |
---|
1256 | simplicity is due to an appropriate choice of defaults. One extra |
---|
1257 | note: due to the explicit recursion num() is more fragile than sym(): |
---|
1258 | we need to explicitly check for the type of $a and $b. If components |
---|
1259 | $a and $b happen to be of some related type, this may lead to problems. |
---|
1260 | |
---|
1261 | =head2 I<Really> symbolic calculator |
---|
1262 | |
---|
1263 | One may wonder why we call the above calculator symbolic. The reason |
---|
1264 | is that the actual calculation of the value of expression is postponed |
---|
1265 | until the value is I<used>. |
---|
1266 | |
---|
1267 | To see it in action, add a method |
---|
1268 | |
---|
1269 | sub STORE { |
---|
1270 | my $obj = shift; |
---|
1271 | $#$obj = 1; |
---|
1272 | @$obj->[0,1] = ('=', shift); |
---|
1273 | } |
---|
1274 | |
---|
1275 | to the package C<symbolic>. After this change one can do |
---|
1276 | |
---|
1277 | my $a = new symbolic 3; |
---|
1278 | my $b = new symbolic 4; |
---|
1279 | my $c = sqrt($a**2 + $b**2); |
---|
1280 | |
---|
1281 | and the numeric value of $c becomes 5. However, after calling |
---|
1282 | |
---|
1283 | $a->STORE(12); $b->STORE(5); |
---|
1284 | |
---|
1285 | the numeric value of $c becomes 13. There is no doubt now that the module |
---|
1286 | symbolic provides a I<symbolic> calculator indeed. |
---|
1287 | |
---|
1288 | To hide the rough edges under the hood, provide a tie()d interface to the |
---|
1289 | package C<symbolic> (compare with L<Metaphor clash>). Add methods |
---|
1290 | |
---|
1291 | sub TIESCALAR { my $pack = shift; $pack->new(@_) } |
---|
1292 | sub FETCH { shift } |
---|
1293 | sub nop { } # Around a bug |
---|
1294 | |
---|
1295 | (the bug is described in L<"BUGS">). One can use this new interface as |
---|
1296 | |
---|
1297 | tie $a, 'symbolic', 3; |
---|
1298 | tie $b, 'symbolic', 4; |
---|
1299 | $a->nop; $b->nop; # Around a bug |
---|
1300 | |
---|
1301 | my $c = sqrt($a**2 + $b**2); |
---|
1302 | |
---|
1303 | Now numeric value of $c is 5. After C<$a = 12; $b = 5> the numeric value |
---|
1304 | of $c becomes 13. To insulate the user of the module add a method |
---|
1305 | |
---|
1306 | sub vars { my $p = shift; tie($_, $p), $_->nop foreach @_; } |
---|
1307 | |
---|
1308 | Now |
---|
1309 | |
---|
1310 | my ($a, $b); |
---|
1311 | symbolic->vars($a, $b); |
---|
1312 | my $c = sqrt($a**2 + $b**2); |
---|
1313 | |
---|
1314 | $a = 3; $b = 4; |
---|
1315 | printf "c5 %s=%f\n", $c, $c; |
---|
1316 | |
---|
1317 | $a = 12; $b = 5; |
---|
1318 | printf "c13 %s=%f\n", $c, $c; |
---|
1319 | |
---|
1320 | shows that the numeric value of $c follows changes to the values of $a |
---|
1321 | and $b. |
---|
1322 | |
---|
1323 | =head1 AUTHOR |
---|
1324 | |
---|
1325 | Ilya Zakharevich E<lt>F<ilya@math.mps.ohio-state.edu>E<gt>. |
---|
1326 | |
---|
1327 | =head1 DIAGNOSTICS |
---|
1328 | |
---|
1329 | When Perl is run with the B<-Do> switch or its equivalent, overloading |
---|
1330 | induces diagnostic messages. |
---|
1331 | |
---|
1332 | Using the C<m> command of Perl debugger (see L<perldebug>) one can |
---|
1333 | deduce which operations are overloaded (and which ancestor triggers |
---|
1334 | this overloading). Say, if C<eq> is overloaded, then the method C<(eq> |
---|
1335 | is shown by debugger. The method C<()> corresponds to the C<fallback> |
---|
1336 | key (in fact a presence of this method shows that this package has |
---|
1337 | overloading enabled, and it is what is used by the C<Overloaded> |
---|
1338 | function of module C<overload>). |
---|
1339 | |
---|
1340 | =head1 BUGS |
---|
1341 | |
---|
1342 | Because it is used for overloading, the per-package hash %OVERLOAD now |
---|
1343 | has a special meaning in Perl. The symbol table is filled with names |
---|
1344 | looking like line-noise. |
---|
1345 | |
---|
1346 | For the purpose of inheritance every overloaded package behaves as if |
---|
1347 | C<fallback> is present (possibly undefined). This may create |
---|
1348 | interesting effects if some package is not overloaded, but inherits |
---|
1349 | from two overloaded packages. |
---|
1350 | |
---|
1351 | Relation between overloading and tie()ing is broken. Overloading is |
---|
1352 | triggered or not basing on the I<previous> class of tie()d value. |
---|
1353 | |
---|
1354 | This happens because the presence of overloading is checked too early, |
---|
1355 | before any tie()d access is attempted. If the FETCH()ed class of the |
---|
1356 | tie()d value does not change, a simple workaround is to access the value |
---|
1357 | immediately after tie()ing, so that after this call the I<previous> class |
---|
1358 | coincides with the current one. |
---|
1359 | |
---|
1360 | B<Needed:> a way to fix this without a speed penalty. |
---|
1361 | |
---|
1362 | Barewords are not covered by overloaded string constants. |
---|
1363 | |
---|
1364 | This document is confusing. There are grammos and misleading language |
---|
1365 | used in places. It would seem a total rewrite is needed. |
---|
1366 | |
---|
1367 | =cut |
---|
1368 | |
---|