[14489] | 1 | =head1 NAME
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| 2 | X<tie>
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| 3 |
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| 4 | perltie - how to hide an object class in a simple variable
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| 5 |
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| 6 | =head1 SYNOPSIS
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| 7 |
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| 8 | tie VARIABLE, CLASSNAME, LIST
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| 9 |
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| 10 | $object = tied VARIABLE
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| 11 |
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| 12 | untie VARIABLE
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| 13 |
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| 14 | =head1 DESCRIPTION
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| 15 |
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| 16 | Prior to release 5.0 of Perl, a programmer could use dbmopen()
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| 17 | to connect an on-disk database in the standard Unix dbm(3x)
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| 18 | format magically to a %HASH in their program. However, their Perl was either
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| 19 | built with one particular dbm library or another, but not both, and
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| 20 | you couldn't extend this mechanism to other packages or types of variables.
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| 21 |
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| 22 | Now you can.
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| 23 |
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| 24 | The tie() function binds a variable to a class (package) that will provide
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| 25 | the implementation for access methods for that variable. Once this magic
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| 26 | has been performed, accessing a tied variable automatically triggers
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| 27 | method calls in the proper class. The complexity of the class is
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| 28 | hidden behind magic methods calls. The method names are in ALL CAPS,
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| 29 | which is a convention that Perl uses to indicate that they're called
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| 30 | implicitly rather than explicitly--just like the BEGIN() and END()
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| 31 | functions.
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| 32 |
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| 33 | In the tie() call, C<VARIABLE> is the name of the variable to be
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| 34 | enchanted. C<CLASSNAME> is the name of a class implementing objects of
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| 35 | the correct type. Any additional arguments in the C<LIST> are passed to
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| 36 | the appropriate constructor method for that class--meaning TIESCALAR(),
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| 37 | TIEARRAY(), TIEHASH(), or TIEHANDLE(). (Typically these are arguments
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| 38 | such as might be passed to the dbminit() function of C.) The object
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| 39 | returned by the "new" method is also returned by the tie() function,
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| 40 | which would be useful if you wanted to access other methods in
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| 41 | C<CLASSNAME>. (You don't actually have to return a reference to a right
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| 42 | "type" (e.g., HASH or C<CLASSNAME>) so long as it's a properly blessed
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| 43 | object.) You can also retrieve a reference to the underlying object
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| 44 | using the tied() function.
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| 45 |
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| 46 | Unlike dbmopen(), the tie() function will not C<use> or C<require> a module
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| 47 | for you--you need to do that explicitly yourself.
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| 48 |
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| 49 | =head2 Tying Scalars
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| 50 | X<scalar, tying>
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| 51 |
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| 52 | A class implementing a tied scalar should define the following methods:
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| 53 | TIESCALAR, FETCH, STORE, and possibly UNTIE and/or DESTROY.
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| 54 |
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| 55 | Let's look at each in turn, using as an example a tie class for
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| 56 | scalars that allows the user to do something like:
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| 57 |
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| 58 | tie $his_speed, 'Nice', getppid();
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| 59 | tie $my_speed, 'Nice', $$;
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| 60 |
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| 61 | And now whenever either of those variables is accessed, its current
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| 62 | system priority is retrieved and returned. If those variables are set,
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| 63 | then the process's priority is changed!
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| 64 |
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| 65 | We'll use Jarkko Hietaniemi <F<[email protected]>>'s BSD::Resource class (not
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| 66 | included) to access the PRIO_PROCESS, PRIO_MIN, and PRIO_MAX constants
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| 67 | from your system, as well as the getpriority() and setpriority() system
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| 68 | calls. Here's the preamble of the class.
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| 69 |
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| 70 | package Nice;
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| 71 | use Carp;
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| 72 | use BSD::Resource;
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| 73 | use strict;
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| 74 | $Nice::DEBUG = 0 unless defined $Nice::DEBUG;
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| 75 |
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| 76 | =over 4
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| 77 |
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| 78 | =item TIESCALAR classname, LIST
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| 79 | X<TIESCALAR>
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| 80 |
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| 81 | This is the constructor for the class. That means it is
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| 82 | expected to return a blessed reference to a new scalar
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| 83 | (probably anonymous) that it's creating. For example:
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| 84 |
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| 85 | sub TIESCALAR {
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| 86 | my $class = shift;
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| 87 | my $pid = shift || $$; # 0 means me
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| 88 |
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| 89 | if ($pid !~ /^\d+$/) {
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| 90 | carp "Nice::Tie::Scalar got non-numeric pid $pid" if $^W;
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| 91 | return undef;
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| 92 | }
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| 93 |
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| 94 | unless (kill 0, $pid) { # EPERM or ERSCH, no doubt
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| 95 | carp "Nice::Tie::Scalar got bad pid $pid: $!" if $^W;
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| 96 | return undef;
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| 97 | }
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| 98 |
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| 99 | return bless \$pid, $class;
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| 100 | }
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| 101 |
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| 102 | This tie class has chosen to return an error rather than raising an
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| 103 | exception if its constructor should fail. While this is how dbmopen() works,
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| 104 | other classes may well not wish to be so forgiving. It checks the global
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| 105 | variable C<$^W> to see whether to emit a bit of noise anyway.
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| 106 |
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| 107 | =item FETCH this
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| 108 | X<FETCH>
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| 109 |
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| 110 | This method will be triggered every time the tied variable is accessed
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| 111 | (read). It takes no arguments beyond its self reference, which is the
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| 112 | object representing the scalar we're dealing with. Because in this case
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| 113 | we're using just a SCALAR ref for the tied scalar object, a simple $$self
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| 114 | allows the method to get at the real value stored there. In our example
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| 115 | below, that real value is the process ID to which we've tied our variable.
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| 116 |
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| 117 | sub FETCH {
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| 118 | my $self = shift;
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| 119 | confess "wrong type" unless ref $self;
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| 120 | croak "usage error" if @_;
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| 121 | my $nicety;
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| 122 | local($!) = 0;
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| 123 | $nicety = getpriority(PRIO_PROCESS, $$self);
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| 124 | if ($!) { croak "getpriority failed: $!" }
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| 125 | return $nicety;
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| 126 | }
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| 127 |
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| 128 | This time we've decided to blow up (raise an exception) if the renice
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| 129 | fails--there's no place for us to return an error otherwise, and it's
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| 130 | probably the right thing to do.
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| 131 |
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| 132 | =item STORE this, value
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| 133 | X<STORE>
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| 134 |
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| 135 | This method will be triggered every time the tied variable is set
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| 136 | (assigned). Beyond its self reference, it also expects one (and only one)
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| 137 | argument--the new value the user is trying to assign. Don't worry about
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| 138 | returning a value from STORE -- the semantic of assignment returning the
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| 139 | assigned value is implemented with FETCH.
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| 140 |
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| 141 | sub STORE {
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| 142 | my $self = shift;
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| 143 | confess "wrong type" unless ref $self;
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| 144 | my $new_nicety = shift;
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| 145 | croak "usage error" if @_;
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| 146 |
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| 147 | if ($new_nicety < PRIO_MIN) {
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| 148 | carp sprintf
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| 149 | "WARNING: priority %d less than minimum system priority %d",
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| 150 | $new_nicety, PRIO_MIN if $^W;
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| 151 | $new_nicety = PRIO_MIN;
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| 152 | }
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| 153 |
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| 154 | if ($new_nicety > PRIO_MAX) {
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| 155 | carp sprintf
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| 156 | "WARNING: priority %d greater than maximum system priority %d",
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| 157 | $new_nicety, PRIO_MAX if $^W;
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| 158 | $new_nicety = PRIO_MAX;
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| 159 | }
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| 160 |
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| 161 | unless (defined setpriority(PRIO_PROCESS, $$self, $new_nicety)) {
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| 162 | confess "setpriority failed: $!";
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| 163 | }
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| 164 | }
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| 165 |
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| 166 | =item UNTIE this
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| 167 | X<UNTIE>
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| 168 |
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| 169 | This method will be triggered when the C<untie> occurs. This can be useful
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| 170 | if the class needs to know when no further calls will be made. (Except DESTROY
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| 171 | of course.) See L<The C<untie> Gotcha> below for more details.
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| 172 |
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| 173 | =item DESTROY this
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| 174 | X<DESTROY>
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| 175 |
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| 176 | This method will be triggered when the tied variable needs to be destructed.
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| 177 | As with other object classes, such a method is seldom necessary, because Perl
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| 178 | deallocates its moribund object's memory for you automatically--this isn't
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| 179 | C++, you know. We'll use a DESTROY method here for debugging purposes only.
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| 180 |
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| 181 | sub DESTROY {
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| 182 | my $self = shift;
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| 183 | confess "wrong type" unless ref $self;
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| 184 | carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG;
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| 185 | }
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| 186 |
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| 187 | =back
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| 188 |
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| 189 | That's about all there is to it. Actually, it's more than all there
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| 190 | is to it, because we've done a few nice things here for the sake
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| 191 | of completeness, robustness, and general aesthetics. Simpler
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| 192 | TIESCALAR classes are certainly possible.
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| 193 |
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| 194 | =head2 Tying Arrays
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| 195 | X<array, tying>
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| 196 |
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| 197 | A class implementing a tied ordinary array should define the following
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| 198 | methods: TIEARRAY, FETCH, STORE, FETCHSIZE, STORESIZE and perhaps UNTIE and/or DESTROY.
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| 199 |
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| 200 | FETCHSIZE and STORESIZE are used to provide C<$#array> and
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| 201 | equivalent C<scalar(@array)> access.
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| 202 |
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| 203 | The methods POP, PUSH, SHIFT, UNSHIFT, SPLICE, DELETE, and EXISTS are
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| 204 | required if the perl operator with the corresponding (but lowercase) name
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| 205 | is to operate on the tied array. The B<Tie::Array> class can be used as a
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| 206 | base class to implement the first five of these in terms of the basic
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| 207 | methods above. The default implementations of DELETE and EXISTS in
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| 208 | B<Tie::Array> simply C<croak>.
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| 209 |
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| 210 | In addition EXTEND will be called when perl would have pre-extended
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| 211 | allocation in a real array.
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| 212 |
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| 213 | For this discussion, we'll implement an array whose elements are a fixed
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| 214 | size at creation. If you try to create an element larger than the fixed
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| 215 | size, you'll take an exception. For example:
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| 216 |
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| 217 | use FixedElem_Array;
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| 218 | tie @array, 'FixedElem_Array', 3;
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| 219 | $array[0] = 'cat'; # ok.
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| 220 | $array[1] = 'dogs'; # exception, length('dogs') > 3.
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| 221 |
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| 222 | The preamble code for the class is as follows:
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| 223 |
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| 224 | package FixedElem_Array;
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| 225 | use Carp;
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| 226 | use strict;
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| 227 |
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| 228 | =over 4
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| 229 |
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| 230 | =item TIEARRAY classname, LIST
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| 231 | X<TIEARRAY>
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| 232 |
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| 233 | This is the constructor for the class. That means it is expected to
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| 234 | return a blessed reference through which the new array (probably an
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| 235 | anonymous ARRAY ref) will be accessed.
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| 236 |
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| 237 | In our example, just to show you that you don't I<really> have to return an
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| 238 | ARRAY reference, we'll choose a HASH reference to represent our object.
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| 239 | A HASH works out well as a generic record type: the C<{ELEMSIZE}> field will
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| 240 | store the maximum element size allowed, and the C<{ARRAY}> field will hold the
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| 241 | true ARRAY ref. If someone outside the class tries to dereference the
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| 242 | object returned (doubtless thinking it an ARRAY ref), they'll blow up.
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| 243 | This just goes to show you that you should respect an object's privacy.
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| 244 |
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| 245 | sub TIEARRAY {
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| 246 | my $class = shift;
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| 247 | my $elemsize = shift;
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| 248 | if ( @_ || $elemsize =~ /\D/ ) {
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| 249 | croak "usage: tie ARRAY, '" . __PACKAGE__ . "', elem_size";
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| 250 | }
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| 251 | return bless {
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| 252 | ELEMSIZE => $elemsize,
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| 253 | ARRAY => [],
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| 254 | }, $class;
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| 255 | }
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| 256 |
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| 257 | =item FETCH this, index
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| 258 | X<FETCH>
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| 259 |
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| 260 | This method will be triggered every time an individual element the tied array
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| 261 | is accessed (read). It takes one argument beyond its self reference: the
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| 262 | index whose value we're trying to fetch.
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| 263 |
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| 264 | sub FETCH {
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| 265 | my $self = shift;
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| 266 | my $index = shift;
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| 267 | return $self->{ARRAY}->[$index];
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| 268 | }
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| 269 |
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| 270 | If a negative array index is used to read from an array, the index
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| 271 | will be translated to a positive one internally by calling FETCHSIZE
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| 272 | before being passed to FETCH. You may disable this feature by
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| 273 | assigning a true value to the variable C<$NEGATIVE_INDICES> in the
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| 274 | tied array class.
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| 275 |
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| 276 | As you may have noticed, the name of the FETCH method (et al.) is the same
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| 277 | for all accesses, even though the constructors differ in names (TIESCALAR
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| 278 | vs TIEARRAY). While in theory you could have the same class servicing
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| 279 | several tied types, in practice this becomes cumbersome, and it's easiest
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| 280 | to keep them at simply one tie type per class.
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| 281 |
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| 282 | =item STORE this, index, value
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| 283 | X<STORE>
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| 284 |
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| 285 | This method will be triggered every time an element in the tied array is set
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| 286 | (written). It takes two arguments beyond its self reference: the index at
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| 287 | which we're trying to store something and the value we're trying to put
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| 288 | there.
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| 289 |
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| 290 | In our example, C<undef> is really C<$self-E<gt>{ELEMSIZE}> number of
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| 291 | spaces so we have a little more work to do here:
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| 292 |
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| 293 | sub STORE {
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| 294 | my $self = shift;
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| 295 | my( $index, $value ) = @_;
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| 296 | if ( length $value > $self->{ELEMSIZE} ) {
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| 297 | croak "length of $value is greater than $self->{ELEMSIZE}";
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| 298 | }
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| 299 | # fill in the blanks
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| 300 | $self->EXTEND( $index ) if $index > $self->FETCHSIZE();
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| 301 | # right justify to keep element size for smaller elements
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| 302 | $self->{ARRAY}->[$index] = sprintf "%$self->{ELEMSIZE}s", $value;
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| 303 | }
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| 304 |
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| 305 | Negative indexes are treated the same as with FETCH.
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| 306 |
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| 307 | =item FETCHSIZE this
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| 308 | X<FETCHSIZE>
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| 309 |
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| 310 | Returns the total number of items in the tied array associated with
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| 311 | object I<this>. (Equivalent to C<scalar(@array)>). For example:
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| 312 |
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| 313 | sub FETCHSIZE {
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| 314 | my $self = shift;
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| 315 | return scalar @{$self->{ARRAY}};
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| 316 | }
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| 317 |
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| 318 | =item STORESIZE this, count
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| 319 | X<STORESIZE>
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| 320 |
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| 321 | Sets the total number of items in the tied array associated with
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| 322 | object I<this> to be I<count>. If this makes the array larger then
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| 323 | class's mapping of C<undef> should be returned for new positions.
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| 324 | If the array becomes smaller then entries beyond count should be
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| 325 | deleted.
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| 326 |
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| 327 | In our example, 'undef' is really an element containing
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| 328 | C<$self-E<gt>{ELEMSIZE}> number of spaces. Observe:
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| 329 |
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| 330 | sub STORESIZE {
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| 331 | my $self = shift;
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| 332 | my $count = shift;
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| 333 | if ( $count > $self->FETCHSIZE() ) {
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| 334 | foreach ( $count - $self->FETCHSIZE() .. $count ) {
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| 335 | $self->STORE( $_, '' );
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| 336 | }
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| 337 | } elsif ( $count < $self->FETCHSIZE() ) {
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| 338 | foreach ( 0 .. $self->FETCHSIZE() - $count - 2 ) {
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| 339 | $self->POP();
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| 340 | }
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| 341 | }
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| 342 | }
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| 343 |
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| 344 | =item EXTEND this, count
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| 345 | X<EXTEND>
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| 346 |
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| 347 | Informative call that array is likely to grow to have I<count> entries.
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| 348 | Can be used to optimize allocation. This method need do nothing.
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| 349 |
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| 350 | In our example, we want to make sure there are no blank (C<undef>)
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| 351 | entries, so C<EXTEND> will make use of C<STORESIZE> to fill elements
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| 352 | as needed:
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| 353 |
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| 354 | sub EXTEND {
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| 355 | my $self = shift;
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| 356 | my $count = shift;
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| 357 | $self->STORESIZE( $count );
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| 358 | }
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| 359 |
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| 360 | =item EXISTS this, key
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| 361 | X<EXISTS>
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| 362 |
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| 363 | Verify that the element at index I<key> exists in the tied array I<this>.
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| 364 |
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| 365 | In our example, we will determine that if an element consists of
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| 366 | C<$self-E<gt>{ELEMSIZE}> spaces only, it does not exist:
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| 367 |
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| 368 | sub EXISTS {
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| 369 | my $self = shift;
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| 370 | my $index = shift;
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| 371 | return 0 if ! defined $self->{ARRAY}->[$index] ||
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| 372 | $self->{ARRAY}->[$index] eq ' ' x $self->{ELEMSIZE};
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| 373 | return 1;
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| 374 | }
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| 375 |
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| 376 | =item DELETE this, key
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| 377 | X<DELETE>
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| 378 |
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| 379 | Delete the element at index I<key> from the tied array I<this>.
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| 380 |
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| 381 | In our example, a deleted item is C<$self-E<gt>{ELEMSIZE}> spaces:
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| 382 |
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| 383 | sub DELETE {
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| 384 | my $self = shift;
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| 385 | my $index = shift;
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| 386 | return $self->STORE( $index, '' );
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| 387 | }
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| 388 |
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| 389 | =item CLEAR this
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| 390 | X<CLEAR>
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| 391 |
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| 392 | Clear (remove, delete, ...) all values from the tied array associated with
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| 393 | object I<this>. For example:
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| 394 |
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| 395 | sub CLEAR {
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| 396 | my $self = shift;
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| 397 | return $self->{ARRAY} = [];
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| 398 | }
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| 399 |
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| 400 | =item PUSH this, LIST
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| 401 | X<PUSH>
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| 402 |
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| 403 | Append elements of I<LIST> to the array. For example:
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| 404 |
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| 405 | sub PUSH {
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| 406 | my $self = shift;
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| 407 | my @list = @_;
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| 408 | my $last = $self->FETCHSIZE();
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| 409 | $self->STORE( $last + $_, $list[$_] ) foreach 0 .. $#list;
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| 410 | return $self->FETCHSIZE();
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| 411 | }
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| 412 |
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| 413 | =item POP this
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| 414 | X<POP>
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| 415 |
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| 416 | Remove last element of the array and return it. For example:
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| 417 |
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| 418 | sub POP {
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| 419 | my $self = shift;
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| 420 | return pop @{$self->{ARRAY}};
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| 421 | }
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| 422 |
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| 423 | =item SHIFT this
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| 424 | X<SHIFT>
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| 425 |
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| 426 | Remove the first element of the array (shifting other elements down)
|
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| 427 | and return it. For example:
|
---|
| 428 |
|
---|
| 429 | sub SHIFT {
|
---|
| 430 | my $self = shift;
|
---|
| 431 | return shift @{$self->{ARRAY}};
|
---|
| 432 | }
|
---|
| 433 |
|
---|
| 434 | =item UNSHIFT this, LIST
|
---|
| 435 | X<UNSHIFT>
|
---|
| 436 |
|
---|
| 437 | Insert LIST elements at the beginning of the array, moving existing elements
|
---|
| 438 | up to make room. For example:
|
---|
| 439 |
|
---|
| 440 | sub UNSHIFT {
|
---|
| 441 | my $self = shift;
|
---|
| 442 | my @list = @_;
|
---|
| 443 | my $size = scalar( @list );
|
---|
| 444 | # make room for our list
|
---|
| 445 | @{$self->{ARRAY}}[ $size .. $#{$self->{ARRAY}} + $size ]
|
---|
| 446 | = @{$self->{ARRAY}};
|
---|
| 447 | $self->STORE( $_, $list[$_] ) foreach 0 .. $#list;
|
---|
| 448 | }
|
---|
| 449 |
|
---|
| 450 | =item SPLICE this, offset, length, LIST
|
---|
| 451 | X<SPLICE>
|
---|
| 452 |
|
---|
| 453 | Perform the equivalent of C<splice> on the array.
|
---|
| 454 |
|
---|
| 455 | I<offset> is optional and defaults to zero, negative values count back
|
---|
| 456 | from the end of the array.
|
---|
| 457 |
|
---|
| 458 | I<length> is optional and defaults to rest of the array.
|
---|
| 459 |
|
---|
| 460 | I<LIST> may be empty.
|
---|
| 461 |
|
---|
| 462 | Returns a list of the original I<length> elements at I<offset>.
|
---|
| 463 |
|
---|
| 464 | In our example, we'll use a little shortcut if there is a I<LIST>:
|
---|
| 465 |
|
---|
| 466 | sub SPLICE {
|
---|
| 467 | my $self = shift;
|
---|
| 468 | my $offset = shift || 0;
|
---|
| 469 | my $length = shift || $self->FETCHSIZE() - $offset;
|
---|
| 470 | my @list = ();
|
---|
| 471 | if ( @_ ) {
|
---|
| 472 | tie @list, __PACKAGE__, $self->{ELEMSIZE};
|
---|
| 473 | @list = @_;
|
---|
| 474 | }
|
---|
| 475 | return splice @{$self->{ARRAY}}, $offset, $length, @list;
|
---|
| 476 | }
|
---|
| 477 |
|
---|
| 478 | =item UNTIE this
|
---|
| 479 | X<UNTIE>
|
---|
| 480 |
|
---|
| 481 | Will be called when C<untie> happens. (See L<The C<untie> Gotcha> below.)
|
---|
| 482 |
|
---|
| 483 | =item DESTROY this
|
---|
| 484 | X<DESTROY>
|
---|
| 485 |
|
---|
| 486 | This method will be triggered when the tied variable needs to be destructed.
|
---|
| 487 | As with the scalar tie class, this is almost never needed in a
|
---|
| 488 | language that does its own garbage collection, so this time we'll
|
---|
| 489 | just leave it out.
|
---|
| 490 |
|
---|
| 491 | =back
|
---|
| 492 |
|
---|
| 493 | =head2 Tying Hashes
|
---|
| 494 | X<hash, tying>
|
---|
| 495 |
|
---|
| 496 | Hashes were the first Perl data type to be tied (see dbmopen()). A class
|
---|
| 497 | implementing a tied hash should define the following methods: TIEHASH is
|
---|
| 498 | the constructor. FETCH and STORE access the key and value pairs. EXISTS
|
---|
| 499 | reports whether a key is present in the hash, and DELETE deletes one.
|
---|
| 500 | CLEAR empties the hash by deleting all the key and value pairs. FIRSTKEY
|
---|
| 501 | and NEXTKEY implement the keys() and each() functions to iterate over all
|
---|
| 502 | the keys. SCALAR is triggered when the tied hash is evaluated in scalar
|
---|
| 503 | context. UNTIE is called when C<untie> happens, and DESTROY is called when
|
---|
| 504 | the tied variable is garbage collected.
|
---|
| 505 |
|
---|
| 506 | If this seems like a lot, then feel free to inherit from merely the
|
---|
| 507 | standard Tie::StdHash module for most of your methods, redefining only the
|
---|
| 508 | interesting ones. See L<Tie::Hash> for details.
|
---|
| 509 |
|
---|
| 510 | Remember that Perl distinguishes between a key not existing in the hash,
|
---|
| 511 | and the key existing in the hash but having a corresponding value of
|
---|
| 512 | C<undef>. The two possibilities can be tested with the C<exists()> and
|
---|
| 513 | C<defined()> functions.
|
---|
| 514 |
|
---|
| 515 | Here's an example of a somewhat interesting tied hash class: it gives you
|
---|
| 516 | a hash representing a particular user's dot files. You index into the hash
|
---|
| 517 | with the name of the file (minus the dot) and you get back that dot file's
|
---|
| 518 | contents. For example:
|
---|
| 519 |
|
---|
| 520 | use DotFiles;
|
---|
| 521 | tie %dot, 'DotFiles';
|
---|
| 522 | if ( $dot{profile} =~ /MANPATH/ ||
|
---|
| 523 | $dot{login} =~ /MANPATH/ ||
|
---|
| 524 | $dot{cshrc} =~ /MANPATH/ )
|
---|
| 525 | {
|
---|
| 526 | print "you seem to set your MANPATH\n";
|
---|
| 527 | }
|
---|
| 528 |
|
---|
| 529 | Or here's another sample of using our tied class:
|
---|
| 530 |
|
---|
| 531 | tie %him, 'DotFiles', 'daemon';
|
---|
| 532 | foreach $f ( keys %him ) {
|
---|
| 533 | printf "daemon dot file %s is size %d\n",
|
---|
| 534 | $f, length $him{$f};
|
---|
| 535 | }
|
---|
| 536 |
|
---|
| 537 | In our tied hash DotFiles example, we use a regular
|
---|
| 538 | hash for the object containing several important
|
---|
| 539 | fields, of which only the C<{LIST}> field will be what the
|
---|
| 540 | user thinks of as the real hash.
|
---|
| 541 |
|
---|
| 542 | =over 5
|
---|
| 543 |
|
---|
| 544 | =item USER
|
---|
| 545 |
|
---|
| 546 | whose dot files this object represents
|
---|
| 547 |
|
---|
| 548 | =item HOME
|
---|
| 549 |
|
---|
| 550 | where those dot files live
|
---|
| 551 |
|
---|
| 552 | =item CLOBBER
|
---|
| 553 |
|
---|
| 554 | whether we should try to change or remove those dot files
|
---|
| 555 |
|
---|
| 556 | =item LIST
|
---|
| 557 |
|
---|
| 558 | the hash of dot file names and content mappings
|
---|
| 559 |
|
---|
| 560 | =back
|
---|
| 561 |
|
---|
| 562 | Here's the start of F<Dotfiles.pm>:
|
---|
| 563 |
|
---|
| 564 | package DotFiles;
|
---|
| 565 | use Carp;
|
---|
| 566 | sub whowasi { (caller(1))[3] . '()' }
|
---|
| 567 | my $DEBUG = 0;
|
---|
| 568 | sub debug { $DEBUG = @_ ? shift : 1 }
|
---|
| 569 |
|
---|
| 570 | For our example, we want to be able to emit debugging info to help in tracing
|
---|
| 571 | during development. We keep also one convenience function around
|
---|
| 572 | internally to help print out warnings; whowasi() returns the function name
|
---|
| 573 | that calls it.
|
---|
| 574 |
|
---|
| 575 | Here are the methods for the DotFiles tied hash.
|
---|
| 576 |
|
---|
| 577 | =over 4
|
---|
| 578 |
|
---|
| 579 | =item TIEHASH classname, LIST
|
---|
| 580 | X<TIEHASH>
|
---|
| 581 |
|
---|
| 582 | This is the constructor for the class. That means it is expected to
|
---|
| 583 | return a blessed reference through which the new object (probably but not
|
---|
| 584 | necessarily an anonymous hash) will be accessed.
|
---|
| 585 |
|
---|
| 586 | Here's the constructor:
|
---|
| 587 |
|
---|
| 588 | sub TIEHASH {
|
---|
| 589 | my $self = shift;
|
---|
| 590 | my $user = shift || $>;
|
---|
| 591 | my $dotdir = shift || '';
|
---|
| 592 | croak "usage: @{[&whowasi]} [USER [DOTDIR]]" if @_;
|
---|
| 593 | $user = getpwuid($user) if $user =~ /^\d+$/;
|
---|
| 594 | my $dir = (getpwnam($user))[7]
|
---|
| 595 | || croak "@{[&whowasi]}: no user $user";
|
---|
| 596 | $dir .= "/$dotdir" if $dotdir;
|
---|
| 597 |
|
---|
| 598 | my $node = {
|
---|
| 599 | USER => $user,
|
---|
| 600 | HOME => $dir,
|
---|
| 601 | LIST => {},
|
---|
| 602 | CLOBBER => 0,
|
---|
| 603 | };
|
---|
| 604 |
|
---|
| 605 | opendir(DIR, $dir)
|
---|
| 606 | || croak "@{[&whowasi]}: can't opendir $dir: $!";
|
---|
| 607 | foreach $dot ( grep /^\./ && -f "$dir/$_", readdir(DIR)) {
|
---|
| 608 | $dot =~ s/^\.//;
|
---|
| 609 | $node->{LIST}{$dot} = undef;
|
---|
| 610 | }
|
---|
| 611 | closedir DIR;
|
---|
| 612 | return bless $node, $self;
|
---|
| 613 | }
|
---|
| 614 |
|
---|
| 615 | It's probably worth mentioning that if you're going to filetest the
|
---|
| 616 | return values out of a readdir, you'd better prepend the directory
|
---|
| 617 | in question. Otherwise, because we didn't chdir() there, it would
|
---|
| 618 | have been testing the wrong file.
|
---|
| 619 |
|
---|
| 620 | =item FETCH this, key
|
---|
| 621 | X<FETCH>
|
---|
| 622 |
|
---|
| 623 | This method will be triggered every time an element in the tied hash is
|
---|
| 624 | accessed (read). It takes one argument beyond its self reference: the key
|
---|
| 625 | whose value we're trying to fetch.
|
---|
| 626 |
|
---|
| 627 | Here's the fetch for our DotFiles example.
|
---|
| 628 |
|
---|
| 629 | sub FETCH {
|
---|
| 630 | carp &whowasi if $DEBUG;
|
---|
| 631 | my $self = shift;
|
---|
| 632 | my $dot = shift;
|
---|
| 633 | my $dir = $self->{HOME};
|
---|
| 634 | my $file = "$dir/.$dot";
|
---|
| 635 |
|
---|
| 636 | unless (exists $self->{LIST}->{$dot} || -f $file) {
|
---|
| 637 | carp "@{[&whowasi]}: no $dot file" if $DEBUG;
|
---|
| 638 | return undef;
|
---|
| 639 | }
|
---|
| 640 |
|
---|
| 641 | if (defined $self->{LIST}->{$dot}) {
|
---|
| 642 | return $self->{LIST}->{$dot};
|
---|
| 643 | } else {
|
---|
| 644 | return $self->{LIST}->{$dot} = `cat $dir/.$dot`;
|
---|
| 645 | }
|
---|
| 646 | }
|
---|
| 647 |
|
---|
| 648 | It was easy to write by having it call the Unix cat(1) command, but it
|
---|
| 649 | would probably be more portable to open the file manually (and somewhat
|
---|
| 650 | more efficient). Of course, because dot files are a Unixy concept, we're
|
---|
| 651 | not that concerned.
|
---|
| 652 |
|
---|
| 653 | =item STORE this, key, value
|
---|
| 654 | X<STORE>
|
---|
| 655 |
|
---|
| 656 | This method will be triggered every time an element in the tied hash is set
|
---|
| 657 | (written). It takes two arguments beyond its self reference: the index at
|
---|
| 658 | which we're trying to store something, and the value we're trying to put
|
---|
| 659 | there.
|
---|
| 660 |
|
---|
| 661 | Here in our DotFiles example, we'll be careful not to let
|
---|
| 662 | them try to overwrite the file unless they've called the clobber()
|
---|
| 663 | method on the original object reference returned by tie().
|
---|
| 664 |
|
---|
| 665 | sub STORE {
|
---|
| 666 | carp &whowasi if $DEBUG;
|
---|
| 667 | my $self = shift;
|
---|
| 668 | my $dot = shift;
|
---|
| 669 | my $value = shift;
|
---|
| 670 | my $file = $self->{HOME} . "/.$dot";
|
---|
| 671 | my $user = $self->{USER};
|
---|
| 672 |
|
---|
| 673 | croak "@{[&whowasi]}: $file not clobberable"
|
---|
| 674 | unless $self->{CLOBBER};
|
---|
| 675 |
|
---|
| 676 | open(F, "> $file") || croak "can't open $file: $!";
|
---|
| 677 | print F $value;
|
---|
| 678 | close(F);
|
---|
| 679 | }
|
---|
| 680 |
|
---|
| 681 | If they wanted to clobber something, they might say:
|
---|
| 682 |
|
---|
| 683 | $ob = tie %daemon_dots, 'daemon';
|
---|
| 684 | $ob->clobber(1);
|
---|
| 685 | $daemon_dots{signature} = "A true daemon\n";
|
---|
| 686 |
|
---|
| 687 | Another way to lay hands on a reference to the underlying object is to
|
---|
| 688 | use the tied() function, so they might alternately have set clobber
|
---|
| 689 | using:
|
---|
| 690 |
|
---|
| 691 | tie %daemon_dots, 'daemon';
|
---|
| 692 | tied(%daemon_dots)->clobber(1);
|
---|
| 693 |
|
---|
| 694 | The clobber method is simply:
|
---|
| 695 |
|
---|
| 696 | sub clobber {
|
---|
| 697 | my $self = shift;
|
---|
| 698 | $self->{CLOBBER} = @_ ? shift : 1;
|
---|
| 699 | }
|
---|
| 700 |
|
---|
| 701 | =item DELETE this, key
|
---|
| 702 | X<DELETE>
|
---|
| 703 |
|
---|
| 704 | This method is triggered when we remove an element from the hash,
|
---|
| 705 | typically by using the delete() function. Again, we'll
|
---|
| 706 | be careful to check whether they really want to clobber files.
|
---|
| 707 |
|
---|
| 708 | sub DELETE {
|
---|
| 709 | carp &whowasi if $DEBUG;
|
---|
| 710 |
|
---|
| 711 | my $self = shift;
|
---|
| 712 | my $dot = shift;
|
---|
| 713 | my $file = $self->{HOME} . "/.$dot";
|
---|
| 714 | croak "@{[&whowasi]}: won't remove file $file"
|
---|
| 715 | unless $self->{CLOBBER};
|
---|
| 716 | delete $self->{LIST}->{$dot};
|
---|
| 717 | my $success = unlink($file);
|
---|
| 718 | carp "@{[&whowasi]}: can't unlink $file: $!" unless $success;
|
---|
| 719 | $success;
|
---|
| 720 | }
|
---|
| 721 |
|
---|
| 722 | The value returned by DELETE becomes the return value of the call
|
---|
| 723 | to delete(). If you want to emulate the normal behavior of delete(),
|
---|
| 724 | you should return whatever FETCH would have returned for this key.
|
---|
| 725 | In this example, we have chosen instead to return a value which tells
|
---|
| 726 | the caller whether the file was successfully deleted.
|
---|
| 727 |
|
---|
| 728 | =item CLEAR this
|
---|
| 729 | X<CLEAR>
|
---|
| 730 |
|
---|
| 731 | This method is triggered when the whole hash is to be cleared, usually by
|
---|
| 732 | assigning the empty list to it.
|
---|
| 733 |
|
---|
| 734 | In our example, that would remove all the user's dot files! It's such a
|
---|
| 735 | dangerous thing that they'll have to set CLOBBER to something higher than
|
---|
| 736 | 1 to make it happen.
|
---|
| 737 |
|
---|
| 738 | sub CLEAR {
|
---|
| 739 | carp &whowasi if $DEBUG;
|
---|
| 740 | my $self = shift;
|
---|
| 741 | croak "@{[&whowasi]}: won't remove all dot files for $self->{USER}"
|
---|
| 742 | unless $self->{CLOBBER} > 1;
|
---|
| 743 | my $dot;
|
---|
| 744 | foreach $dot ( keys %{$self->{LIST}}) {
|
---|
| 745 | $self->DELETE($dot);
|
---|
| 746 | }
|
---|
| 747 | }
|
---|
| 748 |
|
---|
| 749 | =item EXISTS this, key
|
---|
| 750 | X<EXISTS>
|
---|
| 751 |
|
---|
| 752 | This method is triggered when the user uses the exists() function
|
---|
| 753 | on a particular hash. In our example, we'll look at the C<{LIST}>
|
---|
| 754 | hash element for this:
|
---|
| 755 |
|
---|
| 756 | sub EXISTS {
|
---|
| 757 | carp &whowasi if $DEBUG;
|
---|
| 758 | my $self = shift;
|
---|
| 759 | my $dot = shift;
|
---|
| 760 | return exists $self->{LIST}->{$dot};
|
---|
| 761 | }
|
---|
| 762 |
|
---|
| 763 | =item FIRSTKEY this
|
---|
| 764 | X<FIRSTKEY>
|
---|
| 765 |
|
---|
| 766 | This method will be triggered when the user is going
|
---|
| 767 | to iterate through the hash, such as via a keys() or each()
|
---|
| 768 | call.
|
---|
| 769 |
|
---|
| 770 | sub FIRSTKEY {
|
---|
| 771 | carp &whowasi if $DEBUG;
|
---|
| 772 | my $self = shift;
|
---|
| 773 | my $a = keys %{$self->{LIST}}; # reset each() iterator
|
---|
| 774 | each %{$self->{LIST}}
|
---|
| 775 | }
|
---|
| 776 |
|
---|
| 777 | =item NEXTKEY this, lastkey
|
---|
| 778 | X<NEXTKEY>
|
---|
| 779 |
|
---|
| 780 | This method gets triggered during a keys() or each() iteration. It has a
|
---|
| 781 | second argument which is the last key that had been accessed. This is
|
---|
| 782 | useful if you're carrying about ordering or calling the iterator from more
|
---|
| 783 | than one sequence, or not really storing things in a hash anywhere.
|
---|
| 784 |
|
---|
| 785 | For our example, we're using a real hash so we'll do just the simple
|
---|
| 786 | thing, but we'll have to go through the LIST field indirectly.
|
---|
| 787 |
|
---|
| 788 | sub NEXTKEY {
|
---|
| 789 | carp &whowasi if $DEBUG;
|
---|
| 790 | my $self = shift;
|
---|
| 791 | return each %{ $self->{LIST} }
|
---|
| 792 | }
|
---|
| 793 |
|
---|
| 794 | =item SCALAR this
|
---|
| 795 | X<SCALAR>
|
---|
| 796 |
|
---|
| 797 | This is called when the hash is evaluated in scalar context. In order
|
---|
| 798 | to mimic the behaviour of untied hashes, this method should return a
|
---|
| 799 | false value when the tied hash is considered empty. If this method does
|
---|
| 800 | not exist, perl will make some educated guesses and return true when
|
---|
| 801 | the hash is inside an iteration. If this isn't the case, FIRSTKEY is
|
---|
| 802 | called, and the result will be a false value if FIRSTKEY returns the empty
|
---|
| 803 | list, true otherwise.
|
---|
| 804 |
|
---|
| 805 | However, you should B<not> blindly rely on perl always doing the right
|
---|
| 806 | thing. Particularly, perl will mistakenly return true when you clear the
|
---|
| 807 | hash by repeatedly calling DELETE until it is empty. You are therefore
|
---|
| 808 | advised to supply your own SCALAR method when you want to be absolutely
|
---|
| 809 | sure that your hash behaves nicely in scalar context.
|
---|
| 810 |
|
---|
| 811 | In our example we can just call C<scalar> on the underlying hash
|
---|
| 812 | referenced by C<$self-E<gt>{LIST}>:
|
---|
| 813 |
|
---|
| 814 | sub SCALAR {
|
---|
| 815 | carp &whowasi if $DEBUG;
|
---|
| 816 | my $self = shift;
|
---|
| 817 | return scalar %{ $self->{LIST} }
|
---|
| 818 | }
|
---|
| 819 |
|
---|
| 820 | =item UNTIE this
|
---|
| 821 | X<UNTIE>
|
---|
| 822 |
|
---|
| 823 | This is called when C<untie> occurs. See L<The C<untie> Gotcha> below.
|
---|
| 824 |
|
---|
| 825 | =item DESTROY this
|
---|
| 826 | X<DESTROY>
|
---|
| 827 |
|
---|
| 828 | This method is triggered when a tied hash is about to go out of
|
---|
| 829 | scope. You don't really need it unless you're trying to add debugging
|
---|
| 830 | or have auxiliary state to clean up. Here's a very simple function:
|
---|
| 831 |
|
---|
| 832 | sub DESTROY {
|
---|
| 833 | carp &whowasi if $DEBUG;
|
---|
| 834 | }
|
---|
| 835 |
|
---|
| 836 | =back
|
---|
| 837 |
|
---|
| 838 | Note that functions such as keys() and values() may return huge lists
|
---|
| 839 | when used on large objects, like DBM files. You may prefer to use the
|
---|
| 840 | each() function to iterate over such. Example:
|
---|
| 841 |
|
---|
| 842 | # print out history file offsets
|
---|
| 843 | use NDBM_File;
|
---|
| 844 | tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
|
---|
| 845 | while (($key,$val) = each %HIST) {
|
---|
| 846 | print $key, ' = ', unpack('L',$val), "\n";
|
---|
| 847 | }
|
---|
| 848 | untie(%HIST);
|
---|
| 849 |
|
---|
| 850 | =head2 Tying FileHandles
|
---|
| 851 | X<filehandle, tying>
|
---|
| 852 |
|
---|
| 853 | This is partially implemented now.
|
---|
| 854 |
|
---|
| 855 | A class implementing a tied filehandle should define the following
|
---|
| 856 | methods: TIEHANDLE, at least one of PRINT, PRINTF, WRITE, READLINE, GETC,
|
---|
| 857 | READ, and possibly CLOSE, UNTIE and DESTROY. The class can also provide: BINMODE,
|
---|
| 858 | OPEN, EOF, FILENO, SEEK, TELL - if the corresponding perl operators are
|
---|
| 859 | used on the handle.
|
---|
| 860 |
|
---|
| 861 | When STDERR is tied, its PRINT method will be called to issue warnings
|
---|
| 862 | and error messages. This feature is temporarily disabled during the call,
|
---|
| 863 | which means you can use C<warn()> inside PRINT without starting a recursive
|
---|
| 864 | loop. And just like C<__WARN__> and C<__DIE__> handlers, STDERR's PRINT
|
---|
| 865 | method may be called to report parser errors, so the caveats mentioned under
|
---|
| 866 | L<perlvar/%SIG> apply.
|
---|
| 867 |
|
---|
| 868 | All of this is especially useful when perl is embedded in some other
|
---|
| 869 | program, where output to STDOUT and STDERR may have to be redirected
|
---|
| 870 | in some special way. See nvi and the Apache module for examples.
|
---|
| 871 |
|
---|
| 872 | In our example we're going to create a shouting handle.
|
---|
| 873 |
|
---|
| 874 | package Shout;
|
---|
| 875 |
|
---|
| 876 | =over 4
|
---|
| 877 |
|
---|
| 878 | =item TIEHANDLE classname, LIST
|
---|
| 879 | X<TIEHANDLE>
|
---|
| 880 |
|
---|
| 881 | This is the constructor for the class. That means it is expected to
|
---|
| 882 | return a blessed reference of some sort. The reference can be used to
|
---|
| 883 | hold some internal information.
|
---|
| 884 |
|
---|
| 885 | sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift }
|
---|
| 886 |
|
---|
| 887 | =item WRITE this, LIST
|
---|
| 888 | X<WRITE>
|
---|
| 889 |
|
---|
| 890 | This method will be called when the handle is written to via the
|
---|
| 891 | C<syswrite> function.
|
---|
| 892 |
|
---|
| 893 | sub WRITE {
|
---|
| 894 | $r = shift;
|
---|
| 895 | my($buf,$len,$offset) = @_;
|
---|
| 896 | print "WRITE called, \$buf=$buf, \$len=$len, \$offset=$offset";
|
---|
| 897 | }
|
---|
| 898 |
|
---|
| 899 | =item PRINT this, LIST
|
---|
| 900 | X<PRINT>
|
---|
| 901 |
|
---|
| 902 | This method will be triggered every time the tied handle is printed to
|
---|
| 903 | with the C<print()> function.
|
---|
| 904 | Beyond its self reference it also expects the list that was passed to
|
---|
| 905 | the print function.
|
---|
| 906 |
|
---|
| 907 | sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ }
|
---|
| 908 |
|
---|
| 909 | =item PRINTF this, LIST
|
---|
| 910 | X<PRINTF>
|
---|
| 911 |
|
---|
| 912 | This method will be triggered every time the tied handle is printed to
|
---|
| 913 | with the C<printf()> function.
|
---|
| 914 | Beyond its self reference it also expects the format and list that was
|
---|
| 915 | passed to the printf function.
|
---|
| 916 |
|
---|
| 917 | sub PRINTF {
|
---|
| 918 | shift;
|
---|
| 919 | my $fmt = shift;
|
---|
| 920 | print sprintf($fmt, @_);
|
---|
| 921 | }
|
---|
| 922 |
|
---|
| 923 | =item READ this, LIST
|
---|
| 924 | X<READ>
|
---|
| 925 |
|
---|
| 926 | This method will be called when the handle is read from via the C<read>
|
---|
| 927 | or C<sysread> functions.
|
---|
| 928 |
|
---|
| 929 | sub READ {
|
---|
| 930 | my $self = shift;
|
---|
| 931 | my $bufref = \$_[0];
|
---|
| 932 | my(undef,$len,$offset) = @_;
|
---|
| 933 | print "READ called, \$buf=$bufref, \$len=$len, \$offset=$offset";
|
---|
| 934 | # add to $$bufref, set $len to number of characters read
|
---|
| 935 | $len;
|
---|
| 936 | }
|
---|
| 937 |
|
---|
| 938 | =item READLINE this
|
---|
| 939 | X<READLINE>
|
---|
| 940 |
|
---|
| 941 | This method will be called when the handle is read from via <HANDLE>.
|
---|
| 942 | The method should return undef when there is no more data.
|
---|
| 943 |
|
---|
| 944 | sub READLINE { $r = shift; "READLINE called $$r times\n"; }
|
---|
| 945 |
|
---|
| 946 | =item GETC this
|
---|
| 947 | X<GETC>
|
---|
| 948 |
|
---|
| 949 | This method will be called when the C<getc> function is called.
|
---|
| 950 |
|
---|
| 951 | sub GETC { print "Don't GETC, Get Perl"; return "a"; }
|
---|
| 952 |
|
---|
| 953 | =item CLOSE this
|
---|
| 954 | X<CLOSE>
|
---|
| 955 |
|
---|
| 956 | This method will be called when the handle is closed via the C<close>
|
---|
| 957 | function.
|
---|
| 958 |
|
---|
| 959 | sub CLOSE { print "CLOSE called.\n" }
|
---|
| 960 |
|
---|
| 961 | =item UNTIE this
|
---|
| 962 | X<UNTIE>
|
---|
| 963 |
|
---|
| 964 | As with the other types of ties, this method will be called when C<untie> happens.
|
---|
| 965 | It may be appropriate to "auto CLOSE" when this occurs. See
|
---|
| 966 | L<The C<untie> Gotcha> below.
|
---|
| 967 |
|
---|
| 968 | =item DESTROY this
|
---|
| 969 | X<DESTROY>
|
---|
| 970 |
|
---|
| 971 | As with the other types of ties, this method will be called when the
|
---|
| 972 | tied handle is about to be destroyed. This is useful for debugging and
|
---|
| 973 | possibly cleaning up.
|
---|
| 974 |
|
---|
| 975 | sub DESTROY { print "</shout>\n" }
|
---|
| 976 |
|
---|
| 977 | =back
|
---|
| 978 |
|
---|
| 979 | Here's how to use our little example:
|
---|
| 980 |
|
---|
| 981 | tie(*FOO,'Shout');
|
---|
| 982 | print FOO "hello\n";
|
---|
| 983 | $a = 4; $b = 6;
|
---|
| 984 | print FOO $a, " plus ", $b, " equals ", $a + $b, "\n";
|
---|
| 985 | print <FOO>;
|
---|
| 986 |
|
---|
| 987 | =head2 UNTIE this
|
---|
| 988 | X<UNTIE>
|
---|
| 989 |
|
---|
| 990 | You can define for all tie types an UNTIE method that will be called
|
---|
| 991 | at untie(). See L<The C<untie> Gotcha> below.
|
---|
| 992 |
|
---|
| 993 | =head2 The C<untie> Gotcha
|
---|
| 994 | X<untie>
|
---|
| 995 |
|
---|
| 996 | If you intend making use of the object returned from either tie() or
|
---|
| 997 | tied(), and if the tie's target class defines a destructor, there is a
|
---|
| 998 | subtle gotcha you I<must> guard against.
|
---|
| 999 |
|
---|
| 1000 | As setup, consider this (admittedly rather contrived) example of a
|
---|
| 1001 | tie; all it does is use a file to keep a log of the values assigned to
|
---|
| 1002 | a scalar.
|
---|
| 1003 |
|
---|
| 1004 | package Remember;
|
---|
| 1005 |
|
---|
| 1006 | use strict;
|
---|
| 1007 | use warnings;
|
---|
| 1008 | use IO::File;
|
---|
| 1009 |
|
---|
| 1010 | sub TIESCALAR {
|
---|
| 1011 | my $class = shift;
|
---|
| 1012 | my $filename = shift;
|
---|
| 1013 | my $handle = new IO::File "> $filename"
|
---|
| 1014 | or die "Cannot open $filename: $!\n";
|
---|
| 1015 |
|
---|
| 1016 | print $handle "The Start\n";
|
---|
| 1017 | bless {FH => $handle, Value => 0}, $class;
|
---|
| 1018 | }
|
---|
| 1019 |
|
---|
| 1020 | sub FETCH {
|
---|
| 1021 | my $self = shift;
|
---|
| 1022 | return $self->{Value};
|
---|
| 1023 | }
|
---|
| 1024 |
|
---|
| 1025 | sub STORE {
|
---|
| 1026 | my $self = shift;
|
---|
| 1027 | my $value = shift;
|
---|
| 1028 | my $handle = $self->{FH};
|
---|
| 1029 | print $handle "$value\n";
|
---|
| 1030 | $self->{Value} = $value;
|
---|
| 1031 | }
|
---|
| 1032 |
|
---|
| 1033 | sub DESTROY {
|
---|
| 1034 | my $self = shift;
|
---|
| 1035 | my $handle = $self->{FH};
|
---|
| 1036 | print $handle "The End\n";
|
---|
| 1037 | close $handle;
|
---|
| 1038 | }
|
---|
| 1039 |
|
---|
| 1040 | 1;
|
---|
| 1041 |
|
---|
| 1042 | Here is an example that makes use of this tie:
|
---|
| 1043 |
|
---|
| 1044 | use strict;
|
---|
| 1045 | use Remember;
|
---|
| 1046 |
|
---|
| 1047 | my $fred;
|
---|
| 1048 | tie $fred, 'Remember', 'myfile.txt';
|
---|
| 1049 | $fred = 1;
|
---|
| 1050 | $fred = 4;
|
---|
| 1051 | $fred = 5;
|
---|
| 1052 | untie $fred;
|
---|
| 1053 | system "cat myfile.txt";
|
---|
| 1054 |
|
---|
| 1055 | This is the output when it is executed:
|
---|
| 1056 |
|
---|
| 1057 | The Start
|
---|
| 1058 | 1
|
---|
| 1059 | 4
|
---|
| 1060 | 5
|
---|
| 1061 | The End
|
---|
| 1062 |
|
---|
| 1063 | So far so good. Those of you who have been paying attention will have
|
---|
| 1064 | spotted that the tied object hasn't been used so far. So lets add an
|
---|
| 1065 | extra method to the Remember class to allow comments to be included in
|
---|
| 1066 | the file -- say, something like this:
|
---|
| 1067 |
|
---|
| 1068 | sub comment {
|
---|
| 1069 | my $self = shift;
|
---|
| 1070 | my $text = shift;
|
---|
| 1071 | my $handle = $self->{FH};
|
---|
| 1072 | print $handle $text, "\n";
|
---|
| 1073 | }
|
---|
| 1074 |
|
---|
| 1075 | And here is the previous example modified to use the C<comment> method
|
---|
| 1076 | (which requires the tied object):
|
---|
| 1077 |
|
---|
| 1078 | use strict;
|
---|
| 1079 | use Remember;
|
---|
| 1080 |
|
---|
| 1081 | my ($fred, $x);
|
---|
| 1082 | $x = tie $fred, 'Remember', 'myfile.txt';
|
---|
| 1083 | $fred = 1;
|
---|
| 1084 | $fred = 4;
|
---|
| 1085 | comment $x "changing...";
|
---|
| 1086 | $fred = 5;
|
---|
| 1087 | untie $fred;
|
---|
| 1088 | system "cat myfile.txt";
|
---|
| 1089 |
|
---|
| 1090 | When this code is executed there is no output. Here's why:
|
---|
| 1091 |
|
---|
| 1092 | When a variable is tied, it is associated with the object which is the
|
---|
| 1093 | return value of the TIESCALAR, TIEARRAY, or TIEHASH function. This
|
---|
| 1094 | object normally has only one reference, namely, the implicit reference
|
---|
| 1095 | from the tied variable. When untie() is called, that reference is
|
---|
| 1096 | destroyed. Then, as in the first example above, the object's
|
---|
| 1097 | destructor (DESTROY) is called, which is normal for objects that have
|
---|
| 1098 | no more valid references; and thus the file is closed.
|
---|
| 1099 |
|
---|
| 1100 | In the second example, however, we have stored another reference to
|
---|
| 1101 | the tied object in $x. That means that when untie() gets called
|
---|
| 1102 | there will still be a valid reference to the object in existence, so
|
---|
| 1103 | the destructor is not called at that time, and thus the file is not
|
---|
| 1104 | closed. The reason there is no output is because the file buffers
|
---|
| 1105 | have not been flushed to disk.
|
---|
| 1106 |
|
---|
| 1107 | Now that you know what the problem is, what can you do to avoid it?
|
---|
| 1108 | Prior to the introduction of the optional UNTIE method the only way
|
---|
| 1109 | was the good old C<-w> flag. Which will spot any instances where you call
|
---|
| 1110 | untie() and there are still valid references to the tied object. If
|
---|
| 1111 | the second script above this near the top C<use warnings 'untie'>
|
---|
| 1112 | or was run with the C<-w> flag, Perl prints this
|
---|
| 1113 | warning message:
|
---|
| 1114 |
|
---|
| 1115 | untie attempted while 1 inner references still exist
|
---|
| 1116 |
|
---|
| 1117 | To get the script to work properly and silence the warning make sure
|
---|
| 1118 | there are no valid references to the tied object I<before> untie() is
|
---|
| 1119 | called:
|
---|
| 1120 |
|
---|
| 1121 | undef $x;
|
---|
| 1122 | untie $fred;
|
---|
| 1123 |
|
---|
| 1124 | Now that UNTIE exists the class designer can decide which parts of the
|
---|
| 1125 | class functionality are really associated with C<untie> and which with
|
---|
| 1126 | the object being destroyed. What makes sense for a given class depends
|
---|
| 1127 | on whether the inner references are being kept so that non-tie-related
|
---|
| 1128 | methods can be called on the object. But in most cases it probably makes
|
---|
| 1129 | sense to move the functionality that would have been in DESTROY to the UNTIE
|
---|
| 1130 | method.
|
---|
| 1131 |
|
---|
| 1132 | If the UNTIE method exists then the warning above does not occur. Instead the
|
---|
| 1133 | UNTIE method is passed the count of "extra" references and can issue its own
|
---|
| 1134 | warning if appropriate. e.g. to replicate the no UNTIE case this method can
|
---|
| 1135 | be used:
|
---|
| 1136 |
|
---|
| 1137 | sub UNTIE
|
---|
| 1138 | {
|
---|
| 1139 | my ($obj,$count) = @_;
|
---|
| 1140 | carp "untie attempted while $count inner references still exist" if $count;
|
---|
| 1141 | }
|
---|
| 1142 |
|
---|
| 1143 | =head1 SEE ALSO
|
---|
| 1144 |
|
---|
| 1145 | See L<DB_File> or L<Config> for some interesting tie() implementations.
|
---|
| 1146 | A good starting point for many tie() implementations is with one of the
|
---|
| 1147 | modules L<Tie::Scalar>, L<Tie::Array>, L<Tie::Hash>, or L<Tie::Handle>.
|
---|
| 1148 |
|
---|
| 1149 | =head1 BUGS
|
---|
| 1150 |
|
---|
| 1151 | The bucket usage information provided by C<scalar(%hash)> is not
|
---|
| 1152 | available. What this means is that using %tied_hash in boolean
|
---|
| 1153 | context doesn't work right (currently this always tests false,
|
---|
| 1154 | regardless of whether the hash is empty or hash elements).
|
---|
| 1155 |
|
---|
| 1156 | Localizing tied arrays or hashes does not work. After exiting the
|
---|
| 1157 | scope the arrays or the hashes are not restored.
|
---|
| 1158 |
|
---|
| 1159 | Counting the number of entries in a hash via C<scalar(keys(%hash))>
|
---|
| 1160 | or C<scalar(values(%hash)>) is inefficient since it needs to iterate
|
---|
| 1161 | through all the entries with FIRSTKEY/NEXTKEY.
|
---|
| 1162 |
|
---|
| 1163 | Tied hash/array slices cause multiple FETCH/STORE pairs, there are no
|
---|
| 1164 | tie methods for slice operations.
|
---|
| 1165 |
|
---|
| 1166 | You cannot easily tie a multilevel data structure (such as a hash of
|
---|
| 1167 | hashes) to a dbm file. The first problem is that all but GDBM and
|
---|
| 1168 | Berkeley DB have size limitations, but beyond that, you also have problems
|
---|
| 1169 | with how references are to be represented on disk. One experimental
|
---|
| 1170 | module that does attempt to address this need is DBM::Deep. Check your
|
---|
| 1171 | nearest CPAN site as described in L<perlmodlib> for source code. Note
|
---|
| 1172 | that despite its name, DBM::Deep does not use dbm. Another earlier attempt
|
---|
| 1173 | at solving the problem is MLDBM, which is also available on the CPAN, but
|
---|
| 1174 | which has some fairly serious limitations.
|
---|
| 1175 |
|
---|
| 1176 | Tied filehandles are still incomplete. sysopen(), truncate(),
|
---|
| 1177 | flock(), fcntl(), stat() and -X can't currently be trapped.
|
---|
| 1178 |
|
---|
| 1179 | =head1 AUTHOR
|
---|
| 1180 |
|
---|
| 1181 | Tom Christiansen
|
---|
| 1182 |
|
---|
| 1183 | TIEHANDLE by Sven Verdoolaege <F<[email protected]>> and Doug MacEachern <F<[email protected]>>
|
---|
| 1184 |
|
---|
| 1185 | UNTIE by Nick Ing-Simmons <F<[email protected]>>
|
---|
| 1186 |
|
---|
| 1187 | SCALAR by Tassilo von Parseval <F<[email protected]>>
|
---|
| 1188 |
|
---|
| 1189 | Tying Arrays by Casey West <F<[email protected]>>
|
---|