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Comparing Guard/Guard.pm (file contents):
Revision 1.19 by root, Sat Apr 11 04:42:29 2009 UTC vs.
Revision 1.22 by root, Sun Jul 26 08:07:11 2009 UTC

… …
13 chdir "/etc"; 13 chdir "/etc";
14 14
15 code_that_might_die_or_does_other_fun_stuff; 15 code_that_might_die_or_does_other_fun_stuff;
16 } 16 }
17 17
18 # create an object that, when the last reference to it is gone,
19 # invokes the given codeblock:
20 my $guard = guard { print "destroyed!\n" };
21 undef $guard; # probably destroyed here
22
18=head1 DESCRIPTION 23=head1 DESCRIPTION
19 24
20This module implements so-called "guards". A guard is something (usually 25This module implements so-called "guards". A guard is something (usually
21an object) that "guards" a resource, ensuring that it is cleaned up when 26an object) that "guards" a resource, ensuring that it is cleaned up when
22expected. 27expected.
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37package Guard; 42package Guard;
38 43
39no warnings; 44no warnings;
40 45
41BEGIN { 46BEGIN {
42 $VERSION = '1.02'; 47 $VERSION = '1.021';
43 @ISA = qw(Exporter); 48 @ISA = qw(Exporter);
44 @EXPORT = qw(guard scope_guard); 49 @EXPORT = qw(guard scope_guard);
45 50
46 require Exporter; 51 require Exporter;
47 52
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103Behaves the same as C<scope_guard>, except that instead of executing 108Behaves the same as C<scope_guard>, except that instead of executing
104the block on scope exit, it returns an object whose lifetime determines 109the block on scope exit, it returns an object whose lifetime determines
105when the BLOCK gets executed: when the last reference to the object gets 110when the BLOCK gets executed: when the last reference to the object gets
106destroyed, the BLOCK gets executed as with C<scope_guard>. 111destroyed, the BLOCK gets executed as with C<scope_guard>.
107 112
108The returned object can be copied as many times as you want.
109
110See the EXCEPTIONS section for an explanation of how exceptions 113See the EXCEPTIONS section for an explanation of how exceptions
111(i.e. C<die>) are handled inside guard blocks. 114(i.e. C<die>) are handled inside guard blocks.
112 115
113Example: acquire a Coro::Semaphore for a second by registering a 116Example: acquire a Coro::Semaphore for a second by registering a
114timer. The timer callback references the guard used to unlock it 117timer. The timer callback references the guard used to unlock it
115again. (Please ignore the fact that C<Coro::Semaphore> has a C<guard> 118again. (Please ignore the fact that C<Coro::Semaphore> has a C<guard>
116method that does this already): 119method that does this already):
117 120
118 use Guard; 121 use Guard;
119 use AnyEvent; 122 use Coro::AnyEvent;
120 use Coro::Semaphore; 123 use Coro::Semaphore;
121 124
122 my $sem = new Coro::Semaphore; 125 my $sem = new Coro::Semaphore;
123 126
124 sub lock_for_a_second { 127 sub lock_for_a_second {
125 $sem->down; 128 $sem->down;
126 my $guard = guard { $sem->up }; 129 my $guard = guard { $sem->up };
127 130
128 my $timer; 131 Coro::AnyEvent::sleep 1;
129 $timer = AnyEvent->timer (after => 1, sub { 132
130 # do something 133 # $sem->up gets executed when returning
131 undef $sem;
132 undef $timer;
133 });
134 } 134 }
135 135
136The advantage of doing this with a guard instead of simply calling C<< 136The advantage of doing this with a guard instead of simply calling C<<
137$sem->down >> in the callback is that you can opt not to create the timer, 137$sem->down >> in the callback is that you can opt not to create the timer,
138or your code can throw an exception before it can create the timer, or you 138or your code can throw an exception before it can create the timer (or
139can create multiple timers or other event watchers and only when the last 139the thread gets canceled), or you can create multiple timers or other
140one gets executed will the lock be unlocked. Using the C<guard>, you do 140event watchers and only when the last one gets executed will the lock be
141not have to worry about catching all the places where you have to unlock 141unlocked. Using the C<guard>, you do not have to worry about catching all
142the semaphore. 142the places where you have to unlock the semaphore.
143 143
144=item $guard->cancel 144=item $guard->cancel
145 145
146Calling this function will "disable" the guard object returned by the 146Calling this function will "disable" the guard object returned by the
147C<guard> function, i.e. it will free the BLOCK originally passed to 147C<guard> function, i.e. it will free the BLOCK originally passed to
148C<guard >and will arrange for the BLOCK not to be executed. 148C<guard >and will arrange for the BLOCK not to be executed.
149 149
150This can be useful when you use C<guard> to create a fatal cleanup handler 150This can be useful when you use C<guard> to create a cleanup handler to be
151and later decide it is no longer needed. 151called under fatal conditions and later decide it is no longer needed.
152 152
153=cut 153=cut
154 154
1551; 1551;
156 156
157=back 157=back
158 158
159=head1 EXCEPTIONS 159=head1 EXCEPTIONS
160 160
161Guard blocks should not normally throw exceptions (that is, C<die>). After 161Guard blocks should not normally throw exceptions (that is, C<die>). After
162all, they are usually used to clean up after such exceptions. However, if 162all, they are usually used to clean up after such exceptions. However,
163something truly exceptional is happening, a guard block should be allowed 163if something truly exceptional is happening, a guard block should of
164to die. Also, programming errors are a large source of exceptions, and the 164course be allowed to die. Also, programming errors are a large source of
165programmer certainly wants to know about those. 165exceptions, and the programmer certainly wants to know about those.
166 166
167Since in most cases, the block executing when the guard gets executed does 167Since in most cases, the block executing when the guard gets executed does
168not know or does not care about the guard blocks, it makes little sense to 168not know or does not care about the guard blocks, it makes little sense to
169let containing code handle the exception. 169let containing code handle the exception.
170 170
171Therefore, whenever a guard block throws an exception, it will be caught, 171Therefore, whenever a guard block throws an exception, it will be caught
172followed by calling the code reference stored in C<$Guard::DIED> (with 172by Guard, followed by calling the code reference stored in C<$Guard::DIED>
173C<$@> set to the actual exception), which is similar to how most event 173(with C<$@> set to the actual exception), which is similar to how most
174loops handle this case. 174event loops handle this case.
175 175
176The default for C<$Guard::DIED> is to call C<warn "$@">. 176The default for C<$Guard::DIED> is to call C<warn "$@">, i.e. the error is
177printed as a warning and the program continues.
177 178
178The C<$@> variable will be restored to its value before the guard call in 179The C<$@> variable will be restored to its value before the guard call in
179all cases, so guards will not disturb C<$@> in any way. 180all cases, so guards will not disturb C<$@> in any way.
180 181
181The code reference stored in C<$Guard::DIED> should not die (behaviour is 182The code reference stored in C<$Guard::DIED> should not die (behaviour is
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192solution to the problem of exceptions. 193solution to the problem of exceptions.
193 194
194=head1 SEE ALSO 195=head1 SEE ALSO
195 196
196L<Scope::Guard> and L<Sub::ScopeFinalizer>, which actually implement 197L<Scope::Guard> and L<Sub::ScopeFinalizer>, which actually implement
197dynamic, not scoped guards, and have a lot higher CPU, memory and typing 198dynamic guards only, not scoped guards, and have a lot higher CPU, memory
198overhead. 199and typing overhead.
199 200
200L<Hook::Scope>, which has apparently never been finished and corrupts 201L<Hook::Scope>, which has apparently never been finished and can corrupt
201memory when used. 202memory when used.
202 203
203=cut 204=cut
204 205

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