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Revision 1.5 by root, Sun Jul 19 05:44:10 2009 UTC vs.
Revision 1.6 by root, Fri Mar 12 17:25:58 2010 UTC

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83 the block on scope exit, it returns an object whose lifetime 83 the block on scope exit, it returns an object whose lifetime
84 determines when the BLOCK gets executed: when the last reference to 84 determines when the BLOCK gets executed: when the last reference to
85 the object gets destroyed, the BLOCK gets executed as with 85 the object gets destroyed, the BLOCK gets executed as with
86 "scope_guard". 86 "scope_guard".
87 87
88 The returned object can be copied as many times as you want.
89
90 See the EXCEPTIONS section for an explanation of how exceptions 88 See the EXCEPTIONS section for an explanation of how exceptions
91 (i.e. "die") are handled inside guard blocks. 89 (i.e. "die") are handled inside guard blocks.
92 90
93 Example: acquire a Coro::Semaphore for a second by registering a 91 Example: acquire a Coro::Semaphore for a second by registering a
94 timer. The timer callback references the guard used to unlock it 92 timer. The timer callback references the guard used to unlock it
95 again. (Please ignore the fact that "Coro::Semaphore" has a "guard" 93 again. (Please ignore the fact that "Coro::Semaphore" has a "guard"
96 method that does this already): 94 method that does this already):
97 95
98 use Guard; 96 use Guard;
99 use AnyEvent; 97 use Coro::AnyEvent;
100 use Coro::Semaphore; 98 use Coro::Semaphore;
101 99
102 my $sem = new Coro::Semaphore; 100 my $sem = new Coro::Semaphore;
103 101
104 sub lock_for_a_second { 102 sub lock_for_a_second {
105 $sem->down; 103 $sem->down;
106 my $guard = guard { $sem->up }; 104 my $guard = guard { $sem->up };
107 105
108 my $timer; 106 Coro::AnyEvent::sleep 1;
109 $timer = AnyEvent->timer (after => 1, sub { 107
110 # do something 108 # $sem->up gets executed when returning
111 undef $sem;
112 undef $timer;
113 });
114 } 109 }
115 110
116 The advantage of doing this with a guard instead of simply calling 111 The advantage of doing this with a guard instead of simply calling
117 "$sem->down" in the callback is that you can opt not to create the 112 "$sem->down" in the callback is that you can opt not to create the
118 timer, or your code can throw an exception before it can create the 113 timer, or your code can throw an exception before it can create the
119 timer, or you can create multiple timers or other event watchers and 114 timer (or the thread gets canceled), or you can create multiple
120 only when the last one gets executed will the lock be unlocked. 115 timers or other event watchers and only when the last one gets
121 Using the "guard", you do not have to worry about catching all the 116 executed will the lock be unlocked. Using the "guard", you do not
122 places where you have to unlock the semaphore. 117 have to worry about catching all the places where you have to unlock
118 the semaphore.
123 119
124 $guard->cancel 120 $guard->cancel
125 Calling this function will "disable" the guard object returned by 121 Calling this function will "disable" the guard object returned by
126 the "guard" function, i.e. it will free the BLOCK originally passed 122 the "guard" function, i.e. it will free the BLOCK originally passed
127 to "guard "and will arrange for the BLOCK not to be executed. 123 to "guard "and will arrange for the BLOCK not to be executed.
128 124
129 This can be useful when you use "guard" to create a fatal cleanup 125 This can be useful when you use "guard" to create a cleanup handler
130 handler and later decide it is no longer needed. 126 to be called under fatal conditions and later decide it is no longer
127 needed.
131 128
132EXCEPTIONS 129EXCEPTIONS
133 Guard blocks should not normally throw exceptions (that is, "die"). 130 Guard blocks should not normally throw exceptions (that is, "die").
134 After all, they are usually used to clean up after such exceptions. 131 After all, they are usually used to clean up after such exceptions.
135 However, if something truly exceptional is happening, a guard block 132 However, if something truly exceptional is happening, a guard block
136 should be allowed to die. Also, programming errors are a large source of 133 should of course be allowed to die. Also, programming errors are a large
137 exceptions, and the programmer certainly wants to know about those. 134 source of exceptions, and the programmer certainly wants to know about
135 those.
138 136
139 Since in most cases, the block executing when the guard gets executed 137 Since in most cases, the block executing when the guard gets executed
140 does not know or does not care about the guard blocks, it makes little 138 does not know or does not care about the guard blocks, it makes little
141 sense to let containing code handle the exception. 139 sense to let containing code handle the exception.
142 140
143 Therefore, whenever a guard block throws an exception, it will be 141 Therefore, whenever a guard block throws an exception, it will be caught
144 caught, followed by calling the code reference stored in $Guard::DIED 142 by Guard, followed by calling the code reference stored in $Guard::DIED
145 (with $@ set to the actual exception), which is similar to how most 143 (with $@ set to the actual exception), which is similar to how most
146 event loops handle this case. 144 event loops handle this case.
147 145
148 The default for $Guard::DIED is to call "warn "$@"". 146 The default for $Guard::DIED is to call "warn "$@"", i.e. the error is
147 printed as a warning and the program continues.
149 148
150 The $@ variable will be restored to its value before the guard call in 149 The $@ variable will be restored to its value before the guard call in
151 all cases, so guards will not disturb $@ in any way. 150 all cases, so guards will not disturb $@ in any way.
152 151
153 The code reference stored in $Guard::DIED should not die (behaviour is 152 The code reference stored in $Guard::DIED should not die (behaviour is
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160THANKS 159THANKS
161 Thanks to Marco Maisenhelder, who reminded me of the $Guard::DIED 160 Thanks to Marco Maisenhelder, who reminded me of the $Guard::DIED
162 solution to the problem of exceptions. 161 solution to the problem of exceptions.
163 162
164SEE ALSO 163SEE ALSO
165 Scope::Guard and Sub::ScopeFinalizer, which actually implement dynamic, 164 Scope::Guard and Sub::ScopeFinalizer, which actually implement dynamic
166 not scoped guards, and have a lot higher CPU, memory and typing 165 guards only, not scoped guards, and have a lot higher CPU, memory and
167 overhead. 166 typing overhead.
168 167
169 Hook::Scope, which has apparently never been finished and corrupts 168 Hook::Scope, which has apparently never been finished and can corrupt
170 memory when used. 169 memory when used.
171 170

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