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POHow do I safely release an object shared between threads using boost::shared_ptr?
 

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  1. POHow do I safely release an object shared between threads using boost::shared_ptr?
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    <p>I wonder, is it safe to implement like this? :</p> <pre><code>typedef shared_ptr&lt;Foo&gt; FooPtr; FooPtr *gPtrToFooPtr // global variable // init (before any thread has been created) void init() { gPtrToFooPtr = new FooPtr(new Foo); } // thread A, B, C, ..., K // Once thread Z execute read_and_drop(), // no more call to read() from any thread. // But it is possible even after read_and_drop() has returned, // some thread is still in read() function. void read() { FooPtr a = *gPtrToFooPtr; // do useful things (read only) } // thread Z (executed once) void read_and_drop() { FooPtr b = *gPtrToFooPtr; // do useful things with a (read only) b.reset(); } </code></pre> <p>We do not know which thread would do the actual realease. Does boost's <code>shared_ptr</code> do the release safely under circumstance like this?</p> <p>According to boost's document, thread safety of <code>shared_ptr</code> is:</p> <blockquote> <p>A <code>shared_ptr</code> instance can be "read" (accessed using only const operations) simultaneously by multiple threads. <strong><em>Different</em></strong> <code>shared_ptr</code> instances can be "written to" (accessed using mutable operations such as operator<code>=</code> or <code>reset</code>) simultaneosly by multiple threads.</p> </blockquote> <p>As far as I am concerned, the code above does not violate any of thread safety criteria I mentioned above. And I believe the code should run fine. Does anyone tell me if I am right or wrong?</p> <p>Thanks in advance.</p> <hr> <h2><strong>Editted 2012-06-20 01:00 UTC+9</strong></h2> <p>The pseudo code above works fine. The <code>shared_ptr</code> implementation guarantees to work correctly under circumstances where multiple thread is accessing instances of it (each thread <strong>MUST</strong> access its own instance of <code>shared_ptr</code> instantiated by using <em>copy constructor</em>). </p> <p>Note that in the pseudo code above, you must <code>delete gPtrToFooPtr</code> to have the <code>shared_ptr</code> implementation finally release (drop the reference count by one) the object it owns(not proper expression since it is not an <code>auto_ptr</code>, but who cares ;) ). And in this case, you must be aware of the fact that it may cause SIGSEGV in multithreaded application.</p>
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