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5485556
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2011-03-30T11:14:13.760
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2011-03-30T11:14:13.760
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Here is pseudo-code for how I would do it. I am assuming the existing of a <code>mutex</code> class with <code>lock()</code> and <code>unlock()</code> operations. <pre><code>// This forward declaration helps with declaration // of the "friend" status for the nested class. class DoItOnce; class DoItOnce { private: bool m_amFirst; mutex m_mutex; friend class ::DoItOnce::Op; public: DoItOnce() { m_amFirst = true; init(m_mutex); } ~DoItOnce() { destroy(m_mutex); } void reset() { m_mutex.lock(); m_amFirst = true; m_mutex.lock(); } //-------- // Nested class //-------- class Op { public: Op(DoItOnce & sync) : m_sync(sync) { m_sync.m_mutex.lock(); m_amFirst = m_sync.m_amFirst; m_sync.m_amFirst = false; } ~Op() { m_sync.m_mutex.unlock(); } bool amFirst() { return m_amFirst; } private: DoItOnce & m_sync; bool m_amFirst; }; // end of nested class }; // end of outer class </code></pre> Here is an example to illustrate its intended use. You will implement the <code>doWork()</code> operation and have all your threads invoke it. <pre><code>class WorkToBeDoneOnce { private: DoItOnce m_sync; public: bool doWork() { DoItOnce::Op scopedLock(m_sync); if (!scopedLock.amFirst()) { // The work has already been done. return true; } ... // Do the work return true; } void resetAmFirstFlag() { m_sync.reset(); } } </code></pre> If you are confused by my use of the <code>DoItOnce::Op</code> nested class, then you can find an explanation of this coding idiom in my Generic Synchronisation Policies paper, which is available <a href="http://www.ciaranmchale.com/training-courses.html#training-gsp" rel="nofollow">here</a> in various formats (HTML, PDF and slides).
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1. This table or related slice is empty.
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1. POMultiple threads queuing for global lock should all return true once first lock acquired
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1. PTAnswer
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1. USCiaran McHale
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