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    <p>Fraser's answer is great, but when I looked at this I felt a working example would be nice.</p> <p>I made some changes:</p> <ul> <li><p>Personally, I like to use RAII to lock/unlock so that you don't forget to do so (even if it means extra scopes). In the past, I've also had race conditions that I couldn't figure out, and switching to the RAII approach has often made those disappear as well... It's just easier, so do it ;)</p></li> <li><p>I like to see when the cars leave, so I add I/O for that too.</p></li> </ul> <p>Here's the working code example for the stated problem. FYI, I use clang 3.1 with libc++:</p> <pre><code>#include &lt;iostream&gt; #include &lt;thread&gt; #include &lt;mutex&gt; #include &lt;condition_variable&gt; #include &lt;chrono&gt; using namespace std; int cars=0; int max_cars=5; mutex cars_mux; condition_variable cars_cond; bool pred() { return cars &lt; max_cars; } void task() { { unique_lock&lt;mutex&gt; carlock(cars_mux); cars_cond.wait(carlock,pred); cars++; cout &lt;&lt; "Thread " &lt;&lt; this_thread::get_id() &lt;&lt; " has parked. There are " &lt;&lt; cars &lt;&lt; " parked cars." &lt;&lt; endl; } this_thread::sleep_for(chrono::seconds(1)); { unique_lock&lt;mutex&gt; carlock(cars_mux); cars--; cout &lt;&lt; "Thread " &lt;&lt; this_thread::get_id() &lt;&lt; " has left. There are " &lt;&lt; cars &lt;&lt; " parked cars." &lt;&lt; endl; cars_cond.notify_one(); } } int main(int argc, char** argv) { const int NumThreads = 10; thread t[NumThreads]; for(int i=0; i&lt;NumThreads; i++) t[i]=thread(task); for(int i=0; i&lt;NumThreads; i++) t[i].join(); return 0; } </code></pre> <p>Edit: Simplified code as per Rami Al Zuhouri's suggestion.</p>
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    1. COThe output always occurs when the mutex is locked, I don't see the reason for a mutex for I/O.I tried Fraser's suggestions and the code does the job.
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    2. CO@RamyAlZuhouri you're absolutely right. I had overcomplicated things, so I've edited the code to simplify it back down.
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