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    <p>The lock should not be part of the protected ressource but of the caller as you have one caller by thread. They <strong>must</strong> use different unique_lock.</p> <p>The purpose of unique_lock is to lock and release the mutex with RAII, so you don't have to call unlock explicitly. </p> <p>When the unique_lock is declared inside a method body, it will belong to the calling thread stack.</p> <p>So a more correct use is :</p> <pre><code>#include &lt;boost/thread/recursive_mutex.hpp&gt; #include &lt;iostream&gt; template&lt;class T&gt; class FFTBuf { public : FFTBuf() { } // this can be called by any thread void exemple() const { boost::recursive_mutex::scoped_lock lock( mut ); std::cerr &lt;&lt; "Locked" &lt;&lt; std::endl; // we are safe here std::cout &lt;&lt; "exemple" &lt;&lt; std::endl ; std::cerr &lt;&lt; "Unlocking ( by RAII)" &lt;&lt; std::endl; } // this is mutable to allow lock of const FFTBuf mutable boost::recursive_mutex mut; }; int main( void ) { FFTBuf&lt; int &gt; b ; { boost::recursive_mutex::scoped_lock lock1( b.mut ); std::cerr &lt;&lt; "Locking 1" &lt;&lt; std::endl; // here the mutex is locked 1 times { boost::recursive_mutex::scoped_lock lock2( b.mut ); std::cerr &lt;&lt; "Locking 2" &lt;&lt; std::endl; // here the mutex is locked 2 times std::cerr &lt;&lt; "Auto UnLocking 2 ( by RAII) " &lt;&lt; std::endl; } b.exemple(); // here the mutex is locked 1 times std::cerr &lt;&lt; "Auto UnLocking 1 ( by RAII) " &lt;&lt; std::endl; } return 0; } </code></pre> <p>Note the mutable on the mutex for const methods.</p> <p>And the boost mutex types have a scoped_lock typedef which is the good unique_lock type.</p>
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