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POThread safe lazy construction of a singleton in C++
 

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  1. POThread safe lazy construction of a singleton in C++
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    <p>Is there a way to implement a singleton object in C++ that is:</p> <ol> <li>Lazily constructed in a thread safe manner (two threads might simultaneously be the first user of the singleton - it should still only be constructed once).</li> <li>Doesn't rely on static variables being constructed beforehand (so the singleton object is itself safe to use during the construction of static variables).</li> </ol> <p>(I don't know my C++ well enough, but is it the case that integral and constant static variables are initialized before any code is executed (ie, even before static constructors are executed - their values may already be "initialized" in the program image)? If so - perhaps this can be exploited to implement a singleton mutex - which can in turn be used to guard the creation of the real singleton..)</p> <hr> <p>Excellent, it seems that I have a couple of good answers now (shame I can't mark 2 or 3 as being <strong>the answer</strong>). There appears to be two broad solutions:</p> <ol> <li>Use static initialisation (as opposed to dynamic initialisation) of a POD static variable, and implementing my own mutex with that using the builtin atomic instructions. This was the type of solution I was hinting at in my question, and I believe I knew already.</li> <li>Use some other library function like <a href="http://opengroup.org/onlinepubs/007908799/xsh/pthread_once.html" rel="noreferrer">pthread_once</a> or <a href="http://www.boost.org/doc/libs/1_33_1/doc/html/call_once.html" rel="noreferrer">boost::call_once</a>. These I certainly didn't know about - and am very grateful for the answers posted.</li> </ol>
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