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    <p>In <a href="http://en.wikipedia.org/wiki/C++11" rel="noreferrer">C++11</a>, it is thread safe. According to the <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3337.pdf" rel="noreferrer">standard</a>, <code>§6.7 [stmt.dcl] p4</code>:</p> <blockquote> <p>If control enters the declaration concurrently while the variable is being initialized, the <strong>concurrent execution shall wait</strong> for completion of the initialization.</p> </blockquote> <p>GCC and VS support for the feature (<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2660.htm" rel="noreferrer">Dynamic Initialization and Destruction with Concurrency</a>, also known as <a href="https://msdn.microsoft.com/en-us/library/hh567368.aspx" rel="noreferrer">Magic Statics on MSDN</a>) is as follows:</p> <ul> <li>Visual Studio: supported since <a href="https://msdn.microsoft.com/en-us/library/hh567368(v=vs.140).aspx" rel="noreferrer">Visual Studio 2015</a></li> <li>GCC: supported since <a href="https://gcc.gnu.org/gcc-4.3/changes.html" rel="noreferrer">GCC 4.3</a></li> </ul> <p>Thanks to @Mankarse and @olen_gam for their comments.</p> <hr /> <p>In <a href="http://en.wikipedia.org/wiki/C++03" rel="noreferrer">C++03</a>, this code wasn't thread safe. There is an article by Meyers called <a href="http://www.aristeia.com/Papers/DDJ_Jul_Aug_2004_revised.pdf" rel="noreferrer">"C++ and the Perils of Double-Checked Locking"</a> which discusses thread safe implementations of the pattern, and the conclusion is, more or less, that (in C++03) full locking around the instantiating method is basically the simplest way to ensure proper concurrency on all platforms, while most forms of double-checked locking pattern variants may suffer from <a href="https://stackoverflow.com/a/6623662/69809">race conditions on certain architectures</a>, unless instructions are interleaved with strategically places memory barriers.</p>
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    1. POIs Meyers' implementation of the Singleton pattern thread safe?
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    1. POIs Meyers' implementation of the Singleton pattern thread safe?
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    1. COThere is also an extensive discussion on the Singleton Pattern (lifetime and threadsafety) by Alexandrescu in Modern C++ Design. See Loki's site: http://loki-lib.sourceforge.net/index.php?n=Pattern.Singleton
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    2. COYou can create a thread-safe singleton with boost::call_once.
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    3. COThis answer is no longer correct in C++11 (if you take "thread-safe" to mean "will correctly construct a single `Singleton`, even when `instance()` is called from multiple threads simultaneously"). See `C++11:[stmt.dcl]/4`.
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