Date: Sat, 17 Nov 2012 13:51:48 +0100 From: "Vicente J. Botet Escriba" <vicente.botet@wanadoo.fr>
Hi, your code suffers from the problem you try to avoid "But we all know this is not thread safe, at least for the first call" at least in c++98 as you are using static variables. Actually, my post described the circumstances under which it does in fact work: when VarType has a built-in type that is initialized before program execution, or on C++11 with a constexpr constructor.
here they are some pointers that while not responding exactly to your question them could help you.
http://www.justsoftwaresolutions.co.uk/threading/multithreading-in-c++0x-par... Interesting, the post gets it wrong. A unique_ptr has a constructor (in C++03) so still has the problem. Or, maybe the article is just not clear on how it is to be used: if you must initialize an instance of lazy_init (to the null pointer internally) _before_ multiple threads ask it to be "initialized", that just pushes the problem back to the earlier step. I can see the usefulness though, if you just want to put off the expensive "real" initialization until needed, but can arrange the pre-initialization to be done for-sure earlier.
http://stackoverflow.com/questions/12302057/c11-safe-double-checked-locking-...
This points out that under C++11 rules, static locals are thread safe for the first-time initialization. That explains why there is no fancy construct for doing so; it's just there! Nice. Or it will be, some day. I looked at the code generated by Microsoft VS2010 and it does not give this feature, alas. It does tell me though that a compatibility construct like I'm looking for ought to expand into a plain static local for compilers that do work that way; just let the compiler do it. All the more reason to abstract the issue for current uses! —John