Le 17/11/12 10:54, John M. Dlugosz a écrit :
Consider the issue of a static variable inside a function: the idea is to initialize on first use (if any). But we all know this is not thread safe, at least for the first call.
Since the C++11 standard library includes a feature just like boost::call_once, that would seem to be the proper way to address this moving forward. And, I don't see anything in the standard that would be more powerful and direct for this purpose, which leaves me to wonder if there is still more that should have been done.
But, my question is, what is the "best" or slickest way to use it (for this purpose)? If Lambda functions are available (which is not the case on every platform my code needs to support) _and_ the type of variable in question has some kind of "dead" state available, it's not too bad to write something like this:
VarType obtain_singleton() { static boost::once_flag f=BOOST_ONCE_INIT; static VarType retval; boost::call_once (f, [] { retval=the_real_initialization(); }); return retval; }
//...later // static VarType val= the_real_initialization; // !!not thread safe on first call!! static VarType val= obtain_singleton(); // do this instead (and write it in another function for conciseness)
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.
However, it's more awkward if I can't declare VarType retval to be pre-initialized to some "dead" value, or if it has any constructor at all and would itself re-introduce the thread safety issue! In short, it only works correctly (without the availability of the constexpr feature and types having constructors of such a nature) for primitive types.
I've worked around it using a pointer and having the real_init function itself contain a static variable. That makes me think that there should be a simpler way to do it. Or, once the "best way" has been determined, why can't it be done once and succinctly reused?
Anyone have one to share?
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... http://stackoverflow.com/questions/12302057/c11-safe-double-checked-locking-... Best, Vicente