[Boost-users] What is the best idiom for using call_once ?

Subject: [Boost-users] What is the best idiom for using call_once ?
From: John M. Dlugosz (mpbecey7gu_at_[hidden])
Date: 2012-11-17 04:54:06

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)

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?