c++ - How to ensure std::call_once really is only called once -

some code i'm working uses std::call_once initialization occurs once. however, there global objects constructors can end calling initialization code.

in following sample, call_once gets called twice. guess it's because once_flag constructor hasn't ran before gets used. there way around initialization code gets called once without having prohibit globals?

#include <mutex> #include <iostream>  using namespace std;  void init();  class global { public:     global()     {         init();     } };  global global;  once_flag flag;  void init() {     call_once(flag, []{  cout << "hello" << endl;  }); }    int main(int argc, char* argv[]) {     init();     return 0; } 

output is:

hello hello 

according specs, once_flag should have trivial constexpr constructor (for example see here - http://en.cppreference.com/w/cpp/thread/once_flag ). in place, if global/static, not "constructed" (no actual function executed), more "value initialized" - global/static pod type. in case not possible have constructor run "before" once_flag initialized. given comment using msvc, guess can bug in implementation...

edit: according comment below, constexpr not supported on msvc @ all, options limited in here... if have in 1 file, put once_flag "above" uses - constructors in file executed in order of declaration of objects. if have users spread across different files, option use function provides access static internal once_flag - in answer http://www.parashift.com/c++-faq/static-init-order-on-first-use.html .