1、once_flag源码

struct once_flag
  {
  private:
    typedef __gthread_once_t __native_type;
    __native_type  _M_once = __GTHREAD_ONCE_INIT;

  public:
    /// Constructor
    constexpr once_flag() noexcept = default;

    /// Deleted copy constructor
    once_flag(const once_flag&) = delete;
    /// Deleted assignment operator
    once_flag& operator=(const once_flag&) = delete;

    template<typename _Callable, typename... _Args>
      friend void
      call_once(once_flag& __once, _Callable&& __f, _Args&&... __args);
  };

2、call_once源码

  template<typename _Callable, typename... _Args>
    void
    call_once(once_flag& __once, _Callable&& __f, _Args&&... __args)
    {
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 2442. call_once() shouldn't DECAY_COPY()
      auto __callable = [&] {
	  std::__invoke(std::forward<_Callable>(__f),
			std::forward<_Args>(__args)...);
      };
      
      __once_callable = std::__addressof(__callable);
      __once_call = []{ (*(decltype(__callable)*)__once_callable)(); };

      int __e = __gthread_once(&__once._M_once, &__once_proxy);

      if (__e)
	__throw_system_error(__e);
    }

3、试用

#include <iostream>
#include <mutex>
#include <memory>
#include <thread>

using namespace std;

struct some_res
{
    some_res()
    {
        cout << std::this_thread::get_id() << " : " << __func__ << " : " << "start init" << endl;
    }

private:
    // big-data
};

std::shared_ptr<some_res> res_ptr;
std::once_flag res_flag;

void foo()
{
    std::call_once(res_flag, [](){
        cout << std::this_thread::get_id() << " : " << "start init" << endl;
        res_ptr = make_shared<some_res>();
        cout << std::this_thread::get_id() << " : " << "end init" << endl;
    });
}

void test_static_init()
{
    static some_res res;
}

int main(int argc, char const *argv[])
{
    std::thread t1(foo);
    std::thread t2(foo);

    t1.join();
    t2.join();

    // C++11 内存模型 保证初始化只能在一个线程中执行
    std::thread t3(test_static_init);
    std::thread t4(test_static_init);

    t3.join();
    t4.join();

    return 0;
}

执行结果：

0x800083f90 : start init
0x800083f90 : some_res : start init
0x800083f90 : end init
0x8000841e0 : some_res : start init