[/
  (C) Copyright 2007-8 Anthony Williams.
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE_1_0.txt or copy at
  http://www.boost.org/LICENSE_1_0.txt).
]

[section:once One-time Initialization]

  #include <boost/thread/once.hpp>

  namespace boost
  {
    struct once_flag;
    template<typename Function, class ...ArgTypes>
    inline void call_once(once_flag& flag, Function&& f, ArgTypes&&... args);

  #if defined BOOST_THREAD_PROVIDES_DEPRECATED_FEATURES_SINCE_V3_0_0
    void call_once(void (*func)(),once_flag& flag);
  #endif

  }

[warning the variadic prototype is provided only on C++11 compilers supporting variadic templates, otherwise the interface is limited up to 3 parameters.]

[warning the move semantics is ensured only on C++11 compilers supporting SFINAE expression, decltype N3276 and auto. Waiting for a boost::bind that is move aware.]


`boost::call_once` provides a mechanism for ensuring that an initialization routine is run exactly once without data races or deadlocks.

[section:once_flag Typedef `once_flag`]

    #ifdef BOOST_THREAD_PROVIDES_ONCE_CXX11
    struct once_flag
    {
      constexpr once_flag() noexcept;
      once_flag(const once_flag&) = delete;
      once_flag& operator=(const once_flag&) = delete;
    };
    #else
    typedef platform-specific-type once_flag;
    #define BOOST_ONCE_INIT platform-specific-initializer
    #endif

Objects of type `boost::once_flag` shall be initialized with `BOOST_ONCE_INIT` if BOOST_THREAD_PROVIDES_ONCE_CXX11 is not defined

    boost::once_flag f=BOOST_ONCE_INIT;

[endsect]

[section:call_once Non-member function `call_once`]

    template<typename Function, class ...ArgTypes>
    inline void call_once(once_flag& flag, Function&& f, ArgTypes&&... args);

[variablelist

[[Requires:] [`Function` and each or the `ArgTypes` are `MoveConstructible` and `invoke(decay_copy(boost::forward<Function>(f)), decay_copy(boost::forward<ArgTypes>(args))...)` shall be well formed. ]]

[[Effects:] [Calls to `call_once` on the same `once_flag` object are serialized. If there has been no prior effective `call_once` on
the same `once_flag` object, the argument `func` is called as-if by invoking `invoke(decay_copy(boost::forward<Function>(f)), decay_copy(boost::forward<ArgTypes>(args))...)`, and the invocation of
`call_once` is effective if and only if `invoke(decay_copy(boost::forward<Function>(f)), decay_copy(boost::forward<ArgTypes>(args))...)` returns without exception. If an exception is thrown, the exception is propagated to the caller. If there has been a prior effective `call_once` on the same `once_flag` object, the `call_once` returns
without invoking `func`. ]]

[[Synchronization:] [The completion of an effective `call_once` invocation on a `once_flag` object, synchronizes with
all subsequent `call_once` invocations on the same `once_flag` object. ]]

[[Throws:] [`thread_resource_error` when the effects cannot be achieved or any exception propagated from `func`.]]

[[Note:] [The function passed to `call_once` must not also call
`call_once` passing the same `once_flag` object. This may cause
deadlock, or invoking the passed function a second time. The
alternative is to allow the second call to return immediately, but
that assumes the code knows it has been called recursively, and can
proceed even though the call to `call_once` didn't actually call the
function, in which case it could also avoid calling `call_once`
recursively.]]


[[Note:] [On some compilers this function has some restrictions, e.g. if variadic templates are not supported the number of arguments is limited to 3; .]]

]

    void call_once(void (*func)(),once_flag& flag);
    
This second overload is provided for backwards compatibility and is deprecated. The effects of `call_once(func,flag)` shall be the same as those of
`call_once(flag,func)`.

[endsect]
[endsect]
