third_party/protobuf/3.6.1/src/google/protobuf/stubs/once.h

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// Author: kenton@google.com (Kenton Varda)
//
// emulates google3/base/once.h
//
// This header is intended to be included only by internal .cc files and
// generated .pb.cc files.  Users should not use this directly.
//
// This is basically a portable version of pthread_once().
//
// This header declares:
// * A type called ProtobufOnceType.
// * A macro GOOGLE_PROTOBUF_DECLARE_ONCE() which declares a variable of type
//   ProtobufOnceType.  This is the only legal way to declare such a variable.
//   The macro may only be used at the global scope (you cannot create local or
//   class member variables of this type).
// * A function GoogleOnceInit(ProtobufOnceType* once, void (*init_func)()).
//   This function, when invoked multiple times given the same ProtobufOnceType
//   object, will invoke init_func on the first call only, and will make sure
//   none of the calls return before that first call to init_func has finished.
// * The user can provide a parameter which GoogleOnceInit() forwards to the
//   user-provided function when it is called. Usage example:
//     int a = 10;
//     GoogleOnceInit(&my_once, &MyFunctionExpectingIntArgument, &a);
// * This implementation guarantees that ProtobufOnceType is a POD (i.e. no
//   static initializer generated).
//
// This implements a way to perform lazy initialization.  It's more efficient
// than using mutexes as no lock is needed if initialization has already
// happened.
//
// Example usage:
//   void Init();
//   GOOGLE_PROTOBUF_DECLARE_ONCE(once_init);
//
//   // Calls Init() exactly once.
//   void InitOnce() {
//     GoogleOnceInit(&once_init, &Init);
//   }
//
// Note that if GoogleOnceInit() is called before main() has begun, it must
// only be called by the thread that will eventually call main() -- that is,
// the thread that performs dynamic initialization.  In general this is a safe
// assumption since people don't usually construct threads before main() starts,
// but it is technically not guaranteed.  Unfortunately, Win32 provides no way
// whatsoever to statically-initialize its synchronization primitives, so our
// only choice is to assume that dynamic initialization is single-threaded.

#ifndef GOOGLE_PROTOBUF_STUBS_ONCE_H__
#define GOOGLE_PROTOBUF_STUBS_ONCE_H__

#include <mutex>
#include <utility>

namespace google {
namespace protobuf {
namespace internal {

using once_flag = std::once_flag;
template <typename... Args>
void call_once(Args&&... args ) {
  std::call_once(std::forward<Args>(args)...);
}

}  // namespace internal

// TODO(gerbens) remove this once third_party is fully extracted
using ProtobufOnceType = internal::once_flag;

inline void GoogleOnceInit(ProtobufOnceType* once, void (*init_func)()) {
  std::call_once(*once, init_func);
}

template <typename Arg>
inline void GoogleOnceInitArg(ProtobufOnceType* once, void (*init_func)(Arg*),
                              Arg* arg) {
  std::call_once(*once, init_func, arg);
}

class GoogleOnceDynamic {
 public:
  // If this->Init() has not been called before by any thread,
  // execute (*func_with_arg)(arg) then return.
  // Otherwise, wait until that prior invocation has finished
  // executing its function, then return.
  template<typename T>
  void Init(void (*func_with_arg)(T*), T* arg) {
    GoogleOnceInitArg<T>(&this->state_, func_with_arg, arg);
  }
 private:
  ProtobufOnceType state_;
};

#define GOOGLE_PROTOBUF_ONCE_TYPE ::google::protobuf::ProtobufOnceType
#define GOOGLE_PROTOBUF_DECLARE_ONCE(NAME) \
  ::google::protobuf::ProtobufOnceType NAME

}  // namespace protobuf
}  // namespace google

#endif  // GOOGLE_PROTOBUF_STUBS_ONCE_H__