// Protocol Buffers - Google's data interchange format

// Copyright 2008 Google Inc.  All rights reserved.

//

// Use of this source code is governed by a BSD-style

// license that can be found in the LICENSE file or at

// https://developers.google.com/open-source/licenses/bsd

#ifndef GOOGLE_PROTOBUF_EXPLICITLY_CONSTRUCTED_H__

#define GOOGLE_PROTOBUF_EXPLICITLY_CONSTRUCTED_H__

#include <stdint.h>

#include <string>

#include <utility>

// clang-format off

#include "google/protobuf/port_def.inc"

// clang-format on

namespace google {

namespace protobuf {

namespace internal {

// Wraps a variable whose constructor and destructor are explicitly

// called. It is particularly useful for a global variable, without its

// constructor and destructor run on start and end of the program lifetime.

// This circumvents the initial construction order fiasco, while keeping

// the address of the empty string a compile time constant.

//

// Pay special attention to the initialization state of the object.

// 1. The object is "uninitialized" to begin with.

// 2. Call Construct() or DefaultConstruct() only if the object is

//    uninitialized. After the call, the object becomes "initialized".

// 3. Call get() and get_mutable() only if the object is initialized.

// 4. Call Destruct() only if the object is initialized.

//    After the call, the object becomes uninitialized.

template <typename T, size_t min_align = 1>

class ExplicitlyConstructed {

 public:

  void DefaultConstruct() { new (&union_) T(); }

  template <typename... Args>

  void Construct(Args&&... args) {

    new (&union_) T(std::forward<Args>(args)...);

  }

  void Destruct() { get_mutable()->~T(); }

  constexpr const T& get() const { return reinterpret_cast<const T&>(union_); }

  T* get_mutable() { return reinterpret_cast<T*>(&union_); }

 private:

  union AlignedUnion {

    alignas(min_align > alignof(T) ? min_align

                                   : alignof(T)) char space[sizeof(T)];

    int64_t align_to_int64;

    void* align_to_ptr;

  } union_;

};

// ArenaStringPtr compatible explicitly constructed string type.

// This empty string type is aligned with a minimum alignment of 8 bytes

// which is the minimum requirement of ArenaStringPtr

using ExplicitlyConstructedArenaString = ExplicitlyConstructed<std::string, 8>;

}  // namespace internal

}  // namespace protobuf

}  // namespace google

#include "google/protobuf/port_undef.inc"

#endif  // GOOGLE_PROTOBUF_EXPLICITLY_CONSTRUCTED_H__