1// Protocol Buffers - Google's data interchange format
2// Copyright 2008 Google Inc. All rights reserved.
3// https://developers.google.com/protocol-buffers/
4//
5// Redistribution and use in source and binary forms, with or without
6// modification, are permitted provided that the following conditions are
7// met:
8//
9// * Redistributions of source code must retain the above copyright
10// notice, this list of conditions and the following disclaimer.
11// * Redistributions in binary form must reproduce the above
12// copyright notice, this list of conditions and the following disclaimer
13// in the documentation and/or other materials provided with the
14// distribution.
15// * Neither the name of Google Inc. nor the names of its
16// contributors may be used to endorse or promote products derived from
17// this software without specific prior written permission.
18//
19// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
20// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
21// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
22// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
23// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
24// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
25// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
26// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
27// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
28// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
29// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
30
31#ifndef GOOGLE_PROTOBUF_EXPLICITLY_CONSTRUCTED_H__
32#define GOOGLE_PROTOBUF_EXPLICITLY_CONSTRUCTED_H__
33
34#include <stdint.h>
35
36#include <utility>
37
38#include <google/protobuf/stubs/logging.h>
39#include <google/protobuf/stubs/common.h>
40
41// clang-format off
42#include <google/protobuf/port_def.inc>
43// clang-format on
44
45namespace google {
46namespace protobuf {
47namespace internal {
48
49// Wraps a variable whose constructor and destructor are explicitly
50// called. It is particularly useful for a global variable, without its
51// constructor and destructor run on start and end of the program lifetime.
52// This circumvents the initial construction order fiasco, while keeping
53// the address of the empty string a compile time constant.
54//
55// Pay special attention to the initialization state of the object.
56// 1. The object is "uninitialized" to begin with.
57// 2. Call Construct() or DefaultConstruct() only if the object is
58// uninitialized. After the call, the object becomes "initialized".
59// 3. Call get() and get_mutable() only if the object is initialized.
60// 4. Call Destruct() only if the object is initialized.
61// After the call, the object becomes uninitialized.
62template <typename T, size_t min_align = 1>
63class ExplicitlyConstructed {
64 public:
65 void DefaultConstruct() { new (&union_) T(); }
66
67 template <typename... Args>
68 void Construct(Args&&... args) {
69 new (&union_) T(std::forward<Args>(args)...);
70 }
71
72 void Destruct() { get_mutable()->~T(); }
73
74 constexpr const T& get() const { return reinterpret_cast<const T&>(union_); }
75 T* get_mutable() { return reinterpret_cast<T*>(&union_); }
76
77 private:
78 union AlignedUnion {
79 alignas(min_align > alignof(T) ? min_align
80 : alignof(T)) char space[sizeof(T)];
81 int64_t align_to_int64;
82 void* align_to_ptr;
83 } union_;
84};
85
86// ArenaStringPtr compatible explicitly constructed string type.
87// This empty string type is aligned with a minimum alignment of 8 bytes
88// which is the minimum requirement of ArenaStringPtr
89using ExplicitlyConstructedArenaString = ExplicitlyConstructed<std::string, 8>;
90
91} // namespace internal
92} // namespace protobuf
93} // namespace google
94
95#include <google/protobuf/port_undef.inc>
96
97#endif // GOOGLE_PROTOBUF_EXPLICITLY_CONSTRUCTED_H__
98