google/protobuf/explicitly_constructed.h

0001 // Protocol Buffers - Google's data interchange format
0002 // Copyright 2008 Google Inc.  All rights reserved.
0003 //
0004 // Use of this source code is governed by a BSD-style
0005 // license that can be found in the LICENSE file or at
0006 // https://developers.google.com/open-source/licenses/bsd
0007
0008 #ifndef GOOGLE_PROTOBUF_EXPLICITLY_CONSTRUCTED_H__
0009 #define GOOGLE_PROTOBUF_EXPLICITLY_CONSTRUCTED_H__
0010
0011 #include <stdint.h>
0012
0013 #include <string>
0014 #include <utility>
0015
0016 // clang-format off
0017 #include "google/protobuf/port_def.inc"
0018 // clang-format on
0019
0020 namespace google {
0021 namespace protobuf {
0022 namespace internal {
0023
0024 // Wraps a variable whose constructor and destructor are explicitly
0025 // called. It is particularly useful for a global variable, without its
0026 // constructor and destructor run on start and end of the program lifetime.
0027 // This circumvents the initial construction order fiasco, while keeping
0028 // the address of the empty string a compile time constant.
0029 //
0030 // Pay special attention to the initialization state of the object.
0031 // 1. The object is "uninitialized" to begin with.
0032 // 2. Call Construct() or DefaultConstruct() only if the object is
0033 //    uninitialized. After the call, the object becomes "initialized".
0034 // 3. Call get() and get_mutable() only if the object is initialized.
0035 // 4. Call Destruct() only if the object is initialized.
0036 //    After the call, the object becomes uninitialized.
0037 template <typename T, size_t min_align = 1>
0038 class ExplicitlyConstructed {
0039  public:
0040   void DefaultConstruct() { new (&union_) T(); }
0041
0042   template <typename... Args>
0043   void Construct(Args&&... args) {
0044     new (&union_) T(std::forward<Args>(args)...);
0045   }
0046
0047   void Destruct() { get_mutable()->~T(); }
0048
0049   constexpr const T& get() const { return reinterpret_cast<const T&>(union_); }
0050   T* get_mutable() { return reinterpret_cast<T*>(&union_); }
0051
0052  private:
0053   union AlignedUnion {
0054     alignas(min_align > alignof(T) ? min_align
0055                                    : alignof(T)) char space[sizeof(T)];
0056     int64_t align_to_int64;
0057     void* align_to_ptr;
0058   } union_;
0059 };
0060
0061 // ArenaStringPtr compatible explicitly constructed string type.
0062 // This empty string type is aligned with a minimum alignment of 8 bytes
0063 // which is the minimum requirement of ArenaStringPtr
0064 using ExplicitlyConstructedArenaString = ExplicitlyConstructed<std::string, 8>;
0065
0066 }  // namespace internal
0067 }  // namespace protobuf
0068 }  // namespace google
0069
0070 #include "google/protobuf/port_undef.inc"
0071
0072 #endif  // GOOGLE_PROTOBUF_EXPLICITLY_CONSTRUCTED_H__