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 45 namespace google { 46 namespace protobuf { 47 namespace 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. 62 template <typename T, size_t min_align = 1> 63 class ExplicitlyConstructed { 64 public: DefaultConstruct()65 void DefaultConstruct() { new (&union_) T(); } 66 67 template <typename... Args> Construct(Args &&...args)68 void Construct(Args&&... args) { 69 new (&union_) T(std::forward<Args>(args)...); 70 } 71 Destruct()72 void Destruct() { get_mutable()->~T(); } 73 get()74 constexpr const T& get() const { return reinterpret_cast<const T&>(union_); } get_mutable()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 89 using 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