73 lines
2.4 KiB
C
73 lines
2.4 KiB
C
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// Protocol Buffers - Google's data interchange format
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// Copyright 2008 Google Inc. All rights reserved.
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//
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file or at
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// https://developers.google.com/open-source/licenses/bsd
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#ifndef GOOGLE_PROTOBUF_EXPLICITLY_CONSTRUCTED_H__
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#define GOOGLE_PROTOBUF_EXPLICITLY_CONSTRUCTED_H__
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#include <stdint.h>
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#include <string>
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#include <utility>
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// clang-format off
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#include "google/protobuf/port_def.inc"
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// clang-format on
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namespace google {
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namespace protobuf {
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namespace internal {
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// Wraps a variable whose constructor and destructor are explicitly
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// called. It is particularly useful for a global variable, without its
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// constructor and destructor run on start and end of the program lifetime.
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// This circumvents the initial construction order fiasco, while keeping
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// the address of the empty string a compile time constant.
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//
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// Pay special attention to the initialization state of the object.
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// 1. The object is "uninitialized" to begin with.
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// 2. Call Construct() or DefaultConstruct() only if the object is
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// uninitialized. After the call, the object becomes "initialized".
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// 3. Call get() and get_mutable() only if the object is initialized.
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// 4. Call Destruct() only if the object is initialized.
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// After the call, the object becomes uninitialized.
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template <typename T, size_t min_align = 1>
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class ExplicitlyConstructed {
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public:
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void DefaultConstruct() { new (&union_) T(); }
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template <typename... Args>
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void Construct(Args&&... args) {
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new (&union_) T(std::forward<Args>(args)...);
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}
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void Destruct() { get_mutable()->~T(); }
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constexpr const T& get() const { return reinterpret_cast<const T&>(union_); }
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T* get_mutable() { return reinterpret_cast<T*>(&union_); }
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private:
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union AlignedUnion {
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alignas(min_align > alignof(T) ? min_align
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: alignof(T)) char space[sizeof(T)];
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int64_t align_to_int64;
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void* align_to_ptr;
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} union_;
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};
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// ArenaStringPtr compatible explicitly constructed string type.
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// This empty string type is aligned with a minimum alignment of 8 bytes
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// which is the minimum requirement of ArenaStringPtr
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using ExplicitlyConstructedArenaString = ExplicitlyConstructed<std::string, 8>;
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} // namespace internal
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} // namespace protobuf
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} // namespace google
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#include "google/protobuf/port_undef.inc"
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#endif // GOOGLE_PROTOBUF_EXPLICITLY_CONSTRUCTED_H__
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