THUAI6/CAPI/cpp/grpc/include/google/protobuf/explicitly_constructed.h

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#ifndef GOOGLE_PROTOBUF_EXPLICITLY_CONSTRUCTED_H__
#define GOOGLE_PROTOBUF_EXPLICITLY_CONSTRUCTED_H__

#include <stdint.h>

#include <utility>

#include <google/protobuf/stubs/logging.h>
#include <google/protobuf/stubs/common.h>

// 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__