使用std :: experimental :: Optional會產生編譯錯誤

Question

我第一次使用可選功能，任何想法在此代碼中出了什么問題，我希望從存儲在retCode的元組中檢索值

#include <iostream>
#include <experimental/optional>
std::experimental::optional<std::tuple<uint16_t, uint32_t, uint32_t>> addEntity();

  std::experimental::optional<std::tuple<uint16_t, uint32_t, uint32_t>> addEntity() {
    uint32_t ipR = 1111;
    uint32_t ipU = 2222;
    uint16_t entityId = 0;
    return std::make_tuple(entityId, ipR, ipU);
  }


int main()
{
  auto retCode =  addEntity();
  std::cout<<std::get<0>(retCode)<<std::endl;
  return 0;

}

編譯錯誤

g++ experiment.cpp
In file included from /usr/include/c++/7/bits/move.h:54:0,
                 from /usr/include/c++/7/bits/nested_exception.h:40,
                 from /usr/include/c++/7/exception:143,
                 from /usr/include/c++/7/ios:39,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from experiment.cpp:1:
/usr/include/c++/7/type_traits: In instantiation of ‘struct std::is_trivially_destructible<std::tuple<short unsigned int, unsigned int, unsigned int> >’:
/usr/include/c++/7/experimental/optional:203:5:   required from ‘class std::experimental::fundamentals_v1::optional<std::tuple<short unsigned int, unsigned int, unsigned int> >’

Answer 1

從optional<T>到T沒有隱式轉換，您需要提取值。

int main()
{
  auto retCode =  addEntity();
  std::cout<<std::get<0>(retCode.value())<<std::endl; // can throw std::bad_optional_access
  return 0;
}

除非您有其他需要 addEntity返回可選元組的地方，而不僅是您在調用addEntity 初始化可選元組的地方，否則它應該只返回一個tuple 。