c++ - 使用 std::experimental::optional 给出编译错误

Question

我第一次使用可选功能，知道这段代码有什么问题，我希望从存储的元组中检索值retCode

#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;

}

编译错误

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

没有从optional<T>to的隐式转换T，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;
}

除非你有其他地方需要 addEntity返回一个可选元组，而不仅仅是你调用addEntity 初始化一个可选元组的地方，它应该只返回一个元组。