c++ - 如何脱离 std::experimental::optional？

Question

使用 Boost，我可以创建一个可选的就地:

boost::optional<boost::asio::io_service::work> work = boost::in_place(boost::ref(io_service));

并通过以下方式解除它:

work = boost::none;

有了 C++14/实验性支持，我可以改为构建一个可选的就地:

std::experimental::optional<boost::asio::io_service::work> work;
work.emplace(boost::asio::io_service::work(io_service));

但我不知道如何解除它...

Answer 1

work = std::experimental::nullopt;

应该停止工作。
library fundamental TS在 [optional.nullopt] 中指定:

The struct nullopt_t is an empty structure type used as a unique type to indicate a disengaged state for optional objects.

有一个合适的赋值运算符，[optional.object.assign]:

optional<T>& operator=(nullopt_t) noexcept;

Effects: If *this is engaged calls val->T::~T() to destroy the contained value; otherwise no effect.

为了避免每次都构造一个nullopt_t对象，已经声明了一个这种类型的常量:

struct nullopt_t{see below};
constexpr nullopt_t nullopt(unspecified);