Why don't std :: future <T> and std :: shared_future <T> not provide membership swap ()?

Question

All classes in the C ++ Standard Library have a member exchange function, including some polymorphic classes, such as std::basic_ios<CharT>. The template class std::shared_future<T>is the type of value, and std::future<T>is the type of value only for moving. Is there any special reason they don't provide a member function swap()?

Answer 1

The swap member was a massive increase in performance before std::movesupport in C ++ 11. For example, you could move one vector to another place. It was also used in sizes vector, and this meant that inserting vectors into a vector was not a complete suicidal capacity.

After std::movearriving in C ++ 11, with many sometimes empty types was implemented by default std::swap:

template<class T>
void swap( T& lhs, T& rhs ) {
  auto tmp = std::move(rhs);
  rhs = std::move(lhs);
  lhs = std::move(tmp);
}

will be basically as fast as custom-written.

Existing types with members swapare unlikely to lose them (at least right away). However, a new type of API extension should be justified.

std::future std::unique_ptr< future_impl >, 4 , 3 . , ¹ (, SSA ²), .swap .

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¹ , , lhs rhs , tmp : , tmp, , no-op dtor.

² , , ( ). .