std::future::operator= (3) Linux Manual Page

std::future<T>::operator= – std::future<T>::operator=

Synopsis

future &operator=(future &&other) noexcept;
(1)(since C++ 11)

    future &
    operator=(const future &other) = delete;
(2)(since C++ 11)

Assigns the contents of another future object.
1) Releases any shared state and move-assigns the contents of other to *this. After the assignment, other.valid() == false and this->valid() will yield the same value as other.valid() before the assignment.
2) std::future is not CopyAssignable.

Parameters

other – a std::future that will transfer state to *this

Return value

*this