std::promise::operator= (3) Linux Manual Page
std::promise<R>::operator= – std::promise<R>::operator=
Synopsis
promise &operator=(promise &&other) noexcept;
(1)(since C++ 11)
promise &
operator=(const promise &rhs) = delete;
(2)(since C++ 11)Assigns the contents.
1) Move assignment operator. First, abandons the shared state (as in ~promise()), then assigns the shared state of other as if by executing std::promise(std::move(other)).swap(*this).
2) promise is not copy-assignable.
Parameters
other – another promise to acquire the state from
Return value
*this