template <class F>
see below then(F&& func);
Requires:
INVOKE(DECAY_COPY (std::forward<F>(func)), std::move(*this)) shall be a valid expression.
Effects:
The function creates a shared state that is associated with the returned future object. Additionally,
When the object's shared state is ready, the continuation INVOKE(DECAY_COPY(std::forward<F>(func)), std::move(*this)) is called on an unspecified thread of execution with the call to DECAY_COPY() being evaluated in the thread that called then.
Any value returned from the continuation is stored as the result in the shared state of the resulting future. Any exception propagated from the execution of the continuation is stored as the exceptional result in the shared state of the resulting future.
Returns:
When result_of_t<decay_t<F>(future<R>)> is future<R2>, for some type R2, the function returns future<R2>. Otherwise, the function returns future<result_of_t<decay_t<F>(future<R>)>>. [ Note: The rule above is referred to as implicit unwrapping. Without this rule, the return type of then taking a callable returning a future<R> would have been future<future<R>>. This rule avoids such nested future objects. The type of f2 below is future<int> and not future<future<int>>:
[ Example:
future<int> f1 = g();
future<int> f2 = f1.then([](future<int> f) {
future<int> f3 = h();
return f3;
});
— end example ]
— end note ]
Postconditions:
valid() == false on the original future. valid() == true on the future returned from then. [ Note: In case of implicit unwrapping, the validity of the future returned from thenfunc cannot be established until after the completion of the continuation. If it is not valid, the resulting future becomes ready with an exception of type std::future_error, with an error condition of std::future_errc::broken_promise. — end note ]
