// forward declaration
template <class T> class Future;
+namespace detail {
+struct EmptyConstruct {};
+template <typename T, typename F>
+class CoreCallbackState;
+}
+
template <class T>
class Promise {
public:
private:
typedef typename Future<T>::corePtr corePtr;
template <class> friend class Future;
+ template <class, class>
+ friend class detail::CoreCallbackState;
// Whether the Future has been retrieved (a one-time operation).
bool retrieved_;
// shared core state object
corePtr core_;
+ explicit Promise(detail::EmptyConstruct) noexcept;
+
void throwIfFulfilled();
void throwIfRetrieved();
void detach();