+/** A value or an exception (or nothing)
+ *
+ * This is similar to `optional<expected<T>>`` but it with a Future/Promise
+ * like API.
+ *
+ * Also the name is not so great.
+ **/
+template<class T>
+class Result {
+ enum class ResultStatus {
+ invalid,
+ value,
+ exception,
+ };
+public:
+ Result() { /* Nothing to do */}
+ ~Result() { this->reset(); }
+
+ // Copy (if T is copyable) and move:
+ Result(Result const& that)
+ {
+ (*this) = that;
+ }
+ Result& operator=(Result const& that)
+ {
+ this->reset();
+ switch (that.status_) {
+ case ResultStatus::invalid:
+ break;
+ case ResultStatus::value:
+ new (&value_) T(that.value);
+ break;
+ case ResultStatus::exception:
+ new (&exception_) T(that.exception);
+ break;
+ default:
+ THROW_IMPOSSIBLE;
+ }
+ return *this;
+ }
+ Result(Result&& that)
+ {
+ *this = std::move(that);
+ }
+ Result& operator=(Result&& that)
+ {
+ this->reset();
+ switch (that.status_) {
+ case ResultStatus::invalid:
+ break;
+ case ResultStatus::value:
+ new (&value_) T(std::move(that.value));
+ that.value.~T();
+ break;
+ case ResultStatus::exception:
+ new (&exception_) T(std::move(that.exception));
+ that.exception.~exception_ptr();
+ break;
+ default:
+ THROW_IMPOSSIBLE;
+ }
+ that.status_ = ResultStatus::invalid;
+ return *this;
+ }
+
+ bool is_valid() const
+ {
+ return status_ != ResultStatus::invalid;
+ }
+ void reset()
+ {
+ switch (status_) {
+ case ResultStatus::invalid:
+ break;
+ case ResultStatus::value:
+ value_.~T();
+ break;
+ case ResultStatus::exception:
+ exception_.~exception_ptr();
+ break;
+ default:
+ THROW_IMPOSSIBLE;
+ }
+ status_ = ResultStatus::invalid;
+ }
+ void set_exception(std::exception_ptr e)
+ {
+ this->reset();
+ new (&exception_) std::exception_ptr(std::move(e));
+ status_ = ResultStatus::exception;
+ }
+ void set_value(T&& value)
+ {
+ this->reset();
+ new (&value_) T(std::move(value));
+ status_ = ResultStatus::value;
+ }
+ void set_value(T const& value)
+ {
+ this->reset();
+ new (&value_) T(value);
+ status_ = ResultStatus::value;
+ }
+
+ /** Extract the value from the future
+ *
+ * After this, the value is invalid.
+ **/
+ T get()
+ {
+ switch (status_) {
+ case ResultStatus::value: {
+ T value = std::move(value_);
+ value_.~T();
+ status_ = ResultStatus::invalid;
+ return std::move(value);
+ }
+ case ResultStatus::exception: {
+ std::exception_ptr exception = std::move(exception_);
+ exception_.~exception_ptr();
+ status_ = ResultStatus::invalid;
+ std::rethrow_exception(std::move(exception));
+ break;
+ }
+ default:
+ throw std::logic_error("Invalid result");
+ }
+ }
+private:
+ ResultStatus status_ = ResultStatus::invalid;
+ union {
+ T value_;
+ std::exception_ptr exception_;
+ };
+};
+
+template<>
+class Result<void> : public Result<std::nullptr_t>
+{
+public:
+ void set_value()
+ {
+ Result<std::nullptr_t>::set_value(nullptr);
+ }
+ void get()
+ {
+ Result<std::nullptr_t>::get();
+ }
+};
+
+template<class T>
+class Result<T&> : public Result<std::reference_wrapper<T>>
+{
+public:
+ void set_value(T& value)
+ {
+ Result<std::reference_wrapper<T>>::set_value(std::ref(value));
+ }
+ T& get()
+ {
+ return Result<std::reference_wrapper<T>>::get();
+ }
+};
+
+/** Execute some code and set a promise or result accordingly
+ *
+ * Roughly this does:
+ *
+ * <pre>
+ * promise.set_value(code());
+ * </pre>
+ *
+ * but it takes care of exceptions and works with `void`.
+ *
+ * We might need this when working with generic code because
+ * the trivial implementation does not work with `void` (before C++1z).
+ *
+ * @param code What we want to do
+ * @param promise Where to want to store the result
+ */