NOT INTENDED FOR LANDING: A promises demo

components/scheduler/promises/template_helpers.h

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef COMPONENTS_SCHEDULER_PROMISES_TEMPLATE_HELPERS_H_
+#define COMPONENTS_SCHEDULER_PROMISES_TEMPLATE_HELPERS_H_
+
+#include <type_traits>
+
+namespace promise {
+namespace internal {
+class PromiseBase;
+
+template <typename R>
+class Promise;
+
+template <typename T, typename ReturnType>
+using enable_if_void_t =
+    typename std::enable_if<std::is_void<T>::value, ReturnType>::type;
+
+template <typename T, typename ReturnType>
+using enable_if_not_void_t =
+    typename std::enable_if<!std::is_void<T>::value, ReturnType>::type;
+
+// A paramater pack introspection template that can determine if the pack
+// contains |TypeBeingChecked|.
+template <typename TypeBeingChecked, typename Head, typename... Tail>
+struct check_any_have_type {
+  enum {
+    value = std::is_same<TypeBeingChecked, Head>::value ||
+            check_any_have_type<TypeBeingChecked, Tail...>::value
+  };
+};
+
+template <typename TypeBeingChecked, typename Head>
+struct check_any_have_type<TypeBeingChecked, Head> {
+  enum { value = std::is_same<TypeBeingChecked, Head>::value };
+};
+
+// A paramater pack introspection template that can determine if the pack
+// only contains |RequiredType|.
+template <typename RequiredType, typename Head, typename... Tail>
+struct check_all_same_type {
+  enum {
+    value = std::is_same<RequiredType, Head>::value &&
+            check_all_same_type<RequiredType, Tail...>::value
+  };
+};
+
+template <typename RequiredType, typename Head>
+struct check_all_same_type<RequiredType, Head> {
+  enum { value = std::is_same<RequiredType, Head>::value };
+};
+
+// This template is for returning the return type of the Nth promise in a
+// parameter pack of scoped_ref_ptrs<Promise<R>>
+template <std::size_t Index, typename T, typename... Ts>
+struct nth_promise_impl {
+  using Type = typename nth_promise_impl<Index - 1, Ts...>::Type;
+};
+
+template <typename T, typename... Ts>
+struct nth_promise_impl<0, T, Ts...> {
+  using Type = typename T::ReturnType;
+};
+
+// This template is for calling |GetResolved| on the Nth promise in a
+// parameter  pack of scoped_ref_ptrs<Promise<R>> where the actual promises are
+// held in a std::vector<scoped_refptr<PromiseBase>>.
+template <std::size_t Index, typename... Promises>
+struct PromiseResolver {
+  using ReturnType = typename nth_promise_impl<Index, Promises...>::Type;
+
+  static ReturnType GetResolved(
+      std::vector<scoped_refptr<PromiseBase>>& promises) {
+    return std::move(static_cast<Promise<ReturnType>*>(promises[Index].get())
+                         ->GetResolved());
+  }
+};
+
+// This template is for generating a parameter pack of N indicies.
+// When C++14 is allowed we can replace with std::integer_sequence.
+template <size_t... Ns>
+struct indices {
+  typedef indices<Ns..., sizeof...(Ns)> next;
+};
+
+template <size_t N>
+struct make_indices {
+  typedef typename make_indices<N - 1>::type::next type;
+};
+
+template <>
+struct make_indices<0> {
+  typedef indices<> type;
+};
+
+}  // namespace internal
+}  // namespace promise
+
+#endif  // COMPONENTS_SCHEDULER_PROMISES_TEMPLATE_HELPERS_H_