Improving flat containers interface.

base/type_traits.h

+// Copyright 2017 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 BASE_TYPE_TRAITS_H_
+#define BASE_TYPE_TRAITS_H_
+
+#include <type_traits>
+
+namespace base {
+
+// C++14 ---------------------------------------------------------------------
+
+template <bool condition, typename T = void>
+using enable_if_t = typename std::enable_if<condition, T>::type;
+
+template <bool condition, typename T, typename U>
+using condtional_t = typename std::conditional<condition, T, U>::type;
+
+// C++17 ----------------------------------------------------------------------
+
+// Detection idiom. We use constexpr function is_detected_c() instead of a
+// variable template.
+
+template <typename...>
+using void_t = void;
+
+struct nonesuch {
+  nonesuch() = delete;
+  ~nonesuch() = delete;
+  nonesuch(nonesuch const&) = delete;
+  void operator=(nonesuch const&) = delete;
+};
+
+namespace internal {
+
+template <typename Default,
+          typename AlwaysVoid,
+          template <typename...> class Op,
+          typename... Args>
+struct detector {
+  using value_t = std::false_type;
+  using type = Default;
+};
+
+template <typename Default, template <typename...> class Op, typename... Args>
+struct detector<Default, void_t<Op<Args...>>, Op, Args...> {
+  using value_t = std::true_type;
+  using type = Op<Args...>;
+};
+
+}  // namespace internal
+
+template <template <class...> class Op, typename... Args>
+using is_detected =
+    typename internal::detector<nonesuch, void, Op, Args...>::value_t;
+
+template <typename Default, template <typename...> class Op, typename... Args>
+using detected_or = internal::detector<Default, void, Op, Args...>;
+
+template <template <typename...> class Op, typename... Args>
+constexpr bool is_detected_c() {
+  return is_detected<Op, Args...>::value;
+}
+
+}  // namespace base
+
+#endif  // BASE_TYPE_TRAITS_H_