Improving flat containers interface.

base/type_traits.h

+// Copyright 2017 The Chromium Authors. All rights reserved.

[jdoerrie, 2017/06/20 23:51:09]

base/template_util.h
(https://codesearch.chromium.org/chromium/src/base/template_util.h) already
tries to provide some missing std type_traits that are not present on old GCC
versions. With the existence of this file these two files should probably be
merged, however doing this in a follow-up CL seems very reasonable to me.

[dyaroshev, 2017/06/22 10:41:43]

Done - merged it here. Also replaced local implementation of the detection idiom
with the one from the future standard.

+// 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 ---------------------------------------------------------------------
+// These can be replaced with the ones from the standard once we have C++14.
+
+// std::enable_if_t: http://en.cppreference.com/w/cpp/types/enable_if
+template <bool condition, typename T = void>
+using enable_if_t = typename std::enable_if<condition, T>::type;
+
+// std::conditional_t: http://en.cppreference.com/w/cpp/types/conditional
+template <bool condition, typename T, typename U>
+using conditional_t = typename std::conditional<condition, T, U>::type;
+
+// C++17 ----------------------------------------------------------------------
+// These can be replaced with the ones from the standard once we have C++17.
+
+// std::void_t: http://en.cppreference.com/w/cpp/types/void_t
+// Needed for the detection idiom.
+template <typename...>
+using void_t = void;
+
+// std::nonesuch: http://en.cppreference.com/w/cpp/experimental/nonesuch
+// Needed for the detection idiom.
+struct nonesuch {
+  nonesuch() = delete;
+  ~nonesuch() = delete;
+  nonesuch(nonesuch const&) = delete;
+  void operator=(nonesuch const&) = delete;
+};
+
+// Implementation of the detection idiom from cppreference:
+// http://en.cppreference.com/w/cpp/experimental/is_detected
+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
+
+// std::is_detected: http://en.cppreference.com/w/cpp/experimental/is_detected
+template <template <class...> class Op, typename... Args>
+using is_detected =
+    typename internal::detector<nonesuch, void, Op, Args...>::value_t;
+
+// std::is_detected_v, implemented as a constexpr function instead of a variable
+// template, since we don't have those.
+// http://en.cppreference.com/w/cpp/experimental/is_detected
+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_