Improving flat containers interface.

base/functional.h

Index: base/functional.h
diff --git a/base/functional.h b/base/functional.h
new file mode 100644
index 0000000000000000000000000000000000000000..487d59de9f3828f38f8e445ebc63049ef4baea3a
--- /dev/null
+++ b/base/functional.h
@@ -0,0 +1,79 @@
+// 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_FUNCTIONAL_H_
+#define BASE_FUNCTIONAL_H_
+
+#include <functional>
+#include <utility>
+
+#include "base/type_traits.h"
+
+namespace base {
+
+namespace internal {
+
+template <typename T>
+using HasGet = decltype(std::declval<T>().get());
+
+template <typename T>
+using HasDereference = decltype(*(std::declval<T>()));
+
+template <typename Lhs, typename Rhs>
+using HaveLess = decltype(std::declval<Lhs>() < std::declval<Rhs>());
+
+}  // namespace internal
+
+// Concepts -------------------------------------------------------------------
+
+template <typename T>
+constexpr bool SmartPointer() {
+  return is_detected_c<internal::HasGet, T>() &&
+         is_detected_c<internal::HasDereference, T>();
+}
+
+template <typename Lhs, typename Rhs>
+constexpr bool Comparable() {
+  return is_detected_c<internal::HaveLess, Lhs, Rhs>();
+}
+
+// Function objects ------------------------------------------------------------
+
+// Variation of std::less<> from C++14.
+// Needed for improved flat containers API.
+//
+// Also supports comparing smart pointers to plain pointers because this is a
+// common use case and makes sets of smart pointers much more useful.
+// Note: we have to use different enable_if techniques, because:
+// a) we want to deduce return value for common case (as the standard one) and
+//    not be extremely ugly while doing so.
+// b) Some weired property doesn't allow us to use 'typename =' as a common
+//    technique:
+//    https://stackoverflow.com/questions/41646253/sfinae-class-member-cannot-be-redeclared
+
+struct less {
+  template <typename T, typename U, typename = enable_if_t<Comparable<T, U>()>>
+  constexpr auto operator()(T&& lhs, U&& rhs) const
+      -> decltype(std::forward<T>(lhs) < std::forward<U>(rhs)) {
+    return std::forward<T>(lhs) < std::forward<U>(rhs);
+  }
+
+  template <typename T, typename U>
+  constexpr enable_if_t<SmartPointer<T>() && !SmartPointer<U>(), bool>
+  operator()(const T& smart_ptr, const U& ptr) const {
+    return smart_ptr.get() < ptr;
+  }
+
+  template <typename T, typename U>
+  constexpr enable_if_t<!SmartPointer<T>() && SmartPointer<U>(), bool>
+  operator()(const T& ptr, const U& smart_ptr) const {
+    return ptr < smart_ptr.get();
+  }
+
+  using is_transparent = int;
+};
+
+}  // base
+
+#endif  // BASE_FUNCTIONAL_H_