A core-cached shared_ptr

[folly.git] / folly / Utility.h

diff --git a/folly/Utility.h b/folly/Utility.h
index a76925057362dadf7a3fbc48f16fd91f287a9b45..4d6725d7070ec70e9684d19db915100c6fc5e9cc 100644 (file)
--- a/folly/Utility.h
+++ b/folly/Utility.h
@@ -16,6 +16,7 @@
 
 #pragma once
 
+#include <cstdint>
 #include <type_traits>
 #include <utility>
 
@@ -68,4 +69,93 @@ constexpr typename std::decay<T>::type copy(T&& value) noexcept(
     noexcept(typename std::decay<T>::type(std::forward<T>(value)))) {
   return std::forward<T>(value);
 }
+
+/**
+ * A simple helper for getting a constant reference to an object.
+ *
+ * Example:
+ *
+ *   std::vector<int> v{1,2,3};
+ *   // The following two lines are equivalent:
+ *   auto a = const_cast<const std::vector<int>&>(v).begin();
+ *   auto b = folly::as_const(v).begin();
+ *
+ * Like C++17's std::as_const. See http://wg21.link/p0007
+ */
+#if __cpp_lib_as_const || _MSC_VER
+
+/* using override */ using std::as_const;
+
+#else
+
+template <class T>
+constexpr T const& as_const(T& t) noexcept {
+  return t;
+}
+
+template <class T>
+void as_const(T const&&) = delete;
+
+#endif
+
+#if __cpp_lib_integer_sequence || _MSC_VER
+
+/* using override */ using std::integer_sequence;
+/* using override */ using std::index_sequence;
+/* using override */ using std::make_index_sequence;
+
+#else
+
+template <class T, T... Ints>
+struct integer_sequence {
+  using value_type = T;
+
+  static constexpr std::size_t size() noexcept {
+    return sizeof...(Ints);
+  }
+};
+
+template <std::size_t... Ints>
+using index_sequence = folly::integer_sequence<std::size_t, Ints...>;
+
+namespace detail {
+template <std::size_t N, std::size_t... Ints>
+struct make_index_sequence
+    : detail::make_index_sequence<N - 1, N - 1, Ints...> {};
+
+template <std::size_t... Ints>
+struct make_index_sequence<0, Ints...> : folly::index_sequence<Ints...> {};
+}
+
+template <std::size_t N>
+using make_index_sequence = detail::make_index_sequence<N>;
+
+#endif
+
+/**
+ * A simple function object that passes its argument through unchanged.
+ *
+ * Example:
+ *
+ *   int i = 42;
+ *   int &j = Identity()(i);
+ *   assert(&i == &j);
+ *
+ * Warning: passing a prvalue through Identity turns it into an xvalue,
+ * which can effect whether lifetime extension occurs or not. For instance:
+ *
+ *   auto&& x = std::make_unique<int>(42);
+ *   cout << *x ; // OK, x refers to a valid unique_ptr.
+ *
+ *   auto&& y = Identity()(std::make_unique<int>(42));
+ *   cout << *y ; // ERROR: y did not lifetime-extend the unique_ptr. It
+ *                // is no longer valid
+ */
+struct Identity {
+  using is_transparent = void;
+  template <class T>
+  constexpr T&& operator()(T&& x) const noexcept {
+    return static_cast<T&&>(x);
+  }
+};
 }