*/
template <class T> struct IsArenaAllocator : std::false_type { };
+/*
+ * folly::enable_shared_from_this
+ *
+ * To be removed once C++17 becomes a minimum requirement for folly.
+ */
+#if __cplusplus >= 201700L || \
+ __cpp_lib_enable_shared_from_this >= 201603L
+
+// Guaranteed to have std::enable_shared_from_this::weak_from_this(). Prefer
+// type alias over our own class.
+/* using override */ using std::enable_shared_from_this;
+
+#else
+
+/**
+ * Extends std::enabled_shared_from_this. Offers weak_from_this() to pre-C++17
+ * code. Use as drop-in replacement for std::enable_shared_from_this.
+ *
+ * C++14 has no direct means of creating a std::weak_ptr, one must always
+ * create a (temporary) std::shared_ptr first. C++17 adds weak_from_this() to
+ * std::enable_shared_from_this to avoid that overhead. Alas code that must
+ * compile under different language versions cannot call
+ * std::enable_shared_from_this::weak_from_this() directly. Hence this class.
+ *
+ * @example
+ * class MyClass : public folly::enable_shared_from_this<MyClass> {};
+ *
+ * int main() {
+ * std::shared_ptr<MyClass> sp = std::make_shared<MyClass>();
+ * std::weak_ptr<MyClass> wp = sp->weak_from_this();
+ * }
+ */
+template <typename T>
+class enable_shared_from_this : public std::enable_shared_from_this<T> {
+public:
+ constexpr enable_shared_from_this() noexcept = default;
+
+ std::weak_ptr<T> weak_from_this() noexcept {
+ return weak_from_this_<T>(this);
+ }
+
+ std::weak_ptr<T const> weak_from_this() const noexcept {
+ return weak_from_this_<T>(this);
+ }
+
+private:
+ // Uses SFINAE to detect and call
+ // std::enable_shared_from_this<T>::weak_from_this() if available. Falls
+ // back to std::enable_shared_from_this<T>::shared_from_this() otherwise.
+ template <typename U>
+ auto weak_from_this_(std::enable_shared_from_this<U>* base_ptr)
+ noexcept -> decltype(base_ptr->weak_from_this()) {
+ return base_ptr->weak_from_this();
+ }
+
+ template <typename U>
+ auto weak_from_this_(std::enable_shared_from_this<U> const* base_ptr)
+ const noexcept -> decltype(base_ptr->weak_from_this()) {
+ return base_ptr->weak_from_this();
+ }
+
+ template <typename U>
+ std::weak_ptr<U> weak_from_this_(...) noexcept {
+ try {
+ return this->shared_from_this();
+ } catch (std::bad_weak_ptr const&) {
+ // C++17 requires that weak_from_this() on an object not owned by a
+ // shared_ptr returns an empty weak_ptr. Sadly, in C++14,
+ // shared_from_this() on such an object is undefined behavior, and there
+ // is nothing we can do to detect and handle the situation in a portable
+ // manner. But in case a compiler is nice enough to implement C++17
+ // semantics of shared_from_this() and throws a bad_weak_ptr, we catch it
+ // and return an empty weak_ptr.
+ return std::weak_ptr<U>{};
+ }
+ }
+
+ template <typename U>
+ std::weak_ptr<U const> weak_from_this_(...) const noexcept {
+ try {
+ return this->shared_from_this();
+ } catch (std::bad_weak_ptr const&) {
+ return std::weak_ptr<U const>{};
+ }
+ }
+};
+
+#endif
+
} // namespace folly