X-Git-Url: http://plrg.eecs.uci.edu/git/?a=blobdiff_plain;f=folly%2FFunction.h;h=263b0af07d8ac940b186038dd9bbd5c9e4b436df;hb=ba690cad3c4138fce4e55b319237bb9e01ca030f;hp=f5926ac343edc9a1967bb02cdd83bab50693b6fb;hpb=ab389fe4f77be8d4e25796d2bb6e8f491018f17e;p=folly.git

diff --git a/folly/Function.h b/folly/Function.h
index f5926ac3..263b0af0 100644
--- a/folly/Function.h
+++ b/folly/Function.h
@@ -1,7 +1,5 @@
 /*
- * Copyright 2016 Facebook, Inc.
- *
- * @author Eric Niebler (eniebler@fb.com), Sven Over (over@fb.com)
+ * Copyright 2017-present Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -14,7 +12,9 @@
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
- *
+ */
+/*
+ * @author Eric Niebler (eniebler@fb.com), Sven Over (over@fb.com)
  * Acknowledgements: Giuseppe Ottaviano (ott@fb.com)
  */
 
@@ -224,17 +224,17 @@
 #include <utility>
 
 #include <folly/CppAttributes.h>
+#include <folly/Portability.h>
+#include <folly/Traits.h>
 
 namespace folly {
 
-namespace impl {
-template <typename FunctionType, bool Const = false>
+template <typename FunctionType>
 class Function;
 
 template <typename ReturnType, typename... Args>
-Function<ReturnType(Args...), true> constCastFunction(
-    Function<ReturnType(Args...), false>&&) noexcept;
-} // impl
+Function<ReturnType(Args...) const> constCastFunction(
+    Function<ReturnType(Args...)>&&) noexcept;
 
 namespace detail {
 namespace function {
@@ -243,22 +243,28 @@ enum class Op { MOVE, NUKE, FULL, HEAP };
 
 union Data {
   void* big;
-  typename std::aligned_storage<6 * sizeof(void*)>::type small;
+  std::aligned_storage<6 * sizeof(void*)>::type tiny;
 };
 
-template <bool If, typename T>
-using ConstIf = typename std::conditional<If, const T, T>::type;
-
 template <typename Fun, typename FunT = typename std::decay<Fun>::type>
-using IsSmall = std::integral_constant<
-    bool,
-    (sizeof(FunT) <= sizeof(Data::small) &&
-     // Same as is_nothrow_move_constructible, but w/ no template instantiation.
-     noexcept(FunT(std::declval<FunT&&>()))
-     )>;
+using IsSmall = Conjunction<
+    std::integral_constant<bool, (sizeof(FunT) <= sizeof(Data::tiny))>,
+    std::is_nothrow_move_constructible<FunT>>;
 using SmallTag = std::true_type;
 using HeapTag = std::false_type;
 
+template <class T>
+struct NotFunction : std::true_type {};
+template <class T>
+struct NotFunction<Function<T>> : std::false_type {};
+
+template <typename Fun, typename FunT = typename std::decay<Fun>::type>
+using DecayIfConstructible = typename std::enable_if<
+    Conjunction<NotFunction<FunT>, std::is_constructible<FunT, Fun>>::value,
+    FunT>::type;
+
+struct CoerceTag {};
+
 template <typename T>
 bool isNullPtrFn(T* p) {
   return p == nullptr;
@@ -268,21 +274,158 @@ std::false_type isNullPtrFn(T&&) {
   return {};
 }
 
-template <typename ReturnType, typename... Args>
-ReturnType uninitCall(Data&, Args&&...) {
-  throw std::bad_function_call();
-}
 inline bool uninitNoop(Op, Data*, Data*) {
   return false;
 }
 
+template <typename FunctionType>
+struct FunctionTraits;
+
+template <typename ReturnType, typename... Args>
+struct FunctionTraits<ReturnType(Args...)> {
+  using Call = ReturnType (*)(Data&, Args&&...);
+  using IsConst = std::false_type;
+  using ConstSignature = ReturnType(Args...) const;
+  using NonConstSignature = ReturnType(Args...);
+  using OtherSignature = ConstSignature;
+
+  template <typename F, typename G = typename std::decay<F>::type>
+  using ResultOf = decltype(
+      static_cast<ReturnType>(std::declval<G&>()(std::declval<Args>()...)));
+
+  template <typename Fun>
+  static ReturnType callSmall(Data& p, Args&&... args) {
+    return static_cast<ReturnType>((*static_cast<Fun*>(
+        static_cast<void*>(&p.tiny)))(static_cast<Args&&>(args)...));
+  }
+
+  template <typename Fun>
+  static ReturnType callBig(Data& p, Args&&... args) {
+    return static_cast<ReturnType>(
+        (*static_cast<Fun*>(p.big))(static_cast<Args&&>(args)...));
+  }
+
+  static ReturnType uninitCall(Data&, Args&&...) {
+    throw std::bad_function_call();
+  }
+
+  ReturnType operator()(Args... args) {
+    auto& fn = *static_cast<Function<ReturnType(Args...)>*>(this);
+    return fn.call_(fn.data_, static_cast<Args&&>(args)...);
+  }
+
+  class SharedProxy {
+    std::shared_ptr<Function<ReturnType(Args...)>> sp_;
+
+   public:
+    explicit SharedProxy(Function<ReturnType(Args...)>&& func)
+        : sp_(std::make_shared<Function<ReturnType(Args...)>>(
+              std::move(func))) {}
+    ReturnType operator()(Args&&... args) const {
+      return (*sp_)(static_cast<Args&&>(args)...);
+    }
+  };
+};
+
+template <typename ReturnType, typename... Args>
+struct FunctionTraits<ReturnType(Args...) const> {
+  using Call = ReturnType (*)(Data&, Args&&...);
+  using IsConst = std::true_type;
+  using ConstSignature = ReturnType(Args...) const;
+  using NonConstSignature = ReturnType(Args...);
+  using OtherSignature = NonConstSignature;
+
+  template <typename F, typename G = typename std::decay<F>::type>
+  using ResultOf = decltype(static_cast<ReturnType>(
+      std::declval<const G&>()(std::declval<Args>()...)));
+
+  template <typename Fun>
+  static ReturnType callSmall(Data& p, Args&&... args) {
+    return static_cast<ReturnType>((*static_cast<const Fun*>(
+        static_cast<void*>(&p.tiny)))(static_cast<Args&&>(args)...));
+  }
+
+  template <typename Fun>
+  static ReturnType callBig(Data& p, Args&&... args) {
+    return static_cast<ReturnType>(
+        (*static_cast<const Fun*>(p.big))(static_cast<Args&&>(args)...));
+  }
+
+  static ReturnType uninitCall(Data&, Args&&...) {
+    throw std::bad_function_call();
+  }
+
+  ReturnType operator()(Args... args) const {
+    auto& fn = *static_cast<const Function<ReturnType(Args...) const>*>(this);
+    return fn.call_(fn.data_, static_cast<Args&&>(args)...);
+  }
+
+  class SharedProxy {
+    std::shared_ptr<Function<ReturnType(Args...) const>> sp_;
+
+   public:
+    explicit SharedProxy(Function<ReturnType(Args...) const>&& func)
+        : sp_(std::make_shared<Function<ReturnType(Args...) const>>(
+              std::move(func))) {}
+    ReturnType operator()(Args&&... args) const {
+      return (*sp_)(static_cast<Args&&>(args)...);
+    }
+  };
+};
+
+template <typename Fun>
+bool execSmall(Op o, Data* src, Data* dst) {
+  switch (o) {
+    case Op::MOVE:
+      ::new (static_cast<void*>(&dst->tiny))
+          Fun(std::move(*static_cast<Fun*>(static_cast<void*>(&src->tiny))));
+      FOLLY_FALLTHROUGH;
+    case Op::NUKE:
+      static_cast<Fun*>(static_cast<void*>(&src->tiny))->~Fun();
+      break;
+    case Op::FULL:
+      return true;
+    case Op::HEAP:
+      break;
+  }
+  return false;
+}
+
+template <typename Fun>
+bool execBig(Op o, Data* src, Data* dst) {
+  switch (o) {
+    case Op::MOVE:
+      dst->big = src->big;
+      src->big = nullptr;
+      break;
+    case Op::NUKE:
+      delete static_cast<Fun*>(src->big);
+      break;
+    case Op::FULL:
+    case Op::HEAP:
+      break;
+  }
+  return true;
+}
+
+// Invoke helper
+template <typename F, typename... Args>
+inline auto invoke(F&& f, Args&&... args)
+    -> decltype(std::forward<F>(f)(std::forward<Args>(args)...)) {
+  return std::forward<F>(f)(std::forward<Args>(args)...);
+}
+
+template <typename M, typename C, typename... Args>
+inline auto invoke(M(C::*d), Args&&... args)
+    -> decltype(std::mem_fn(d)(std::forward<Args>(args)...)) {
+  return std::mem_fn(d)(std::forward<Args>(args)...);
+}
+
 } // namespace function
 } // namespace detail
 
-namespace impl {
-
-template <typename ReturnType, typename... Args, bool Const>
-class Function<ReturnType(Args...), Const> final {
+template <typename FunctionType>
+class Function final : private detail::function::FunctionTraits<FunctionType> {
   // These utility types are defined outside of the template to reduce
   // the number of instantiations, and then imported in the class
   // namespace for convenience.
@@ -290,98 +433,57 @@ class Function<ReturnType(Args...), Const> final {
   using Op = detail::function::Op;
   using SmallTag = detail::function::SmallTag;
   using HeapTag = detail::function::HeapTag;
-  using Call = ReturnType (*)(Data&, Args&&...);
+  using CoerceTag = detail::function::CoerceTag;
+
+  using Traits = detail::function::FunctionTraits<FunctionType>;
+  using Call = typename Traits::Call;
   using Exec = bool (*)(Op, Data*, Data*);
 
-  template <typename T>
-  using ConstIf = detail::function::ConstIf<Const, T>;
   template <typename Fun>
   using IsSmall = detail::function::IsSmall<Fun>;
 
-  /**
-   * @Function is const-safe:
-   * - @call_ takes @Data as non-const param to avoid code/data duplication.
-   * - @data_ can only be mutated if @constCastFunction is used.
-   */
+  // The `data_` member is mutable to allow `constCastFunction` to work without
+  // invoking undefined behavior. Const-correctness is only violated when
+  // `FunctionType` is a const function type (e.g., `int() const`) and `*this`
+  // is the result of calling `constCastFunction`.
   mutable Data data_;
-  Call call_{&detail::function::uninitCall<ReturnType, Args...>};
+  Call call_{&Traits::uninitCall};
   Exec exec_{&detail::function::uninitNoop};
 
-  friend Function<ReturnType(Args...), true> constCastFunction<>(
-      Function<ReturnType(Args...), false>&&) noexcept;
-  friend class Function<ReturnType(Args...), !Const>;
-
-  template <typename Fun>
-  struct OpsSmall {
-    using FunT = typename std::decay<Fun>::type;
-    static ReturnType call(Data& p, Args&&... args) {
-      return static_cast<ReturnType>((*static_cast<ConstIf<FunT>*>(
-          static_cast<void*>(&p.small)))(static_cast<Args&&>(args)...));
-    }
-    static bool exec(Op o, Data* src, Data* dst) {
-      switch (o) {
-        case Op::MOVE:
-          ::new (static_cast<void*>(&dst->small)) FunT(
-              std::move(*static_cast<FunT*>(static_cast<void*>(&src->small))));
-          FOLLY_FALLTHROUGH;
-        case Op::NUKE:
-          static_cast<FunT*>(static_cast<void*>(&src->small))->~FunT();
-          break;
-        case Op::FULL:
-          return true;
-        case Op::HEAP:
-          break;
-      }
-      return false;
-    }
-  };
+  friend Traits;
+  friend Function<typename Traits::ConstSignature> folly::constCastFunction<>(
+      Function<typename Traits::NonConstSignature>&&) noexcept;
+  friend class Function<typename Traits::OtherSignature>;
 
   template <typename Fun>
   Function(Fun&& fun, SmallTag) noexcept {
-    using Ops = OpsSmall<Fun>;
+    using FunT = typename std::decay<Fun>::type;
     if (!detail::function::isNullPtrFn(fun)) {
-      ::new (static_cast<void*>(&data_.small))
-          typename Ops::FunT(static_cast<Fun&&>(fun));
-      exec_ = &Ops::exec;
-      call_ = &Ops::call;
+      ::new (static_cast<void*>(&data_.tiny)) FunT(static_cast<Fun&&>(fun));
+      call_ = &Traits::template callSmall<FunT>;
+      exec_ = &detail::function::execSmall<FunT>;
     }
   }
 
-  template <typename Fun>
-  struct OpsHeap {
-    using FunT = typename std::decay<Fun>::type;
-    static ReturnType call(Data& p, Args&&... args) {
-      return static_cast<ReturnType>(
-          (*static_cast<ConstIf<FunT>*>(p.big))(static_cast<Args&&>(args)...));
-    }
-    static bool exec(Op o, Data* src, Data* dst) {
-      switch (o) {
-        case Op::MOVE:
-          dst->big = src->big;
-          src->big = nullptr;
-          break;
-        case Op::NUKE:
-          delete static_cast<FunT*>(src->big);
-          break;
-        case Op::FULL:
-        case Op::HEAP:
-          break;
-      }
-      return true;
-    }
-  };
-
   template <typename Fun>
   Function(Fun&& fun, HeapTag) {
-    using Ops = OpsHeap<Fun>;
-    data_.big = new typename Ops::FunT(static_cast<Fun&&>(fun));
-    call_ = &Ops::call;
-    exec_ = &Ops::exec;
+    using FunT = typename std::decay<Fun>::type;
+    data_.big = new FunT(static_cast<Fun&&>(fun));
+    call_ = &Traits::template callBig<FunT>;
+    exec_ = &detail::function::execBig<FunT>;
   }
 
-  template <typename F, typename G = typename std::decay<F>::type>
-  using ResultOf = decltype(static_cast<ReturnType>(
-      std::declval<ConstIf<G>&>()(std::declval<Args>()...)));
+  template <typename Signature>
+  Function(Function<Signature>&& that, CoerceTag)
+      : Function(static_cast<Function<Signature>&&>(that), HeapTag{}) {}
+
+  Function(
+      Function<typename Traits::OtherSignature>&& that,
+      CoerceTag) noexcept {
+    that.exec_(Op::MOVE, &that.data_, &data_);
+    std::swap(call_, that.call_);
+    std::swap(exec_, that.exec_);
+  }
 
  public:
   /**
@@ -390,11 +492,6 @@ class Function<ReturnType(Args...), Const> final {
   Function() = default;
 
   // not copyable
-  // NOTE: Deleting the non-const copy constructor is unusual but necessary to
-  // prevent copies from non-const `Function` object from selecting the
-  // perfect forwarding implicit converting constructor below
-  // (i.e., `template <typename Fun> Function(Fun&&)`).
-  Function(Function&) = delete;
   Function(const Function&) = delete;
 
   /**
@@ -412,36 +509,47 @@ class Function<ReturnType(Args...), Const> final {
   /* implicit */ Function(std::nullptr_t) noexcept {}
 
   /**
-   * Constructs a new `Function` from any callable object. This
-   * handles function pointers, pointers to static member functions,
-   * `std::reference_wrapper` objects, `std::function` objects, and arbitrary
-   * objects that implement `operator()` if the parameter signature
-   * matches (i.e. it returns R when called with Args...).
-   * For a `Function` with a const function type, the object must be
-   * callable from a const-reference, i.e. implement `operator() const`.
-   * For a `Function` with a non-const function type, the object will
-   * be called from a non-const reference, which means that it will execute
-   * a non-const `operator()` if it is defined, and falls back to
-   * `operator() const` otherwise.
+   * Constructs a new `Function` from any callable object that is _not_ a
+   * `folly::Function`. This handles function pointers, pointers to static
+   * member functions, `std::reference_wrapper` objects, `std::function`
+   * objects, and arbitrary objects that implement `operator()` if the parameter
+   * signature matches (i.e. it returns an object convertible to `R` when called
+   * with `Args...`).
    *
    * \note `typename = ResultOf<Fun>` prevents this overload from being
    * selected by overload resolution when `fun` is not a compatible function.
+   *
+   * \note The noexcept requires some explanation. IsSmall is true when the
+   * decayed type fits within the internal buffer and is noexcept-movable. But
+   * this ctor might copy, not move. What we need here, if this ctor does a
+   * copy, is that this ctor be noexcept when the copy is noexcept. That is not
+   * checked in IsSmall, and shouldn't be, because once the Function is
+   * constructed, the contained object is never copied. This check is for this
+   * ctor only, in the case that this ctor does a copy.
    */
-  template <class Fun, typename = ResultOf<Fun>>
-  /* implicit */ Function(Fun&& fun) noexcept(IsSmall<Fun>::value)
+  template <
+      typename Fun,
+      typename FunT = detail::function::DecayIfConstructible<Fun>,
+      typename = typename Traits::template ResultOf<Fun>>
+  /* implicit */ Function(Fun&& fun) noexcept(
+      IsSmall<Fun>::value && noexcept(FunT(std::declval<Fun>())))
       : Function(static_cast<Fun&&>(fun), IsSmall<Fun>{}) {}
 
   /**
-   * For moving a `Function<X(Ys..) const>` into a `Function<X(Ys...)>`.
+   * For move-constructing from a `folly::Function<X(Ys...) [const?]>`.
+   * For a `Function` with a `const` function type, the object must be
+   * callable from a `const`-reference, i.e. implement `operator() const`.
+   * For a `Function` with a non-`const` function type, the object will
+   * be called from a non-const reference, which means that it will execute
+   * a non-const `operator()` if it is defined, and falls back to
+   * `operator() const` otherwise.
    */
   template <
-      bool OtherConst,
-      typename std::enable_if<!Const && OtherConst, int>::type = 0>
-  Function(Function<ReturnType(Args...), OtherConst>&& that) noexcept {
-    that.exec_(Op::MOVE, &that.data_, &data_);
-    std::swap(call_, that.call_);
-    std::swap(exec_, that.exec_);
-  }
+      typename Signature,
+      typename = typename Traits::template ResultOf<Function<Signature>>>
+  Function(Function<Signature>&& that) noexcept(
+      noexcept(Function(std::move(that), CoerceTag{})))
+      : Function(std::move(that), CoerceTag{}) {}
 
   /**
    * If `ptr` is null, constructs an empty `Function`. Otherwise,
@@ -463,22 +571,27 @@ class Function<ReturnType(Args...), Const> final {
     exec_(Op::NUKE, &data_, nullptr);
   }
 
-  Function& operator=(Function&) = delete;
   Function& operator=(const Function&) = delete;
 
   /**
    * Move assignment operator
+   *
+   * \note Leaves `that` in a valid but unspecified state. If `&that == this`
+   * then `*this` is left in a valid but unspecified state.
    */
   Function& operator=(Function&& that) noexcept {
-    if (&that != this) {
-      // Q: Why is is safe to destroy and reconstruct this object in place?
-      // A: Two reasons: First, `Function` is a final class, so in doing this
-      //    we aren't slicing off any derived parts. And second, the move
-      //    operation is guaranteed not to throw so we always leave the object
-      //    in a valid state.
-      this->~Function();
-      ::new (this) Function(std::move(that));
-    }
+    // Q: Why is is safe to destroy and reconstruct this object in place?
+    // A: Two reasons: First, `Function` is a final class, so in doing this
+    //    we aren't slicing off any derived parts. And second, the move
+    //    operation is guaranteed not to throw so we always leave the object
+    //    in a valid state.
+    // In the case of self-move (this == &that), this leaves the object in
+    // a default-constructed state. First the object is destroyed, then we
+    // pass the destroyed object to the move constructor. The first thing the
+    // move constructor does is default-construct the object. That object is
+    // "moved" into itself, which is a no-op for a default-constructed Function.
+    this->~Function();
+    ::new (this) Function(std::move(that));
     return *this;
   }
 
@@ -489,7 +602,7 @@ class Function<ReturnType(Args...), Const> final {
    * \note `typename = ResultOf<Fun>` prevents this overload from being
    * selected by overload resolution when `fun` is not a compatible function.
    */
-  template <class Fun, typename = ResultOf<Fun>>
+  template <typename Fun, typename = decltype(Function(std::declval<Fun>()))>
   Function& operator=(Fun&& fun) noexcept(
       noexcept(/* implicit */ Function(std::declval<Fun>()))) {
     // Doing this in place is more efficient when we can do so safely.
@@ -505,6 +618,17 @@ class Function<ReturnType(Args...), Const> final {
     return *this;
   }
 
+  /**
+   * For assigning from a `Function<X(Ys..) [const?]>`.
+   */
+  template <
+      typename Signature,
+      typename = typename Traits::template ResultOf<Function<Signature>>>
+  Function& operator=(Function<Signature>&& that) noexcept(
+      noexcept(Function(std::move(that)))) {
+    return (*this = Function(std::move(that)));
+  }
+
   /**
    * Clears this `Function`.
    */
@@ -526,31 +650,8 @@ class Function<ReturnType(Args...), Const> final {
 
   /**
    * Call the wrapped callable object with the specified arguments.
-   * If this `Function` object is a const `folly::Function` object,
-   * this overload shall not participate in overload resolution.
    */
-  template <
-      // `True` makes `operator()` a template so we can SFINAE on `Const`,
-      // which is non-deduced here.
-      bool True = true,
-      typename std::enable_if<True && !Const, int>::type = 0>
-  ReturnType operator()(Args... args) {
-    return call_(data_, static_cast<Args&&>(args)...);
-  }
-
-  /**
-   * Call the wrapped callable object with the specified arguments.
-   * If this `Function` object is not a const `folly::Function` object,
-   * this overload shall not participate in overload resolution.
-   */
-  template <
-      // `True` makes `operator()` a template so we can SFINAE on `Const`,
-      // which is non-deduced here.
-      bool True = true,
-      typename std::enable_if<True && Const, int>::type = 0>
-  ReturnType operator()(Args... args) const {
-    return call_(data_, static_cast<Args&&>(args)...);
-  }
+  using Traits::operator();
 
   /**
    * Exchanges the callable objects of `*this` and `that`.
@@ -577,85 +678,149 @@ class Function<ReturnType(Args...), Const> final {
     return exec_(Op::HEAP, nullptr, nullptr);
   }
 
+  using typename Traits::SharedProxy;
+
+  /**
+   * Move this `Function` into a copyable callable object, of which all copies
+   * share the state.
+   */
+  SharedProxy asSharedProxy() && {
+    return SharedProxy{std::move(*this)};
+  }
+
   /**
    * Construct a `std::function` by moving in the contents of this `Function`.
    * Note that the returned `std::function` will share its state (i.e. captured
    * data) across all copies you make of it, so be very careful when copying.
    */
-  std::function<ReturnType(Args...)> asStdFunction() && {
-    struct Impl {
-      std::shared_ptr<Function> sp_;
-      ReturnType operator()(Args&&... args) const {
-        return (*sp_)(static_cast<Args&&>(args)...);
-      }
-    };
-    return Impl{std::make_shared<Function>(std::move(*this))};
+  std::function<typename Traits::NonConstSignature> asStdFunction() && {
+    return std::move(*this).asSharedProxy();
   }
 };
 
-template <typename FunctionType, bool Const>
-void swap(
-    Function<FunctionType, Const>& lhs,
-    Function<FunctionType, Const>& rhs) noexcept {
+template <typename FunctionType>
+void swap(Function<FunctionType>& lhs, Function<FunctionType>& rhs) noexcept {
   lhs.swap(rhs);
 }
 
-template <typename FunctionType, bool Const>
-bool operator==(const Function<FunctionType, Const>& fn, std::nullptr_t) {
+template <typename FunctionType>
+bool operator==(const Function<FunctionType>& fn, std::nullptr_t) {
   return !fn;
 }
 
-template <typename FunctionType, bool Const>
-bool operator==(std::nullptr_t, const Function<FunctionType, Const>& fn) {
+template <typename FunctionType>
+bool operator==(std::nullptr_t, const Function<FunctionType>& fn) {
   return !fn;
 }
 
-template <typename FunctionType, bool Const>
-bool operator!=(const Function<FunctionType, Const>& fn, std::nullptr_t) {
+template <typename FunctionType>
+bool operator!=(const Function<FunctionType>& fn, std::nullptr_t) {
   return !(fn == nullptr);
 }
 
-template <typename FunctionType, bool Const>
-bool operator!=(std::nullptr_t, const Function<FunctionType, Const>& fn) {
+template <typename FunctionType>
+bool operator!=(std::nullptr_t, const Function<FunctionType>& fn) {
   return !(nullptr == fn);
 }
 
 /**
- * NOTE: See detailed note about @constCastFunction at the top of the file.
+ * NOTE: See detailed note about `constCastFunction` at the top of the file.
  * This is potentially dangerous and requires the equivalent of a `const_cast`.
  */
 template <typename ReturnType, typename... Args>
-Function<ReturnType(Args...), true> constCastFunction(
-    Function<ReturnType(Args...), false>&& that) noexcept {
-  Function<ReturnType(Args...), true> fn{};
-  that.exec_(detail::function::Op::MOVE, &that.data_, &fn.data_);
-  std::swap(fn.call_, that.call_);
-  std::swap(fn.exec_, that.exec_);
-  return fn;
+Function<ReturnType(Args...) const> constCastFunction(
+    Function<ReturnType(Args...)>&& that) noexcept {
+  return Function<ReturnType(Args...) const>{std::move(that),
+                                             detail::function::CoerceTag{}};
 }
 
-template <typename FunctionType>
-Function<FunctionType, true> constCastFunction(
-    Function<FunctionType, true>&& that) noexcept {
+template <typename ReturnType, typename... Args>
+Function<ReturnType(Args...) const> constCastFunction(
+    Function<ReturnType(Args...) const>&& that) noexcept {
   return std::move(that);
 }
 
+/**
+ * @class FunctionRef
+ *
+ * @brief A reference wrapper for callable objects
+ *
+ * FunctionRef is similar to std::reference_wrapper, but the template parameter
+ * is the function signature type rather than the type of the referenced object.
+ * A folly::FunctionRef is cheap to construct as it contains only a pointer to
+ * the referenced callable and a pointer to a function which invokes the
+ * callable.
+ *
+ * The user of FunctionRef must be aware of the reference semantics: storing a
+ * copy of a FunctionRef is potentially dangerous and should be avoided unless
+ * the referenced object definitely outlives the FunctionRef object. Thus any
+ * function that accepts a FunctionRef parameter should only use it to invoke
+ * the referenced function and not store a copy of it. Knowing that FunctionRef
+ * itself has reference semantics, it is generally okay to use it to reference
+ * lambdas that capture by reference.
+ */
+
 template <typename FunctionType>
-struct MakeFunction {};
+class FunctionRef;
 
 template <typename ReturnType, typename... Args>
-struct MakeFunction<ReturnType(Args...)> {
-  using type = Function<ReturnType(Args...), false>;
-};
+class FunctionRef<ReturnType(Args...)> final {
+  using Call = ReturnType (*)(void*, Args&&...);
 
-template <typename ReturnType, typename... Args>
-struct MakeFunction<ReturnType(Args...) const> {
-  using type = Function<ReturnType(Args...), true>;
-};
-} // namespace impl
+  void* object_{nullptr};
+  Call call_{&FunctionRef::uninitCall};
+
+  static ReturnType uninitCall(void*, Args&&...) {
+    throw std::bad_function_call();
+  }
 
-/* using override */ using impl::constCastFunction;
+  template <typename Fun>
+  static ReturnType call(void* object, Args&&... args) {
+    return static_cast<ReturnType>(detail::function::invoke(
+        *static_cast<Fun*>(object), static_cast<Args&&>(args)...));
+  }
 
-template <typename FunctionType>
-using Function = typename impl::MakeFunction<FunctionType>::type;
-}
+ public:
+  /**
+   * Default constructor. Constructs an empty FunctionRef.
+   *
+   * Invoking it will throw std::bad_function_call.
+   */
+  FunctionRef() = default;
+
+  /**
+   * Construct a FunctionRef from a reference to a callable object.
+   */
+  template <
+      typename Fun,
+      typename std::enable_if<
+          !std::is_same<FunctionRef, typename std::decay<Fun>::type>::value,
+          int>::type = 0>
+  /* implicit */ FunctionRef(Fun&& fun) noexcept {
+    using ReferencedType = typename std::remove_reference<Fun>::type;
+
+    static_assert(
+        std::is_convertible<
+            typename std::result_of<ReferencedType&(Args && ...)>::type,
+            ReturnType>::value,
+        "FunctionRef cannot be constructed from object with "
+        "incompatible function signature");
+
+    // `Fun` may be a const type, in which case we have to do a const_cast
+    // to store the address in a `void*`. This is safe because the `void*`
+    // will be cast back to `Fun*` (which is a const pointer whenever `Fun`
+    // is a const type) inside `FunctionRef::call`
+    object_ = const_cast<void*>(static_cast<void const*>(std::addressof(fun)));
+    call_ = &FunctionRef::call<ReferencedType>;
+  }
+
+  ReturnType operator()(Args... args) const {
+    return call_(object_, static_cast<Args&&>(args)...);
+  }
+
+  explicit operator bool() const {
+    return object_;
+  }
+};
+
+} // namespace folly