folly::rvalue_reference_wrapper for store&forward of rvalue references

Summary:
Class template that wraps a reference to an rvalue. Similar to std::reference_wrapper but with three important differences:
1) folly::rvalue_reference_wrappers can only be moved, not copied;
2) the get() function and the conversion-to-T operator are destructive and not const, they invalidate the wrapper;
3) the constructor-from-T is explicit.
These restrictions are designed to make it harder to accidentally create a a dangling rvalue reference, or to use an rvalue reference multiple times. (Using an rvalue reference typically implies invalidation of the target object, such as move-assignment to another object.)

Reviewed By: yfeldblum

Differential Revision: D4931483

fbshipit-source-id: 68453553bf4656ec41976699669a4491fcab79c9

folly/Functional.h

+/*
+ * 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.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * 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.
+ */
+
+#pragma once
+
+#include <cassert>
+#include <memory>
+#include <utility>
+
+namespace folly {
+
+/**
+ * Class template that wraps a reference to an rvalue. Similar to
+ * std::reference_wrapper but with four important differences:
+ *
+ * 1) folly::rvalue_reference_wrappers can only be moved, not copied;
+ * 2) the get() function and the conversion-to-T operator are destructive and
+ *    not const, they invalidate the wrapper;
+ * 3) the constructor-from-T is explicit.
+ *
+ * These restrictions are designed to make it harder to accidentally create a
+ * a dangling rvalue reference, or to use an rvalue reference multiple times.
+ * (Using an rvalue reference typically implies invalidation of the target
+ * object, such as move-assignment to another object.)
+ *
+ * @seealso folly::rref
+ */
+template <class T>
+class rvalue_reference_wrapper {
+ public:
+  using type = T;
+
+  /**
+   * Default constructor. Creates an invalid reference. Must be move-assigned
+   * to in order to be come valid.
+   */
+  rvalue_reference_wrapper() noexcept : ptr_(nullptr) {}
+
+  /**
+   * Explicit constructor to make it harder to accidentally create a dangling
+   * reference to a temporary.
+   */
+  explicit rvalue_reference_wrapper(T&& ref) noexcept
+      : ptr_(std::addressof(ref)) {}
+
+  /**
+   * No construction from lvalue reference. Use std::move.
+   */
+  explicit rvalue_reference_wrapper(T&) noexcept = delete;
+
+  /**
+   * Destructive move construction.
+   */
+  rvalue_reference_wrapper(rvalue_reference_wrapper<T>&& other) noexcept
+      : ptr_(other.ptr_) {
+    other.ptr_ = nullptr;
+  }
+
+  /**
+   * Destructive move assignment.
+   */
+  rvalue_reference_wrapper& operator=(
+      rvalue_reference_wrapper&& other) noexcept {
+    ptr_ = other.ptr_;
+    other.ptr_ = nullptr;
+    return *this;
+  }
+
+  /**
+   * Implicit conversion to raw reference. Destructive.
+   */
+  /* implicit */ operator T &&() && noexcept {
+    return static_cast<rvalue_reference_wrapper&&>(*this).get();
+  }
+
+  /**
+   * Explicit unwrap. Destructive.
+   */
+  T&& get() && noexcept {
+    assert(valid());
+    T& ref = *ptr_;
+    ptr_ = nullptr;
+    return static_cast<T&&>(ref);
+  }
+
+  /**
+   * Calls the callable object to whom reference is stored. Only available if
+   * the wrapped reference points to a callable object. Destructive.
+   */
+  template <class... Args>
+  decltype(auto) operator()(Args&&... args) &&
+      noexcept(noexcept(std::declval<T>()(std::forward<Args>(args)...))) {
+    return static_cast<rvalue_reference_wrapper&&>(*this).get()(
+        std::forward<Args>(args)...);
+  }
+
+  /**
+   * Check whether wrapped reference is valid.
+   */
+  bool valid() const noexcept {
+    return ptr_ != nullptr;
+  }
+
+ private:
+  // Disallow copy construction and copy assignment, to make it harder to
+  // accidentally use an rvalue reference multiple times.
+  rvalue_reference_wrapper(const rvalue_reference_wrapper&) = delete;
+  rvalue_reference_wrapper& operator=(const rvalue_reference_wrapper&) = delete;
+
+  T* ptr_;
+};
+
+/**
+ * Create a folly::rvalue_reference_wrapper. Analogous to std::ref().
+ *
+ * Warning: folly::rvalue_reference_wrappers are potentially dangerous, because
+ * they can easily be used to capture references to temporary values. Users must
+ * ensure that the target object outlives the reference wrapper.
+ *
+ * @example
+ *   class Object {};
+ *   void f(Object&&);
+ *   // BAD
+ *   void g() {
+ *     auto ref = folly::rref(Object{});  // create reference to temporary
+ *     f(std::move(ref));                 // pass dangling reference
+ *   }
+ *   // GOOD
+ *   void h() {
+ *     Object o;
+ *     auto ref = folly::rref(std::move(o));
+ *     f(std::move(ref));
+ *   }
+ */
+template <typename T>
+rvalue_reference_wrapper<T> rref(T&& value) noexcept {
+  return rvalue_reference_wrapper<T>(std::move(value));
+}
+template <typename T>
+rvalue_reference_wrapper<T> rref(T&) noexcept = delete;
+}

folly/Makefile.am

@@ -394,6 +394,7 @@ nobase_follyinclude_HEADERS = \
 	Try.h \
 	Unicode.h \
 	Function.h \
+	Functional.h \
 	UncaughtExceptions.h \
 	Unit.h \
 	Uri.h \

folly/test/FunctionalTest.cpp

+/*
+ * Copyright 2017 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * 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.
+ */
+
+#include <utility>
+
+#include <folly/Functional.h>
+#include <folly/portability/GTest.h>
+
+TEST(RvalueReferenceWrapper, MoveAndConvert) {
+  using folly::rvalue_reference_wrapper;
+
+  // Destructive moves.
+  int i1 = 0;
+  rvalue_reference_wrapper<int> rref1(std::move(i1));
+  ASSERT_TRUE(rref1.valid());
+  rvalue_reference_wrapper<int> rref0(std::move(rref1));
+  ASSERT_TRUE(rref0.valid());
+  ASSERT_FALSE(rref1.valid());
+  rref1 = std::move(rref0);
+  ASSERT_FALSE(rref0.valid());
+  ASSERT_TRUE(rref1.valid());
+  const int& r1 = std::move(rref1);
+  ASSERT_FALSE(rref1.valid());
+  ASSERT_EQ(&r1, &i1);
+
+  // Destructive unwrap to T&&.
+  int i2 = 0;
+  rvalue_reference_wrapper<int> rref2(std::move(i2));
+  int&& r2 = std::move(rref2);
+  ASSERT_EQ(&r2, &i2);
+
+  // Destructive unwrap to const T&.
+  const int i3 = 0;
+  rvalue_reference_wrapper<const int> rref3(std::move(i3));
+  const int& r3 = std::move(rref3);
+  ASSERT_EQ(&r3, &i3);
+
+  // Destructive unwrap to const T&&.
+  const int i4 = 0;
+  rvalue_reference_wrapper<const int> rref4(std::move(i4));
+  const int&& r4 = std::move(rref4);
+  ASSERT_EQ(&r4, &i4);
+
+  /*
+   * Things that intentionally do not compile. Copy construction, copy
+   * assignment, unwrap of lvalue reference to wrapper, const violations.
+   *
+  int i5;
+  const int i6 = 0;
+  rvalue_reference_wrapper<int> rref5(i5);
+  rvalue_reference_wrapper<const int> rref6(i6);
+  rref1 = rref5;
+  int& r5 = rref5;
+  const int& r6 = rref6;
+  int i7;
+  const rvalue_reference_wrapper<int> rref7(std::move(i7));
+  int& r7 = std::move(rref7);
+  */
+}
+
+TEST(RvalueReferenceWrapper, Call) {
+  int a = 4711, b, c;
+  auto callMe = [&](int x, const int& y, int&& z) -> int {
+    EXPECT_EQ(a, x);
+    EXPECT_EQ(&b, &y);
+    EXPECT_EQ(&c, &z);
+    return a;
+  };
+  int result = folly::rref(std::move(callMe))(a, b, std::move(c));
+  EXPECT_EQ(a, result);
+}

folly/test/Makefile.am

@@ -302,6 +302,11 @@ function_test_SOURCES = \
 function_test_LDADD = libfollytestmain.la
 TESTS += function_test
 
+functional_test_SOURCES = \
+		FunctionalTest.cpp
+functional_test_LDADD = libfollytestmain.la
+TESTS += functional_test
+
 ssl_test_SOURCES = \
 		../ssl/test/OpenSSLHashTest.cpp
 ssl_test_LDADD = libfollytestmain.la -lcrypto