2 * Copyright 2017 Facebook, Inc.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
18 * Discriminated pointer: Type-safe pointer to one of several types.
20 * Similar to boost::variant, but has no space overhead over a raw pointer, as
21 * it relies on the fact that (on x86_64) there are 16 unused bits in a
24 * @author Tudor Bosman (tudorb@fb.com)
32 #include <glog/logging.h>
34 #include <folly/Likely.h>
35 #include <folly/Portability.h>
36 #include <folly/detail/DiscriminatedPtrDetail.h>
38 #if !FOLLY_X64 && !FOLLY_AARCH64 && !FOLLY_PPC64
39 # error "DiscriminatedPtr is x64, arm64 and ppc64 specific code."
45 * Discriminated pointer.
47 * Given a list of types, a DiscriminatedPtr<Types...> may point to an object
48 * of one of the given types, or may be empty. DiscriminatedPtr is type-safe:
49 * you may only get a pointer to the type that you put in, otherwise get
50 * throws an exception (and get_nothrow returns nullptr)
52 * This pointer does not do any kind of lifetime management -- it's not a
53 * "smart" pointer. You are responsible for deallocating any memory used
54 * to hold pointees, if necessary.
56 template <typename... Types>
57 class DiscriminatedPtr {
58 // <, not <=, as our indexes are 1-based (0 means "empty")
59 static_assert(sizeof...(Types) < std::numeric_limits<uint16_t>::max(),
64 * Create an empty DiscriminatedPtr.
66 DiscriminatedPtr() : data_(0) {
70 * Create a DiscriminatedPtr that points to an object of type T.
71 * Fails at compile time if T is not a valid type (listed in Types)
74 explicit DiscriminatedPtr(T* ptr) {
75 set(ptr, typeIndex<T>());
79 * Set this DiscriminatedPtr to point to an object of type T.
80 * Fails at compile time if T is not a valid type (listed in Types)
84 set(ptr, typeIndex<T>());
88 * Get a pointer to the object that this DiscriminatedPtr points to, if it is
89 * of type T. Fails at compile time if T is not a valid type (listed in
90 * Types), and returns nullptr if this DiscriminatedPtr is empty or points to
91 * an object of a different type.
94 T* get_nothrow() noexcept {
95 void* p = LIKELY(hasType<T>()) ? ptr() : nullptr;
96 return static_cast<T*>(p);
100 const T* get_nothrow() const noexcept {
101 const void* p = LIKELY(hasType<T>()) ? ptr() : nullptr;
102 return static_cast<const T*>(p);
106 * Get a pointer to the object that this DiscriminatedPtr points to, if it is
107 * of type T. Fails at compile time if T is not a valid type (listed in
108 * Types), and throws std::invalid_argument if this DiscriminatedPtr is empty
109 * or points to an object of a different type.
111 template <typename T>
113 if (UNLIKELY(!hasType<T>())) {
114 throw std::invalid_argument("Invalid type");
116 return static_cast<T*>(ptr());
119 template <typename T>
120 const T* get() const {
121 if (UNLIKELY(!hasType<T>())) {
122 throw std::invalid_argument("Invalid type");
124 return static_cast<const T*>(ptr());
128 * Return true iff this DiscriminatedPtr is empty.
135 * Return true iff the object pointed by this DiscriminatedPtr has type T,
136 * false otherwise. Fails at compile time if T is not a valid type (listed
139 template <typename T>
140 bool hasType() const {
141 return index() == typeIndex<T>();
145 * Clear this DiscriminatedPtr, making it empty.
152 * Assignment operator from a pointer of type T.
154 template <typename T>
155 DiscriminatedPtr& operator=(T* ptr) {
161 * Apply a visitor to this object, calling the appropriate overload for
162 * the type currently stored in DiscriminatedPtr. Throws invalid_argument
163 * if the DiscriminatedPtr is empty.
165 * The visitor must meet the following requirements:
167 * - The visitor must allow invocation as a function by overloading
168 * operator(), unambiguously accepting all values of type T* (or const T*)
169 * for all T in Types...
170 * - All operations of the function object on T* (or const T*) must
171 * return the same type (or a static_assert will fire).
173 template <typename V>
174 typename dptr_detail::VisitorResult<V, Types...>::type apply(V&& visitor) {
176 if (n == 0) throw std::invalid_argument("Empty DiscriminatedPtr");
177 return dptr_detail::ApplyVisitor<V, Types...>()(
178 n, std::forward<V>(visitor), ptr());
181 template <typename V>
182 typename dptr_detail::ConstVisitorResult<V, Types...>::type apply(V&& visitor)
185 if (n == 0) throw std::invalid_argument("Empty DiscriminatedPtr");
186 return dptr_detail::ApplyConstVisitor<V, Types...>()(
187 n, std::forward<V>(visitor), ptr());
192 * Get the 1-based type index of T in Types.
194 template <typename T>
195 uint16_t typeIndex() const {
196 return uint16_t(dptr_detail::GetTypeIndex<T, Types...>::value);
199 uint16_t index() const { return data_ >> 48; }
201 return reinterpret_cast<void*>(data_ & ((1ULL << 48) - 1));
204 void set(void* p, uint16_t v) {
205 uintptr_t ip = reinterpret_cast<uintptr_t>(p);
207 ip |= static_cast<uintptr_t>(v) << 48;
212 * We store a pointer in the least significant 48 bits of data_, and a type
213 * index (0 = empty, or 1-based index in Types) in the most significant 16
214 * bits. We rely on the fact that pointers have their most significant 16
215 * bits clear on x86_64.
220 template <typename Visitor, typename... Args>
221 decltype(auto) apply_visitor(
223 const DiscriminatedPtr<Args...>& variant) {
224 return variant.apply(std::forward<Visitor>(visitor));
227 template <typename Visitor, typename... Args>
228 decltype(auto) apply_visitor(
230 DiscriminatedPtr<Args...>& variant) {
231 return variant.apply(std::forward<Visitor>(visitor));
234 template <typename Visitor, typename... Args>
235 decltype(auto) apply_visitor(
237 DiscriminatedPtr<Args...>&& variant) {
238 return variant.apply(std::forward<Visitor>(visitor));