2 * Copyright 2014 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)
27 #ifndef FOLLY_DISCRIMINATEDPTR_H_
28 #define FOLLY_DISCRIMINATEDPTR_H_
32 #include <glog/logging.h>
33 #include <folly/Likely.h>
34 #include <folly/Portability.h>
35 #include <folly/detail/DiscriminatedPtrDetail.h>
38 # error "DiscriminatedPtr is x64-specific code."
44 * Discriminated pointer.
46 * Given a list of types, a DiscriminatedPtr<Types...> may point to an object
47 * of one of the given types, or may be empty. DiscriminatedPtr is type-safe:
48 * you may only get a pointer to the type that you put in, otherwise get
49 * throws an exception (and get_nothrow returns nullptr)
51 * This pointer does not do any kind of lifetime management -- it's not a
52 * "smart" pointer. You are responsible for deallocating any memory used
53 * to hold pointees, if necessary.
55 template <typename... Types>
56 class DiscriminatedPtr {
57 // <, not <=, as our indexes are 1-based (0 means "empty")
58 static_assert(sizeof...(Types) < std::numeric_limits<uint16_t>::max(),
63 * Create an empty DiscriminatedPtr.
65 DiscriminatedPtr() : data_(0) {
69 * Create a DiscriminatedPtr that points to an object of type T.
70 * Fails at compile time if T is not a valid type (listed in Types)
73 explicit DiscriminatedPtr(T* ptr) {
74 set(ptr, typeIndex<T>());
78 * Set this DiscriminatedPtr to point to an object of type T.
79 * Fails at compile time if T is not a valid type (listed in Types)
83 set(ptr, typeIndex<T>());
87 * Get a pointer to the object that this DiscriminatedPtr points to, if it is
88 * of type T. Fails at compile time if T is not a valid type (listed in
89 * Types), and returns nullptr if this DiscriminatedPtr is empty or points to
90 * an object of a different type.
93 T* get_nothrow() noexcept {
94 void* p = LIKELY(hasType<T>()) ? ptr() : nullptr;
95 return static_cast<T*>(p);
99 const T* get_nothrow() const noexcept {
100 const void* p = LIKELY(hasType<T>()) ? ptr() : nullptr;
101 return static_cast<const T*>(p);
105 * Get a pointer to the object that this DiscriminatedPtr points to, if it is
106 * of type T. Fails at compile time if T is not a valid type (listed in
107 * Types), and throws std::invalid_argument if this DiscriminatedPtr is empty
108 * or points to an object of a different type.
110 template <typename T>
112 if (UNLIKELY(!hasType<T>())) {
113 throw std::invalid_argument("Invalid type");
115 return static_cast<T*>(ptr());
118 template <typename T>
119 const T* get() const {
120 if (UNLIKELY(!hasType<T>())) {
121 throw std::invalid_argument("Invalid type");
123 return static_cast<const T*>(ptr());
127 * Return true iff this DiscriminatedPtr is empty.
134 * Return true iff the object pointed by this DiscriminatedPtr has type T,
135 * false otherwise. Fails at compile time if T is not a valid type (listed
138 template <typename T>
139 bool hasType() const {
140 return index() == typeIndex<T>();
144 * Clear this DiscriminatedPtr, making it empty.
151 * Assignment operator from a pointer of type T.
153 template <typename T>
154 DiscriminatedPtr& operator=(T* ptr) {
160 * Apply a visitor to this object, calling the appropriate overload for
161 * the type currently stored in DiscriminatedPtr. Throws invalid_argument
162 * if the DiscriminatedPtr is empty.
164 * The visitor must meet the following requirements:
166 * - The visitor must allow invocation as a function by overloading
167 * operator(), unambiguously accepting all values of type T* (or const T*)
168 * for all T in Types...
169 * - All operations of the function object on T* (or const T*) must
170 * return the same type (or a static_assert will fire).
172 template <typename V>
173 typename dptr_detail::VisitorResult<V, Types...>::type apply(V&& visitor) {
175 if (n == 0) throw std::invalid_argument("Empty DiscriminatedPtr");
176 return dptr_detail::ApplyVisitor<V, Types...>()(
177 n, std::forward<V>(visitor), ptr());
180 template <typename V>
181 typename dptr_detail::ConstVisitorResult<V, Types...>::type apply(V&& visitor)
184 if (n == 0) throw std::invalid_argument("Empty DiscriminatedPtr");
185 return dptr_detail::ApplyConstVisitor<V, Types...>()(
186 n, std::forward<V>(visitor), ptr());
191 * Get the 1-based type index of T in Types.
193 template <typename T>
194 size_t typeIndex() const {
195 return dptr_detail::GetTypeIndex<T, Types...>::value;
198 uint16_t index() const { return data_ >> 48; }
200 return reinterpret_cast<void*>(data_ & ((1ULL << 48) - 1));
203 void set(void* p, uint16_t v) {
204 uintptr_t ip = reinterpret_cast<uintptr_t>(p);
206 ip |= static_cast<uintptr_t>(v) << 48;
211 * We store a pointer in the least significant 48 bits of data_, and a type
212 * index (0 = empty, or 1-based index in Types) in the most significant 16
213 * bits. We rely on the fact that pointers have their most significant 16
214 * bits clear on x86_64.
221 #endif /* FOLLY_DISCRIMINATEDPTR_H_ */