1//==- llvm/ADT/IntrusiveRefCntPtr.h - Smart Refcounting Pointer --*- C++ -*-==//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8///
9/// \file
10/// This file defines the RefCountedBase, ThreadSafeRefCountedBase, and
11/// IntrusiveRefCntPtr classes.
12///
13/// IntrusiveRefCntPtr is a smart pointer to an object which maintains a
14/// reference count. (ThreadSafe)RefCountedBase is a mixin class that adds a
15/// refcount member variable and methods for updating the refcount. An object
16/// that inherits from (ThreadSafe)RefCountedBase deletes itself when its
17/// refcount hits zero.
18///
19/// For example:
20///
21/// ```
22/// class MyClass : public RefCountedBase<MyClass> {};
23///
24/// void foo() {
25/// // Constructing an IntrusiveRefCntPtr increases the pointee's refcount
26/// // by 1 (from 0 in this case).
27/// IntrusiveRefCntPtr<MyClass> Ptr1(new MyClass());
28///
29/// // Copying an IntrusiveRefCntPtr increases the pointee's refcount by 1.
30/// IntrusiveRefCntPtr<MyClass> Ptr2(Ptr1);
31///
32/// // Constructing an IntrusiveRefCntPtr has no effect on the object's
33/// // refcount. After a move, the moved-from pointer is null.
34/// IntrusiveRefCntPtr<MyClass> Ptr3(std::move(Ptr1));
35/// assert(Ptr1 == nullptr);
36///
37/// // Clearing an IntrusiveRefCntPtr decreases the pointee's refcount by 1.
38/// Ptr2.reset();
39///
40/// // The object deletes itself when we return from the function, because
41/// // Ptr3's destructor decrements its refcount to 0.
42/// }
43/// ```
44///
45/// You can use IntrusiveRefCntPtr with isa<T>(), dyn_cast<T>(), etc.:
46///
47/// ```
48/// IntrusiveRefCntPtr<MyClass> Ptr(new MyClass());
49/// OtherClass *Other = dyn_cast<OtherClass>(Ptr); // Ptr.get() not required
50/// ```
51///
52/// IntrusiveRefCntPtr works with any class that
53///
54/// - inherits from (ThreadSafe)RefCountedBase,
55/// - has Retain() and Release() methods, or
56/// - specializes IntrusiveRefCntPtrInfo.
57///
58//===----------------------------------------------------------------------===//
59
60#ifndef LLVM_ADT_INTRUSIVEREFCNTPTR_H
61#define LLVM_ADT_INTRUSIVEREFCNTPTR_H
62
63#include <atomic>
64#include <cassert>
65#include <cstddef>
66#include <memory>
67
68namespace llvm {
69
70/// A CRTP mixin class that adds reference counting to a type.
71///
72/// The lifetime of an object which inherits from RefCountedBase is managed by
73/// calls to Release() and Retain(), which increment and decrement the object's
74/// refcount, respectively. When a Release() call decrements the refcount to 0,
75/// the object deletes itself.
76template <class Derived> class RefCountedBase {
77 mutable unsigned RefCount = 0;
78
79protected:
80 RefCountedBase() = default;
81 RefCountedBase(const RefCountedBase &) {}
82 RefCountedBase &operator=(const RefCountedBase &) = delete;
83
84#ifndef NDEBUG
85 ~RefCountedBase() {
86 assert(RefCount == 0 &&
87 "Destruction occurred when there are still references to this.");
88 }
89#else
90 // Default the destructor in release builds, A trivial destructor may enable
91 // better codegen.
92 ~RefCountedBase() = default;
93#endif
94
95public:
96 unsigned UseCount() const { return RefCount; }
97
98 void Retain() const { ++RefCount; }
99
100 void Release() const {
101 assert(RefCount > 0 && "Reference count is already zero.");
102 if (--RefCount == 0)
103 delete static_cast<const Derived *>(this);
104 }
105};
106
107/// A thread-safe version of \c RefCountedBase.
108template <class Derived> class ThreadSafeRefCountedBase {
109 mutable std::atomic<int> RefCount{0};
110
111protected:
112 ThreadSafeRefCountedBase() = default;
113 ThreadSafeRefCountedBase(const ThreadSafeRefCountedBase &) {}
114 ThreadSafeRefCountedBase &
115 operator=(const ThreadSafeRefCountedBase &) = delete;
116
117#ifndef NDEBUG
118 ~ThreadSafeRefCountedBase() {
119 assert(RefCount == 0 &&
120 "Destruction occurred when there are still references to this.");
121 }
122#else
123 // Default the destructor in release builds, A trivial destructor may enable
124 // better codegen.
125 ~ThreadSafeRefCountedBase() = default;
126#endif
127
128public:
129 unsigned UseCount() const { return RefCount.load(m: std::memory_order_relaxed); }
130
131 void Retain() const { RefCount.fetch_add(i: 1, m: std::memory_order_relaxed); }
132
133 void Release() const {
134 int NewRefCount = RefCount.fetch_sub(i: 1, m: std::memory_order_acq_rel) - 1;
135 assert(NewRefCount >= 0 && "Reference count was already zero.");
136 if (NewRefCount == 0)
137 delete static_cast<const Derived *>(this);
138 }
139};
140
141/// Class you can specialize to provide custom retain/release functionality for
142/// a type.
143///
144/// Usually specializing this class is not necessary, as IntrusiveRefCntPtr
145/// works with any type which defines Retain() and Release() functions -- you
146/// can define those functions yourself if RefCountedBase doesn't work for you.
147///
148/// One case when you might want to specialize this type is if you have
149/// - Foo.h defines type Foo and includes Bar.h, and
150/// - Bar.h uses IntrusiveRefCntPtr<Foo> in inline functions.
151///
152/// Because Foo.h includes Bar.h, Bar.h can't include Foo.h in order to pull in
153/// the declaration of Foo. Without the declaration of Foo, normally Bar.h
154/// wouldn't be able to use IntrusiveRefCntPtr<Foo>, which wants to call
155/// T::Retain and T::Release.
156///
157/// To resolve this, Bar.h could include a third header, FooFwd.h, which
158/// forward-declares Foo and specializes IntrusiveRefCntPtrInfo<Foo>. Then
159/// Bar.h could use IntrusiveRefCntPtr<Foo>, although it still couldn't call any
160/// functions on Foo itself, because Foo would be an incomplete type.
161template <typename T> struct IntrusiveRefCntPtrInfo {
162 static unsigned useCount(const T *obj) { return obj->UseCount(); }
163 static void retain(T *obj) { obj->Retain(); }
164 static void release(T *obj) { obj->Release(); }
165};
166
167/// A smart pointer to a reference-counted object that inherits from
168/// RefCountedBase or ThreadSafeRefCountedBase.
169///
170/// This class increments its pointee's reference count when it is created, and
171/// decrements its refcount when it's destroyed (or is changed to point to a
172/// different object).
173template <typename T> class IntrusiveRefCntPtr {
174 T *Obj = nullptr;
175
176public:
177 using element_type = T;
178
179 explicit IntrusiveRefCntPtr() = default;
180 IntrusiveRefCntPtr(T *obj) : Obj(obj) { retain(); }
181 IntrusiveRefCntPtr(const IntrusiveRefCntPtr &S) : Obj(S.Obj) { retain(); }
182 IntrusiveRefCntPtr(IntrusiveRefCntPtr &&S) : Obj(S.Obj) { S.Obj = nullptr; }
183
184 template <class X,
185 std::enable_if_t<std::is_convertible<X *, T *>::value, bool> = true>
186 IntrusiveRefCntPtr(IntrusiveRefCntPtr<X> S) : Obj(S.get()) {
187 S.Obj = nullptr;
188 }
189
190 template <class X,
191 std::enable_if_t<std::is_convertible<X *, T *>::value, bool> = true>
192 IntrusiveRefCntPtr(std::unique_ptr<X> S) : Obj(S.release()) {
193 retain();
194 }
195
196 ~IntrusiveRefCntPtr() { release(); }
197
198 IntrusiveRefCntPtr &operator=(IntrusiveRefCntPtr S) {
199 swap(other&: S);
200 return *this;
201 }
202
203 T &operator*() const { return *Obj; }
204 T *operator->() const { return Obj; }
205 T *get() const { return Obj; }
206 explicit operator bool() const { return Obj; }
207
208 void swap(IntrusiveRefCntPtr &other) {
209 T *tmp = other.Obj;
210 other.Obj = Obj;
211 Obj = tmp;
212 }
213
214 void reset() {
215 release();
216 Obj = nullptr;
217 }
218
219 void resetWithoutRelease() { Obj = nullptr; }
220
221 unsigned useCount() const {
222 return Obj ? IntrusiveRefCntPtrInfo<T>::useCount(Obj) : 0;
223 }
224
225private:
226 void retain() {
227 if (Obj)
228 IntrusiveRefCntPtrInfo<T>::retain(Obj);
229 }
230
231 void release() {
232 if (Obj)
233 IntrusiveRefCntPtrInfo<T>::release(Obj);
234 }
235
236 template <typename X> friend class IntrusiveRefCntPtr;
237};
238
239template <class T, class U>
240inline bool operator==(const IntrusiveRefCntPtr<T> &A,
241 const IntrusiveRefCntPtr<U> &B) {
242 return A.get() == B.get();
243}
244
245template <class T, class U>
246inline bool operator!=(const IntrusiveRefCntPtr<T> &A,
247 const IntrusiveRefCntPtr<U> &B) {
248 return A.get() != B.get();
249}
250
251template <class T, class U>
252inline bool operator==(const IntrusiveRefCntPtr<T> &A, U *B) {
253 return A.get() == B;
254}
255
256template <class T, class U>
257inline bool operator!=(const IntrusiveRefCntPtr<T> &A, U *B) {
258 return A.get() != B;
259}
260
261template <class T, class U>
262inline bool operator==(T *A, const IntrusiveRefCntPtr<U> &B) {
263 return A == B.get();
264}
265
266template <class T, class U>
267inline bool operator!=(T *A, const IntrusiveRefCntPtr<U> &B) {
268 return A != B.get();
269}
270
271template <class T>
272bool operator==(std::nullptr_t, const IntrusiveRefCntPtr<T> &B) {
273 return !B;
274}
275
276template <class T>
277bool operator==(const IntrusiveRefCntPtr<T> &A, std::nullptr_t B) {
278 return B == A;
279}
280
281template <class T>
282bool operator!=(std::nullptr_t A, const IntrusiveRefCntPtr<T> &B) {
283 return !(A == B);
284}
285
286template <class T>
287bool operator!=(const IntrusiveRefCntPtr<T> &A, std::nullptr_t B) {
288 return !(A == B);
289}
290
291// Make IntrusiveRefCntPtr work with dyn_cast, isa, and the other idioms from
292// Casting.h.
293template <typename From> struct simplify_type;
294
295template <class T> struct simplify_type<IntrusiveRefCntPtr<T>> {
296 using SimpleType = T *;
297
298 static SimpleType getSimplifiedValue(IntrusiveRefCntPtr<T> &Val) {
299 return Val.get();
300 }
301};
302
303template <class T> struct simplify_type<const IntrusiveRefCntPtr<T>> {
304 using SimpleType = /*const*/ T *;
305
306 static SimpleType getSimplifiedValue(const IntrusiveRefCntPtr<T> &Val) {
307 return Val.get();
308 }
309};
310
311/// Factory function for creating intrusive ref counted pointers.
312template <typename T, typename... Args>
313IntrusiveRefCntPtr<T> makeIntrusiveRefCnt(Args &&...A) {
314 return IntrusiveRefCntPtr<T>(new T(std::forward<Args>(A)...));
315}
316
317} // end namespace llvm
318
319#endif // LLVM_ADT_INTRUSIVEREFCNTPTR_H
320

source code of include/llvm-20/llvm/ADT/IntrusiveRefCntPtr.h