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[email protected]9fc44162012-01-23 22:56:411// Copyright (c) 2012 The Chromium Authors. All rights reserved.
[email protected]30039e62008-09-08 14:11:132// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5// The LazyInstance<Type, Traits> class manages a single instance of Type,
6// which will be lazily created on the first time it's accessed. This class is
7// useful for places you would normally use a function-level static, but you
8// need to have guaranteed thread-safety. The Type constructor will only ever
9// be called once, even if two threads are racing to create the object. Get()
10// and Pointer() will always return the same, completely initialized instance.
11// When the instance is constructed it is registered with AtExitManager. The
12// destructor will be called on program exit.
13//
14// LazyInstance is completely thread safe, assuming that you create it safely.
15// The class was designed to be POD initialized, so it shouldn't require a
16// static constructor. It really only makes sense to declare a LazyInstance as
[email protected]6de0fd1d2011-11-15 13:31:4917// a global variable using the LAZY_INSTANCE_INITIALIZER initializer.
[email protected]30039e62008-09-08 14:11:1318//
19// LazyInstance is similar to Singleton, except it does not have the singleton
20// property. You can have multiple LazyInstance's of the same type, and each
21// will manage a unique instance. It also preallocates the space for Type, as
22// to avoid allocating the Type instance on the heap. This may help with the
23// performance of creating the instance, and reducing heap fragmentation. This
24// requires that Type be a complete type so we can determine the size.
25//
26// Example usage:
[email protected]6de0fd1d2011-11-15 13:31:4927// static LazyInstance<MyClass> my_instance = LAZY_INSTANCE_INITIALIZER;
[email protected]30039e62008-09-08 14:11:1328// void SomeMethod() {
29// my_instance.Get().SomeMethod(); // MyClass::SomeMethod()
[email protected]52a261f2009-03-03 15:01:1230//
[email protected]30039e62008-09-08 14:11:1331// MyClass* ptr = my_instance.Pointer();
32// ptr->DoDoDo(); // MyClass::DoDoDo
33// }
34
35#ifndef BASE_LAZY_INSTANCE_H_
36#define BASE_LAZY_INSTANCE_H_
37
[email protected]359d2bf2010-11-19 20:34:1838#include <new> // For placement new.
39
[email protected]30039e62008-09-08 14:11:1340#include "base/atomicops.h"
[email protected]0bea7252011-08-05 15:34:0041#include "base/base_export.h"
[email protected]30039e62008-09-08 14:11:1342#include "base/basictypes.h"
[email protected]b2206da52013-06-07 22:09:3243#include "base/debug/leak_annotations.h"
[email protected]e4a638f762011-10-21 19:46:0044#include "base/logging.h"
[email protected]cd924d62012-02-23 17:52:2045#include "base/memory/aligned_memory.h"
[email protected]ee857512010-05-14 08:24:4246#include "base/third_party/dynamic_annotations/dynamic_annotations.h"
[email protected]34b99632011-01-01 01:01:0647#include "base/threading/thread_restrictions.h"
[email protected]30039e62008-09-08 14:11:1348
[email protected]6de0fd1d2011-11-15 13:31:4949// LazyInstance uses its own struct initializer-list style static
50// initialization, as base's LINKER_INITIALIZED requires a constructor and on
51// some compilers (notably gcc 4.4) this still ends up needing runtime
52// initialization.
53#define LAZY_INSTANCE_INITIALIZER {0}
54
[email protected]30039e62008-09-08 14:11:1355namespace base {
56
57template <typename Type>
58struct DefaultLazyInstanceTraits {
[email protected]113eee02011-10-25 19:04:4859 static const bool kRegisterOnExit = true;
[email protected]5a8a8062014-03-06 01:11:5460#ifndef NDEBUG
[email protected]359d2bf2010-11-19 20:34:1861 static const bool kAllowedToAccessOnNonjoinableThread = false;
[email protected]5a8a8062014-03-06 01:11:5462#endif
[email protected]359d2bf2010-11-19 20:34:1863
[email protected]c1aeaac2010-03-12 15:28:4864 static Type* New(void* instance) {
[email protected]cd924d62012-02-23 17:52:2065 DCHECK_EQ(reinterpret_cast<uintptr_t>(instance) & (ALIGNOF(Type) - 1), 0u)
[email protected]e4a638f762011-10-21 19:46:0066 << ": Bad boy, the buffer passed to placement new is not aligned!\n"
67 "This may break some stuff like SSE-based optimizations assuming the "
68 "<Type> objects are word aligned.";
[email protected]30039e62008-09-08 14:11:1369 // Use placement new to initialize our instance in our preallocated space.
70 // The parenthesis is very important here to force POD type initialization.
[email protected]c1aeaac2010-03-12 15:28:4871 return new (instance) Type();
[email protected]30039e62008-09-08 14:11:1372 }
[email protected]113eee02011-10-25 19:04:4873 static void Delete(Type* instance) {
[email protected]30039e62008-09-08 14:11:1374 // Explicitly call the destructor.
[email protected]113eee02011-10-25 19:04:4875 instance->~Type();
[email protected]30039e62008-09-08 14:11:1376 }
77};
78
[email protected]67f92bc32012-01-26 01:56:1979// We pull out some of the functionality into non-templated functions, so we
80// can implement the more complicated pieces out of line in the .cc file.
81namespace internal {
82
[email protected]9fc44162012-01-23 22:56:4183// Use LazyInstance<T>::Leaky for a less-verbose call-site typedef; e.g.:
84// base::LazyInstance<T>::Leaky my_leaky_lazy_instance;
85// instead of:
[email protected]67f92bc32012-01-26 01:56:1986// base::LazyInstance<T, base::internal::LeakyLazyInstanceTraits<T> >
87// my_leaky_lazy_instance;
[email protected]9fc44162012-01-23 22:56:4188// (especially when T is MyLongTypeNameImplClientHolderFactory).
[email protected]67f92bc32012-01-26 01:56:1989// Only use this internal::-qualified verbose form to extend this traits class
90// (depending on its implementation details).
[email protected]dcc69332010-10-21 20:41:4791template <typename Type>
92struct LeakyLazyInstanceTraits {
[email protected]113eee02011-10-25 19:04:4893 static const bool kRegisterOnExit = false;
[email protected]5a8a8062014-03-06 01:11:5494#ifndef NDEBUG
[email protected]359d2bf2010-11-19 20:34:1895 static const bool kAllowedToAccessOnNonjoinableThread = true;
[email protected]5a8a8062014-03-06 01:11:5496#endif
[email protected]359d2bf2010-11-19 20:34:1897
[email protected]dcc69332010-10-21 20:41:4798 static Type* New(void* instance) {
[email protected]b2206da52013-06-07 22:09:3299 ANNOTATE_SCOPED_MEMORY_LEAK;
[email protected]dcc69332010-10-21 20:41:47100 return DefaultLazyInstanceTraits<Type>::New(instance);
101 }
[email protected]113eee02011-10-25 19:04:48102 static void Delete(Type* instance) {
103 }
[email protected]dcc69332010-10-21 20:41:47104};
105
[email protected]6de0fd1d2011-11-15 13:31:49106// Our AtomicWord doubles as a spinlock, where a value of
107// kBeingCreatedMarker means the spinlock is being held for creation.
108static const subtle::AtomicWord kLazyInstanceStateCreating = 1;
[email protected]1b651d52011-05-16 15:01:54109
[email protected]6de0fd1d2011-11-15 13:31:49110// Check if instance needs to be created. If so return true otherwise
111// if another thread has beat us, wait for instance to be created and
112// return false.
113BASE_EXPORT bool NeedsLazyInstance(subtle::AtomicWord* state);
[email protected]30039e62008-09-08 14:11:13114
[email protected]6de0fd1d2011-11-15 13:31:49115// After creating an instance, call this to register the dtor to be called
116// at program exit and to update the atomic state to hold the |new_instance|
117BASE_EXPORT void CompleteLazyInstance(subtle::AtomicWord* state,
118 subtle::AtomicWord new_instance,
119 void* lazy_instance,
120 void (*dtor)(void*));
[email protected]1b651d52011-05-16 15:01:54121
[email protected]6de0fd1d2011-11-15 13:31:49122} // namespace internal
[email protected]30039e62008-09-08 14:11:13123
124template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> >
[email protected]6de0fd1d2011-11-15 13:31:49125class LazyInstance {
[email protected]30039e62008-09-08 14:11:13126 public:
[email protected]6de0fd1d2011-11-15 13:31:49127 // Do not define a destructor, as doing so makes LazyInstance a
128 // non-POD-struct. We don't want that because then a static initializer will
129 // be created to register the (empty) destructor with atexit() under MSVC, for
130 // example. We handle destruction of the contained Type class explicitly via
131 // the OnExit member function, where needed.
[email protected]1b651d52011-05-16 15:01:54132 // ~LazyInstance() {}
[email protected]30039e62008-09-08 14:11:13133
[email protected]9fc44162012-01-23 22:56:41134 // Convenience typedef to avoid having to repeat Type for leaky lazy
135 // instances.
[email protected]67f92bc32012-01-26 01:56:19136 typedef LazyInstance<Type, internal::LeakyLazyInstanceTraits<Type> > Leaky;
[email protected]9fc44162012-01-23 22:56:41137
[email protected]30039e62008-09-08 14:11:13138 Type& Get() {
139 return *Pointer();
140 }
141
142 Type* Pointer() {
[email protected]3c7a7f32011-11-04 17:29:10143#ifndef NDEBUG
144 // Avoid making TLS lookup on release builds.
[email protected]359d2bf2010-11-19 20:34:18145 if (!Traits::kAllowedToAccessOnNonjoinableThread)
[email protected]6de0fd1d2011-11-15 13:31:49146 ThreadRestrictions::AssertSingletonAllowed();
[email protected]3c7a7f32011-11-04 17:29:10147#endif
[email protected]6de0fd1d2011-11-15 13:31:49148 // If any bit in the created mask is true, the instance has already been
149 // fully constructed.
150 static const subtle::AtomicWord kLazyInstanceCreatedMask =
151 ~internal::kLazyInstanceStateCreating;
[email protected]359d2bf2010-11-19 20:34:18152
[email protected]30039e62008-09-08 14:11:13153 // We will hopefully have fast access when the instance is already created.
[email protected]6de0fd1d2011-11-15 13:31:49154 // Since a thread sees private_instance_ == 0 or kLazyInstanceStateCreating
155 // at most once, the load is taken out of NeedsInstance() as a fast-path.
[email protected]1b651d52011-05-16 15:01:54156 // The load has acquire memory ordering as a thread which sees
[email protected]6de0fd1d2011-11-15 13:31:49157 // private_instance_ > creating needs to acquire visibility over
158 // the associated data (private_buf_). Pairing Release_Store is in
159 // CompleteLazyInstance().
160 subtle::AtomicWord value = subtle::Acquire_Load(&private_instance_);
161 if (!(value & kLazyInstanceCreatedMask) &&
162 internal::NeedsLazyInstance(&private_instance_)) {
163 // Create the instance in the space provided by |private_buf_|.
[email protected]cd924d62012-02-23 17:52:20164 value = reinterpret_cast<subtle::AtomicWord>(
165 Traits::New(private_buf_.void_data()));
[email protected]6de0fd1d2011-11-15 13:31:49166 internal::CompleteLazyInstance(&private_instance_, value, this,
167 Traits::kRegisterOnExit ? OnExit : NULL);
[email protected]c1aeaac2010-03-12 15:28:48168 }
[email protected]30039e62008-09-08 14:11:13169
[email protected]001b6942009-06-26 11:28:03170 // This annotation helps race detectors recognize correct lock-less
171 // synchronization between different threads calling Pointer().
[email protected]c1aeaac2010-03-12 15:28:48172 // We suggest dynamic race detection tool that "Traits::New" above
[email protected]6de0fd1d2011-11-15 13:31:49173 // and CompleteLazyInstance(...) happens before "return instance()" below.
174 // See the corresponding HAPPENS_BEFORE in CompleteLazyInstance(...).
175 ANNOTATE_HAPPENS_AFTER(&private_instance_);
176 return instance();
[email protected]30039e62008-09-08 14:11:13177 }
178
[email protected]332710b2011-02-22 19:21:59179 bool operator==(Type* p) {
[email protected]6de0fd1d2011-11-15 13:31:49180 switch (subtle::NoBarrier_Load(&private_instance_)) {
181 case 0:
[email protected]332710b2011-02-22 19:21:59182 return p == NULL;
[email protected]6de0fd1d2011-11-15 13:31:49183 case internal::kLazyInstanceStateCreating:
[email protected]cd924d62012-02-23 17:52:20184 return static_cast<void*>(p) == private_buf_.void_data();
[email protected]332710b2011-02-22 19:21:59185 default:
[email protected]6de0fd1d2011-11-15 13:31:49186 return p == instance();
[email protected]332710b2011-02-22 19:21:59187 }
188 }
189
[email protected]6de0fd1d2011-11-15 13:31:49190 // Effectively private: member data is only public to allow the linker to
[email protected]cd924d62012-02-23 17:52:20191 // statically initialize it and to maintain a POD class. DO NOT USE FROM
192 // OUTSIDE THIS CLASS.
[email protected]6de0fd1d2011-11-15 13:31:49193
[email protected]6de0fd1d2011-11-15 13:31:49194 subtle::AtomicWord private_instance_;
[email protected]cd924d62012-02-23 17:52:20195 // Preallocated space for the Type instance.
196 base::AlignedMemory<sizeof(Type), ALIGNOF(Type)> private_buf_;
[email protected]6de0fd1d2011-11-15 13:31:49197
[email protected]30039e62008-09-08 14:11:13198 private:
[email protected]6de0fd1d2011-11-15 13:31:49199 Type* instance() {
200 return reinterpret_cast<Type*>(subtle::NoBarrier_Load(&private_instance_));
201 }
202
[email protected]625332e02010-12-14 07:48:49203 // Adapter function for use with AtExit. This should be called single
[email protected]113eee02011-10-25 19:04:48204 // threaded, so don't synchronize across threads.
[email protected]625332e02010-12-14 07:48:49205 // Calling OnExit while the instance is in use by other threads is a mistake.
206 static void OnExit(void* lazy_instance) {
207 LazyInstance<Type, Traits>* me =
208 reinterpret_cast<LazyInstance<Type, Traits>*>(lazy_instance);
[email protected]6de0fd1d2011-11-15 13:31:49209 Traits::Delete(me->instance());
[email protected]cd924d62012-02-23 17:52:20210 subtle::NoBarrier_Store(&me->private_instance_, 0);
[email protected]625332e02010-12-14 07:48:49211 }
[email protected]30039e62008-09-08 14:11:13212};
213
214} // namespace base
215
216#endif // BASE_LAZY_INSTANCE_H_