1// Copyright (c) 2012 The Chromium Authors. All rights reserved.
2// 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
17// a global variable using the LAZY_INSTANCE_INITIALIZER initializer.
18//
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:
27//   static LazyInstance<MyClass> my_instance = LAZY_INSTANCE_INITIALIZER;
28//   void SomeMethod() {
29//     my_instance.Get().SomeMethod();  // MyClass::SomeMethod()
30//
31//     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
38#include <new>  // For placement new.
39
40#include "base/atomicops.h"
41#include "base/base_export.h"
42#include "base/debug/leak_annotations.h"
43#include "base/logging.h"
44#include "base/memory/aligned_memory.h"
45#include "base/threading/thread_restrictions.h"
46
47// LazyInstance uses its own struct initializer-list style static
48// initialization, as base's LINKER_INITIALIZED requires a constructor and on
49// some compilers (notably gcc 4.4) this still ends up needing runtime
50// initialization.
51#ifdef __clang__
52  #define LAZY_INSTANCE_INITIALIZER {}
53#else
54  #define LAZY_INSTANCE_INITIALIZER {0, 0}
55#endif
56
57namespace base {
58
59template <typename Type>
60struct DefaultLazyInstanceTraits {
61  static const bool kRegisterOnExit = true;
62#ifndef NDEBUG
63  static const bool kAllowedToAccessOnNonjoinableThread = false;
64#endif
65
66  static Type* New(void* instance) {
67    DCHECK_EQ(reinterpret_cast<uintptr_t>(instance) & (ALIGNOF(Type) - 1), 0u)
68        << ": Bad boy, the buffer passed to placement new is not aligned!\n"
69        "This may break some stuff like SSE-based optimizations assuming the "
70        "<Type> objects are word aligned.";
71    // Use placement new to initialize our instance in our preallocated space.
72    // The parenthesis is very important here to force POD type initialization.
73    return new (instance) Type();
74  }
75  static void Delete(Type* instance) {
76    // Explicitly call the destructor.
77    instance->~Type();
78  }
79};
80
81// We pull out some of the functionality into non-templated functions, so we
82// can implement the more complicated pieces out of line in the .cc file.
83namespace internal {
84
85// Use LazyInstance<T>::Leaky for a less-verbose call-site typedef; e.g.:
86// base::LazyInstance<T>::Leaky my_leaky_lazy_instance;
87// instead of:
88// base::LazyInstance<T, base::internal::LeakyLazyInstanceTraits<T> >
89// my_leaky_lazy_instance;
90// (especially when T is MyLongTypeNameImplClientHolderFactory).
91// Only use this internal::-qualified verbose form to extend this traits class
92// (depending on its implementation details).
93template <typename Type>
94struct LeakyLazyInstanceTraits {
95  static const bool kRegisterOnExit = false;
96#ifndef NDEBUG
97  static const bool kAllowedToAccessOnNonjoinableThread = true;
98#endif
99
100  static Type* New(void* instance) {
101    ANNOTATE_SCOPED_MEMORY_LEAK;
102    return DefaultLazyInstanceTraits<Type>::New(instance);
103  }
104  static void Delete(Type*) {}
105};
106
107// Our AtomicWord doubles as a spinlock, where a value of
108// kBeingCreatedMarker means the spinlock is being held for creation.
109static const subtle::AtomicWord kLazyInstanceStateCreating = 1;
110
111// Check if instance needs to be created. If so return true otherwise
112// if another thread has beat us, wait for instance to be created and
113// return false.
114BASE_EXPORT bool NeedsLazyInstance(subtle::AtomicWord* state);
115
116// After creating an instance, call this to register the dtor to be called
117// at program exit and to update the atomic state to hold the |new_instance|
118BASE_EXPORT void CompleteLazyInstance(subtle::AtomicWord* state,
119                                      subtle::AtomicWord new_instance,
120                                      void* lazy_instance,
121                                      void (*dtor)(void*));
122
123}  // namespace internal
124
125template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> >
126class LazyInstance {
127 public:
128  // Do not define a destructor, as doing so makes LazyInstance a
129  // non-POD-struct. We don't want that because then a static initializer will
130  // be created to register the (empty) destructor with atexit() under MSVC, for
131  // example. We handle destruction of the contained Type class explicitly via
132  // the OnExit member function, where needed.
133  // ~LazyInstance() {}
134
135  // Convenience typedef to avoid having to repeat Type for leaky lazy
136  // instances.
137  typedef LazyInstance<Type, internal::LeakyLazyInstanceTraits<Type> > Leaky;
138
139  Type& Get() {
140    return *Pointer();
141  }
142
143  Type* Pointer() {
144#ifndef NDEBUG
145    // Avoid making TLS lookup on release builds.
146    if (!Traits::kAllowedToAccessOnNonjoinableThread)
147      ThreadRestrictions::AssertSingletonAllowed();
148#endif
149    // If any bit in the created mask is true, the instance has already been
150    // fully constructed.
151    static const subtle::AtomicWord kLazyInstanceCreatedMask =
152        ~internal::kLazyInstanceStateCreating;
153
154    // We will hopefully have fast access when the instance is already created.
155    // Since a thread sees private_instance_ == 0 or kLazyInstanceStateCreating
156    // at most once, the load is taken out of NeedsInstance() as a fast-path.
157    // The load has acquire memory ordering as a thread which sees
158    // private_instance_ > creating needs to acquire visibility over
159    // the associated data (private_buf_). Pairing Release_Store is in
160    // CompleteLazyInstance().
161    subtle::AtomicWord value = subtle::Acquire_Load(&private_instance_);
162    if (!(value & kLazyInstanceCreatedMask) &&
163        internal::NeedsLazyInstance(&private_instance_)) {
164      // Create the instance in the space provided by |private_buf_|.
165      value = reinterpret_cast<subtle::AtomicWord>(
166          Traits::New(private_buf_.void_data()));
167      internal::CompleteLazyInstance(&private_instance_, value, this,
168                                     Traits::kRegisterOnExit ? OnExit : NULL);
169    }
170    return instance();
171  }
172
173  bool operator==(Type* p) {
174    switch (subtle::NoBarrier_Load(&private_instance_)) {
175      case 0:
176        return p == NULL;
177      case internal::kLazyInstanceStateCreating:
178        return static_cast<void*>(p) == private_buf_.void_data();
179      default:
180        return p == instance();
181    }
182  }
183
184  // Effectively private: member data is only public to allow the linker to
185  // statically initialize it and to maintain a POD class. DO NOT USE FROM
186  // OUTSIDE THIS CLASS.
187
188  subtle::AtomicWord private_instance_;
189  // Preallocated space for the Type instance.
190  base::AlignedMemory<sizeof(Type), ALIGNOF(Type)> private_buf_;
191
192 private:
193  Type* instance() {
194    return reinterpret_cast<Type*>(subtle::NoBarrier_Load(&private_instance_));
195  }
196
197  // Adapter function for use with AtExit.  This should be called single
198  // threaded, so don't synchronize across threads.
199  // Calling OnExit while the instance is in use by other threads is a mistake.
200  static void OnExit(void* lazy_instance) {
201    LazyInstance<Type, Traits>* me =
202        reinterpret_cast<LazyInstance<Type, Traits>*>(lazy_instance);
203    Traits::Delete(me->instance());
204    subtle::NoBarrier_Store(&me->private_instance_, 0);
205  }
206};
207
208}  // namespace base
209
210#endif  // BASE_LAZY_INSTANCE_H_
211