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