base/lazy_instance.h

   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/third_party/dynamic_annotations/dynamic_annotations.h"
  47 #include "base/threading/thread_restrictions.h"
  48
  49 // 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
  55 namespace base {
  56
  57 template <typename Type>
  58 struct DefaultLazyInstanceTraits {
  59   static const bool kRegisterOnExit = true;
  60 #ifndef NDEBUG
  61   static const bool kAllowedToAccessOnNonjoinableThread = false;
  62 #endif
  63
  64   static Type* New(void* instance) {
  65     DCHECK_EQ(reinterpret_cast<uintptr_t>(instance) & (ALIGNOF(Type) - 1), 0u)
  66         << ": 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.";
  69     // Use placement new to initialize our instance in our preallocated space.
  70     // The parenthesis is very important here to force POD type initialization.
  71     return new (instance) Type();
  72   }
  73   static void Delete(Type* instance) {
  74     // Explicitly call the destructor.
  75     instance->~Type();
  76   }
  77 };
  78
  79 // 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.
  81 namespace internal {
  82
  83 // 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:
  86 // base::LazyInstance<T, base::internal::LeakyLazyInstanceTraits<T> >
  87 // my_leaky_lazy_instance;
  88 // (especially when T is MyLongTypeNameImplClientHolderFactory).
  89 // Only use this internal::-qualified verbose form to extend this traits class
  90 // (depending on its implementation details).
  91 template <typename Type>
  92 struct LeakyLazyInstanceTraits {
  93   static const bool kRegisterOnExit = false;
  94 #ifndef NDEBUG
  95   static const bool kAllowedToAccessOnNonjoinableThread = true;
  96 #endif
  97
  98   static Type* New(void* instance) {
  99     ANNOTATE_SCOPED_MEMORY_LEAK;
 100     return DefaultLazyInstanceTraits<Type>::New(instance);
 101   }
 102   static void Delete(Type* instance) {
 103   }
 104 };
 105
 106 // Our AtomicWord doubles as a spinlock, where a value of
 107 // kBeingCreatedMarker means the spinlock is being held for creation.
 108 static const subtle::AtomicWord kLazyInstanceStateCreating = 1;
 109
 110 // 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.
 113 BASE_EXPORT bool NeedsLazyInstance(subtle::AtomicWord* state);
 114
 115 // 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|
 117 BASE_EXPORT void CompleteLazyInstance(subtle::AtomicWord* state,
 118                                       subtle::AtomicWord new_instance,
 119                                       void* lazy_instance,
 120                                       void (*dtor)(void*));
 121
 122 }  // namespace internal
 123
 124 template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> >
 125 class LazyInstance {
 126  public:
 127   // 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.
 132   // ~LazyInstance() {}
 133
 134   // Convenience typedef to avoid having to repeat Type for leaky lazy
 135   // instances.
 136   typedef LazyInstance<Type, internal::LeakyLazyInstanceTraits<Type> > Leaky;
 137
 138   Type& Get() {
 139     return *Pointer();
 140   }
 141
 142   Type* Pointer() {
 143 #ifndef NDEBUG
 144     // Avoid making TLS lookup on release builds.
 145     if (!Traits::kAllowedToAccessOnNonjoinableThread)
 146       ThreadRestrictions::AssertSingletonAllowed();
 147 #endif
 148     // 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;
 152
 153     // We will hopefully have fast access when the instance is already created.
 154     // Since a thread sees private_instance_ == 0 or kLazyInstanceStateCreating
 155     // at most once, the load is taken out of NeedsInstance() as a fast-path.
 156     // The load has acquire memory ordering as a thread which sees
 157     // 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_|.
 164       value = reinterpret_cast<subtle::AtomicWord>(
 165           Traits::New(private_buf_.void_data()));
 166       internal::CompleteLazyInstance(&private_instance_, value, this,
 167                                      Traits::kRegisterOnExit ? OnExit : NULL);
 168     }
 169
 170     // This annotation helps race detectors recognize correct lock-less
 171     // synchronization between different threads calling Pointer().
 172     // We suggest dynamic race detection tool that "Traits::New" above
 173     // and CompleteLazyInstance(...) happens before "return instance()" below.
 174     // See the corresponding HAPPENS_BEFORE in CompleteLazyInstance(...).
 175     ANNOTATE_HAPPENS_AFTER(&private_instance_);
 176     return instance();
 177   }
 178
 179   bool operator==(Type* p) {
 180     switch (subtle::NoBarrier_Load(&private_instance_)) {
 181       case 0:
 182         return p == NULL;
 183       case internal::kLazyInstanceStateCreating:
 184         return static_cast<void*>(p) == private_buf_.void_data();
 185       default:
 186         return p == instance();
 187     }
 188   }
 189
 190   // Effectively private: member data is only public to allow the linker to
 191   // statically initialize it and to maintain a POD class. DO NOT USE FROM
 192   // OUTSIDE THIS CLASS.
 193
 194   subtle::AtomicWord private_instance_;
 195   // Preallocated space for the Type instance.
 196   base::AlignedMemory<sizeof(Type), ALIGNOF(Type)> private_buf_;
 197
 198  private:
 199   Type* instance() {
 200     return reinterpret_cast<Type*>(subtle::NoBarrier_Load(&private_instance_));
 201   }
 202
 203   // Adapter function for use with AtExit.  This should be called single
 204   // threaded, so don't synchronize across threads.
 205   // 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);
 209     Traits::Delete(me->instance());
 210     subtle::NoBarrier_Store(&me->private_instance_, 0);
 211   }
 212 };
 213
 214 }  // namespace base
 215
 216 #endif  // BASE_LAZY_INSTANCE_H_