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/logging.h"
  44 #include "base/memory/aligned_memory.h"
  45 #include "base/third_party/dynamic_annotations/dynamic_annotations.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
  54 namespace base {
  55
  56 template <typename Type>
  57 struct DefaultLazyInstanceTraits {
  58   static const bool kRegisterOnExit = true;
  59   static const bool kAllowedToAccessOnNonjoinableThread = false;
  60
  61   static Type* New(void* instance) {
  62     DCHECK_EQ(reinterpret_cast<uintptr_t>(instance) & (ALIGNOF(Type) - 1), 0u)
  63         << ": Bad boy, the buffer passed to placement new is not aligned!\n"
  64         "This may break some stuff like SSE-based optimizations assuming the "
  65         "<Type> objects are word aligned.";
  66     // Use placement new to initialize our instance in our preallocated space.
  67     // The parenthesis is very important here to force POD type initialization.
  68     return new (instance) Type();
  69   }
  70   static void Delete(Type* instance) {
  71     // Explicitly call the destructor.
  72     instance->~Type();
  73   }
  74 };
  75
  76 // We pull out some of the functionality into non-templated functions, so we
  77 // can implement the more complicated pieces out of line in the .cc file.
  78 namespace internal {
  79
  80 // Use LazyInstance<T>::Leaky for a less-verbose call-site typedef; e.g.:
  81 // base::LazyInstance<T>::Leaky my_leaky_lazy_instance;
  82 // instead of:
  83 // base::LazyInstance<T, base::internal::LeakyLazyInstanceTraits<T> >
  84 // my_leaky_lazy_instance;
  85 // (especially when T is MyLongTypeNameImplClientHolderFactory).
  86 // Only use this internal::-qualified verbose form to extend this traits class
  87 // (depending on its implementation details).
  88 template <typename Type>
  89 struct LeakyLazyInstanceTraits {
  90   static const bool kRegisterOnExit = false;
  91   static const bool kAllowedToAccessOnNonjoinableThread = true;
  92
  93   static Type* New(void* instance) {
  94     return DefaultLazyInstanceTraits<Type>::New(instance);
  95   }
  96   static void Delete(Type* instance) {
  97   }
  98 };
  99
 100 // Our AtomicWord doubles as a spinlock, where a value of
 101 // kBeingCreatedMarker means the spinlock is being held for creation.
 102 static const subtle::AtomicWord kLazyInstanceStateCreating = 1;
 103
 104 // Check if instance needs to be created. If so return true otherwise
 105 // if another thread has beat us, wait for instance to be created and
 106 // return false.
 107 BASE_EXPORT bool NeedsLazyInstance(subtle::AtomicWord* state);
 108
 109 // After creating an instance, call this to register the dtor to be called
 110 // at program exit and to update the atomic state to hold the |new_instance|
 111 BASE_EXPORT void CompleteLazyInstance(subtle::AtomicWord* state,
 112                                       subtle::AtomicWord new_instance,
 113                                       void* lazy_instance,
 114                                       void (*dtor)(void*));
 115
 116 }  // namespace internal
 117
 118 template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> >
 119 class LazyInstance {
 120  public:
 121   // Do not define a destructor, as doing so makes LazyInstance a
 122   // non-POD-struct. We don't want that because then a static initializer will
 123   // be created to register the (empty) destructor with atexit() under MSVC, for
 124   // example. We handle destruction of the contained Type class explicitly via
 125   // the OnExit member function, where needed.
 126   // ~LazyInstance() {}
 127
 128   // Convenience typedef to avoid having to repeat Type for leaky lazy
 129   // instances.
 130   typedef LazyInstance<Type, internal::LeakyLazyInstanceTraits<Type> > Leaky;
 131
 132   Type& Get() {
 133     return *Pointer();
 134   }
 135
 136   Type* Pointer() {
 137 #ifndef NDEBUG
 138     // Avoid making TLS lookup on release builds.
 139     if (!Traits::kAllowedToAccessOnNonjoinableThread)
 140       ThreadRestrictions::AssertSingletonAllowed();
 141 #endif
 142     // If any bit in the created mask is true, the instance has already been
 143     // fully constructed.
 144     static const subtle::AtomicWord kLazyInstanceCreatedMask =
 145         ~internal::kLazyInstanceStateCreating;
 146
 147     // We will hopefully have fast access when the instance is already created.
 148     // Since a thread sees private_instance_ == 0 or kLazyInstanceStateCreating
 149     // at most once, the load is taken out of NeedsInstance() as a fast-path.
 150     // The load has acquire memory ordering as a thread which sees
 151     // private_instance_ > creating needs to acquire visibility over
 152     // the associated data (private_buf_). Pairing Release_Store is in
 153     // CompleteLazyInstance().
 154     subtle::AtomicWord value = subtle::Acquire_Load(&private_instance_);
 155     if (!(value & kLazyInstanceCreatedMask) &&
 156         internal::NeedsLazyInstance(&private_instance_)) {
 157       // Create the instance in the space provided by |private_buf_|.
 158       value = reinterpret_cast<subtle::AtomicWord>(
 159           Traits::New(private_buf_.void_data()));
 160       internal::CompleteLazyInstance(&private_instance_, value, this,
 161                                      Traits::kRegisterOnExit ? OnExit : NULL);
 162     }
 163
 164     // This annotation helps race detectors recognize correct lock-less
 165     // synchronization between different threads calling Pointer().
 166     // We suggest dynamic race detection tool that "Traits::New" above
 167     // and CompleteLazyInstance(...) happens before "return instance()" below.
 168     // See the corresponding HAPPENS_BEFORE in CompleteLazyInstance(...).
 169     ANNOTATE_HAPPENS_AFTER(&private_instance_);
 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_