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      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 
     54 namespace base {
     55 
     56 template <typename Type>
     57 struct 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.
     80 namespace 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).
     90 template <typename Type>
     91 struct 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.
    107 static 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.
    112 BASE_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|
    116 BASE_EXPORT void CompleteLazyInstance(subtle::AtomicWord* state,
    117                                       subtle::AtomicWord new_instance,
    118                                       void* lazy_instance,
    119                                       void (*dtor)(void*));
    120 
    121 }  // namespace internal
    122 
    123 template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> >
    124 class 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