michael@0: // Copyright (c) 2012 The Chromium Authors. All rights reserved.
michael@0: // Use of this source code is governed by a BSD-style license that can be
michael@0: // found in the LICENSE file.
michael@0: 
michael@0: // The LazyInstance<Type, Traits> class manages a single instance of Type,
michael@0: // which will be lazily created on the first time it's accessed.  This class is
michael@0: // useful for places you would normally use a function-level static, but you
michael@0: // need to have guaranteed thread-safety.  The Type constructor will only ever
michael@0: // be called once, even if two threads are racing to create the object.  Get()
michael@0: // and Pointer() will always return the same, completely initialized instance.
michael@0: // When the instance is constructed it is registered with AtExitManager.  The
michael@0: // destructor will be called on program exit.
michael@0: //
michael@0: // LazyInstance is completely thread safe, assuming that you create it safely.
michael@0: // The class was designed to be POD initialized, so it shouldn't require a
michael@0: // static constructor.  It really only makes sense to declare a LazyInstance as
michael@0: // a global variable using the LAZY_INSTANCE_INITIALIZER initializer.
michael@0: //
michael@0: // LazyInstance is similar to Singleton, except it does not have the singleton
michael@0: // property.  You can have multiple LazyInstance's of the same type, and each
michael@0: // will manage a unique instance.  It also preallocates the space for Type, as
michael@0: // to avoid allocating the Type instance on the heap.  This may help with the
michael@0: // performance of creating the instance, and reducing heap fragmentation.  This
michael@0: // requires that Type be a complete type so we can determine the size.
michael@0: //
michael@0: // Example usage:
michael@0: //   static LazyInstance<MyClass> my_instance = LAZY_INSTANCE_INITIALIZER;
michael@0: //   void SomeMethod() {
michael@0: //     my_instance.Get().SomeMethod();  // MyClass::SomeMethod()
michael@0: //
michael@0: //     MyClass* ptr = my_instance.Pointer();
michael@0: //     ptr->DoDoDo();  // MyClass::DoDoDo
michael@0: //   }
michael@0: 
michael@0: #ifndef BASE_LAZY_INSTANCE_H_
michael@0: #define BASE_LAZY_INSTANCE_H_
michael@0: 
michael@0: #include <new>  // For placement new.
michael@0: 
michael@0: #include "base/atomicops.h"
michael@0: #include "base/base_export.h"
michael@0: #include "base/basictypes.h"
michael@0: #include "base/debug/leak_annotations.h"
michael@0: #include "base/logging.h"
michael@0: #include "base/memory/aligned_memory.h"
michael@0: #include "base/third_party/dynamic_annotations/dynamic_annotations.h"
michael@0: #include "base/threading/thread_restrictions.h"
michael@0: 
michael@0: // LazyInstance uses its own struct initializer-list style static
michael@0: // initialization, as base's LINKER_INITIALIZED requires a constructor and on
michael@0: // some compilers (notably gcc 4.4) this still ends up needing runtime
michael@0: // initialization.
michael@0: #define LAZY_INSTANCE_INITIALIZER {0}
michael@0: 
michael@0: namespace base {
michael@0: 
michael@0: template <typename Type>
michael@0: struct DefaultLazyInstanceTraits {
michael@0:   static const bool kRegisterOnExit = true;
michael@0:   static const bool kAllowedToAccessOnNonjoinableThread = false;
michael@0: 
michael@0:   static Type* New(void* instance) {
michael@0:     DCHECK_EQ(reinterpret_cast<uintptr_t>(instance) & (ALIGNOF(Type) - 1), 0u)
michael@0:         << ": Bad boy, the buffer passed to placement new is not aligned!\n"
michael@0:         "This may break some stuff like SSE-based optimizations assuming the "
michael@0:         "<Type> objects are word aligned.";
michael@0:     // Use placement new to initialize our instance in our preallocated space.
michael@0:     // The parenthesis is very important here to force POD type initialization.
michael@0:     return new (instance) Type();
michael@0:   }
michael@0:   static void Delete(Type* instance) {
michael@0:     // Explicitly call the destructor.
michael@0:     instance->~Type();
michael@0:   }
michael@0: };
michael@0: 
michael@0: // We pull out some of the functionality into non-templated functions, so we
michael@0: // can implement the more complicated pieces out of line in the .cc file.
michael@0: namespace internal {
michael@0: 
michael@0: // Use LazyInstance<T>::Leaky for a less-verbose call-site typedef; e.g.:
michael@0: // base::LazyInstance<T>::Leaky my_leaky_lazy_instance;
michael@0: // instead of:
michael@0: // base::LazyInstance<T, base::internal::LeakyLazyInstanceTraits<T> >
michael@0: // my_leaky_lazy_instance;
michael@0: // (especially when T is MyLongTypeNameImplClientHolderFactory).
michael@0: // Only use this internal::-qualified verbose form to extend this traits class
michael@0: // (depending on its implementation details).
michael@0: template <typename Type>
michael@0: struct LeakyLazyInstanceTraits {
michael@0:   static const bool kRegisterOnExit = false;
michael@0:   static const bool kAllowedToAccessOnNonjoinableThread = true;
michael@0: 
michael@0:   static Type* New(void* instance) {
michael@0:     ANNOTATE_SCOPED_MEMORY_LEAK;
michael@0:     return DefaultLazyInstanceTraits<Type>::New(instance);
michael@0:   }
michael@0:   static void Delete(Type* instance) {
michael@0:   }
michael@0: };
michael@0: 
michael@0: // Our AtomicWord doubles as a spinlock, where a value of
michael@0: // kBeingCreatedMarker means the spinlock is being held for creation.
michael@0: static const subtle::AtomicWord kLazyInstanceStateCreating = 1;
michael@0: 
michael@0: // Check if instance needs to be created. If so return true otherwise
michael@0: // if another thread has beat us, wait for instance to be created and
michael@0: // return false.
michael@0: BASE_EXPORT bool NeedsLazyInstance(subtle::AtomicWord* state);
michael@0: 
michael@0: // After creating an instance, call this to register the dtor to be called
michael@0: // at program exit and to update the atomic state to hold the |new_instance|
michael@0: BASE_EXPORT void CompleteLazyInstance(subtle::AtomicWord* state,
michael@0:                                       subtle::AtomicWord new_instance,
michael@0:                                       void* lazy_instance,
michael@0:                                       void (*dtor)(void*));
michael@0: 
michael@0: }  // namespace internal
michael@0: 
michael@0: template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> >
michael@0: class LazyInstance {
michael@0:  public:
michael@0:   // Do not define a destructor, as doing so makes LazyInstance a
michael@0:   // non-POD-struct. We don't want that because then a static initializer will
michael@0:   // be created to register the (empty) destructor with atexit() under MSVC, for
michael@0:   // example. We handle destruction of the contained Type class explicitly via
michael@0:   // the OnExit member function, where needed.
michael@0:   // ~LazyInstance() {}
michael@0: 
michael@0:   // Convenience typedef to avoid having to repeat Type for leaky lazy
michael@0:   // instances.
michael@0:   typedef LazyInstance<Type, internal::LeakyLazyInstanceTraits<Type> > Leaky;
michael@0: 
michael@0:   Type& Get() {
michael@0:     return *Pointer();
michael@0:   }
michael@0: 
michael@0:   Type* Pointer() {
michael@0: #ifndef NDEBUG
michael@0:     // Avoid making TLS lookup on release builds.
michael@0:     if (!Traits::kAllowedToAccessOnNonjoinableThread)
michael@0:       ThreadRestrictions::AssertSingletonAllowed();
michael@0: #endif
michael@0:     // If any bit in the created mask is true, the instance has already been
michael@0:     // fully constructed.
michael@0:     static const subtle::AtomicWord kLazyInstanceCreatedMask =
michael@0:         ~internal::kLazyInstanceStateCreating;
michael@0: 
michael@0:     // We will hopefully have fast access when the instance is already created.
michael@0:     // Since a thread sees private_instance_ == 0 or kLazyInstanceStateCreating
michael@0:     // at most once, the load is taken out of NeedsInstance() as a fast-path.
michael@0:     // The load has acquire memory ordering as a thread which sees
michael@0:     // private_instance_ > creating needs to acquire visibility over
michael@0:     // the associated data (private_buf_). Pairing Release_Store is in
michael@0:     // CompleteLazyInstance().
michael@0:     subtle::AtomicWord value = subtle::Acquire_Load(&private_instance_);
michael@0:     if (!(value & kLazyInstanceCreatedMask) &&
michael@0:         internal::NeedsLazyInstance(&private_instance_)) {
michael@0:       // Create the instance in the space provided by |private_buf_|.
michael@0:       value = reinterpret_cast<subtle::AtomicWord>(
michael@0:           Traits::New(private_buf_.void_data()));
michael@0:       internal::CompleteLazyInstance(&private_instance_, value, this,
michael@0:                                      Traits::kRegisterOnExit ? OnExit : NULL);
michael@0:     }
michael@0: 
michael@0:     // This annotation helps race detectors recognize correct lock-less
michael@0:     // synchronization between different threads calling Pointer().
michael@0:     // We suggest dynamic race detection tool that "Traits::New" above
michael@0:     // and CompleteLazyInstance(...) happens before "return instance()" below.
michael@0:     // See the corresponding HAPPENS_BEFORE in CompleteLazyInstance(...).
michael@0:     ANNOTATE_HAPPENS_AFTER(&private_instance_);
michael@0:     return instance();
michael@0:   }
michael@0: 
michael@0:   bool operator==(Type* p) {
michael@0:     switch (subtle::NoBarrier_Load(&private_instance_)) {
michael@0:       case 0:
michael@0:         return p == NULL;
michael@0:       case internal::kLazyInstanceStateCreating:
michael@0:         return static_cast<void*>(p) == private_buf_.void_data();
michael@0:       default:
michael@0:         return p == instance();
michael@0:     }
michael@0:   }
michael@0: 
michael@0:   // Effectively private: member data is only public to allow the linker to
michael@0:   // statically initialize it and to maintain a POD class. DO NOT USE FROM
michael@0:   // OUTSIDE THIS CLASS.
michael@0: 
michael@0:   subtle::AtomicWord private_instance_;
michael@0:   // Preallocated space for the Type instance.
michael@0:   base::AlignedMemory<sizeof(Type), ALIGNOF(Type)> private_buf_;
michael@0: 
michael@0:  private:
michael@0:   Type* instance() {
michael@0:     return reinterpret_cast<Type*>(subtle::NoBarrier_Load(&private_instance_));
michael@0:   }
michael@0: 
michael@0:   // Adapter function for use with AtExit.  This should be called single
michael@0:   // threaded, so don't synchronize across threads.
michael@0:   // Calling OnExit while the instance is in use by other threads is a mistake.
michael@0:   static void OnExit(void* lazy_instance) {
michael@0:     LazyInstance<Type, Traits>* me =
michael@0:         reinterpret_cast<LazyInstance<Type, Traits>*>(lazy_instance);
michael@0:     Traits::Delete(me->instance());
michael@0:     subtle::NoBarrier_Store(&me->private_instance_, 0);
michael@0:   }
michael@0: };
michael@0: 
michael@0: }  // namespace base
michael@0: 
michael@0: #endif  // BASE_LAZY_INSTANCE_H_