Create a LazyInstance abstraction.

base/lazy_instance.h

+// Copyright (c) 2008 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// The LazyInstance<Type, Traits> class manages a single instance of Type,
+// which will be lazily created on the first time it's accessed.  This class is
+// useful for places you would normally use a function-level static, but you
+// need to have guaranteed thread-safety.  The Type constructor will only ever
+// be called once, even if two threads are racing to create the object.  Get()
+// and Pointer() will always return the same, completely initialized instance.
+// When the instance is constructed it is registered with AtExitManager.  The
+// destructor will be called on program exit.
+//
+// LazyInstance is completely thread safe, assuming that you create it safely.
+// The class was designed to be POD initialized, so it shouldn't require a
+// static constructor.  It really only makes sense to declare a LazyInstance as
+// a global variable using the base::LinkerInitialized constructor.
+//
+// LazyInstance is similar to Singleton, except it does not have the singleton
+// property.  You can have multiple LazyInstance's of the same type, and each
+// will manage a unique instance.  It also preallocates the space for Type, as
+// to avoid allocating the Type instance on the heap.  This may help with the
+// performance of creating the instance, and reducing heap fragmentation.  This
+// requires that Type be a complete type so we can determine the size.
+//
+// Example usage:
+//   static LazyInstance<MyClass> my_instance(base::LINKER_INITALIZED);
+//   void SomeMethod() {
+//     my_instance.Get().SomeMethod();  // MyClass::SomeMethod()
+// 
+//     MyClass* ptr = my_instance.Pointer();
+//     ptr->DoDoDo();  // MyClass::DoDoDo
+//   }
+
+#ifndef BASE_LAZY_INSTANCE_H_
+#define BASE_LAZY_INSTANCE_H_
+
+#include "base/atomicops.h"
+#include "base/basictypes.h"
+
+namespace base {
+
+template <typename Type>
+struct DefaultLazyInstanceTraits {
+  static void New(void* instance) {
+    // Use placement new to initialize our instance in our preallocated space.
+    // The parenthesis is very important here to force POD type initialization.
+    new (instance) Type();
+  }
+  static void Delete(void* instance) {
+    // Explicitly call the destructor.
+    reinterpret_cast<Type*>(instance)->~Type();
+  }
+};
+
+// We pull out some of the functionality into a non-templated base, so that we
+// can implement the more complicated pieces out of line in the .cc file.
+class LazyInstanceHelper {
+ protected:
+  enum {
+    STATE_EMPTY    = 0,
+    STATE_CREATING = 1,
+    STATE_CREATED  = 2
+  };
+
+  explicit LazyInstanceHelper(LinkerInitialized x) { /* state_ is 0 */ }
+  ~LazyInstanceHelper() { }
+
+  // Make sure that instance is created, creating or waiting for it to be
+  // created if neccessary.  Constructs with |ctor| in the space provided by
+  // |instance| and registers dtor for destruction at program exit.
+  void EnsureInstance(void* instance, void (*ctor)(void*), void (*dtor)(void*));
+
+  base::subtle::Atomic32 state_;
+};
+
+template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> >
+class LazyInstance : public LazyInstanceHelper {
+ public:
+  explicit LazyInstance(LinkerInitialized x) : LazyInstanceHelper(x) { }
+  ~LazyInstance() { }
+
+  Type& Get() {
+    return *Pointer();
+  }
+
+  Type* Pointer() {
+    Type* instance = reinterpret_cast<Type*>(&buf_);
+
+    // We will hopefully have fast access when the instance is already created.
+    if (base::subtle::NoBarrier_Load(&state_) != STATE_CREATED)
+      EnsureInstance(instance, Traits::New, Traits::Delete);
+
+    return instance;
+  }
+
+ private:
+  int8 buf_[sizeof(Type)];  // Preallocate the space for the Type instance.

[M-A Ruel, 2008/09/08 13:56:35]

I'm just sad about objects requiring 8 bytes alignment.

+
+  DISALLOW_COPY_AND_ASSIGN(LazyInstance);
+};
+
+}  // namespace base
+
+#endif  // BASE_LAZY_INSTANCE_H_