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Unified Diff: base/lazy_instance.h

Issue 1608: Create a LazyInstance abstraction. (Closed) Base URL: svn://chrome-svn/chrome/trunk/src/
Patch Set: '' Created 12 years, 3 months ago
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Index: base/lazy_instance.h
===================================================================
--- base/lazy_instance.h (revision 0)
+++ base/lazy_instance.h (revision 0)
@@ -0,0 +1,105 @@
+// 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 align
+
+ DISALLOW_COPY_AND_ASSIGN(LazyInstance);
+};
+
+} // namespace base
+
+#endif // BASE_LAZY_INSTANCE_H_
Property changes on: base\lazy_instance.h
___________________________________________________________________
Added: svn:eol-style
+ LF
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