The Tor Browser: comparison ipc/chromium/src/base/lazy

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+// 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_INITIALIZED);
+//   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 */ }
+// Declaring a destructor (even if it's empty) will cause MSVC to register a
+// static initializer to register the empty destructor with atexit().
+// 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_;
+private:
+DISALLOW_COPY_AND_ASSIGN(LazyInstanceHelper);
+};
+template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> >
+class LazyInstance : public LazyInstanceHelper {
+public:
+explicit LazyInstance(LinkerInitialized x) : LazyInstanceHelper(x) { }
+// Declaring a destructor (even if it's empty) will cause MSVC to register a
+// static initializer to register the empty destructor with atexit().
+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_t buf_[sizeof(Type)];  // Preallocate the space for the Type instance.
+DISALLOW_COPY_AND_ASSIGN(LazyInstance);
+};
+}  // namespace base
+#endif  // BASE_LAZY_INSTANCE_H_

The Tor Browser / file comparison

comparison: ipc/chromium/src/base/lazy_instance.h

ipc/chromium/src/base/lazy_instance.h