Index: chrome/browser/sync/util/immutable.h |
diff --git a/chrome/browser/sync/util/immutable.h b/chrome/browser/sync/util/immutable.h |
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-// Copyright (c) 2011 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. |
- |
-// Immutable<T> provides an easy, cheap, and thread-safe way to pass |
-// large immutable data around. |
-// |
-// For example, consider the following code: |
-// |
-// typedef std::vector<LargeObject> LargeObjectList; |
-// |
-// void ProcessStuff(const LargeObjectList& stuff) { |
-// for (LargeObjectList::const_iterator it = stuff.begin(); |
-// it != stuff.end(); ++it) { |
-// ... process it ... |
-// } |
-// } |
-// |
-// ... |
-// |
-// LargeObjectList my_stuff; |
-// ... fill my_stuff with lots of LargeObjects ... |
-// some_loop->PostTask(FROM_HERE, base::Bind(&ProcessStuff, my_stuff)); |
-// |
-// The last line incurs the cost of copying my_stuff, which is |
-// undesirable. Here's the above code re-written using Immutable<T>: |
-// |
-// void ProcessStuff( |
-// const browser_sync::Immutable<LargeObjectList>& stuff) { |
-// for (LargeObjectList::const_iterator it = stuff.Get().begin(); |
-// it != stuff.Get().end(); ++it) { |
-// ... process it ... |
-// } |
-// } |
-// |
-// ... |
-// |
-// LargeObjectList my_stuff; |
-// ... fill my_stuff with lots of LargeObjects ... |
-// some_loop->PostTask( |
-// FROM_HERE, base::Bind(&ProcessStuff, MakeImmutable(&my_stuff))); |
-// |
-// The last line, which resets my_stuff to a default-initialized |
-// state, incurs only the cost of a swap of LargeObjectLists, which is |
-// O(1) for most STL container implementations. The data in my_stuff |
-// is ref-counted (thread-safely), so it is freed as soon as |
-// ProcessStuff is finished. |
-// |
-// NOTE: By default, Immutable<T> relies on ADL |
-// (http://en.wikipedia.org/wiki/Argument-dependent_name_lookup) to |
-// find a swap() function for T, falling back to std::swap() when |
-// necessary. If you overload swap() for your type in its namespace, |
-// or if you specialize std::swap() for your type, (see |
-// http://stackoverflow.com/questions/11562/how-to-overload-stdswap |
-// for discussion) Immutable<T> should be able to find it. |
-// |
-// Alternatively, you could explicitly control which swap function is |
-// used by providing your own traits class or using one of the |
-// pre-defined ones below. See comments on traits below for details. |
-// |
-// NOTE: Some complexity is necessary in order to use Immutable<T> |
-// with forward-declared types. See comments on traits below for |
-// details. |
- |
-#ifndef CHROME_BROWSER_SYNC_UTIL_IMMUTABLE_H_ |
-#define CHROME_BROWSER_SYNC_UTIL_IMMUTABLE_H_ |
-#pragma once |
- |
-// For std::swap(). |
-#include <algorithm> |
- |
-#include "base/basictypes.h" |
-#include "base/memory/ref_counted.h" |
- |
-namespace browser_sync { |
- |
-namespace internal { |
-// This class is part of the Immutable implementation. DO NOT USE |
-// THIS CLASS DIRECTLY YOURSELF. |
- |
-template <typename T, typename Traits> |
-class ImmutableCore |
- : public base::RefCountedThreadSafe<ImmutableCore<T, Traits> > { |
- public: |
- // wrapper_ is always explicitly default-initialized to handle |
- // primitive types and the case where Traits::Wrapper == T. |
- |
- ImmutableCore() : wrapper_() { |
- Traits::InitializeWrapper(&wrapper_); |
- } |
- |
- explicit ImmutableCore(T* t) : wrapper_() { |
- Traits::InitializeWrapper(&wrapper_); |
- Traits::Swap(Traits::UnwrapMutable(&wrapper_), t); |
- } |
- |
- const T& Get() const { |
- return Traits::Unwrap(wrapper_); |
- } |
- |
- private: |
- ~ImmutableCore() { |
- Traits::DestroyWrapper(&wrapper_); |
- } |
- friend class base::RefCountedThreadSafe<ImmutableCore<T, Traits> >; |
- |
- // This is semantically const, but we can't mark it a such as we |
- // modify it in the constructor. |
- typename Traits::Wrapper wrapper_; |
- |
- DISALLOW_COPY_AND_ASSIGN(ImmutableCore); |
-}; |
- |
-} // namespace internal |
- |
-// Traits usage notes |
-// ------------------ |
-// The most common reason to use your own traits class is to provide |
-// your own swap method. First, consider the pre-defined traits |
-// classes HasSwapMemFn{ByRef,ByPtr} below. If neither of those work, |
-// then define your own traits class inheriting from |
-// DefaultImmutableTraits<YourType> (to pick up the defaults for |
-// everything else) and provide your own Swap() method. |
-// |
-// Another reason to use your own traits class is to be able to use |
-// Immutable<T> with a forward-declared type (important for protobuf |
-// classes, when you want to avoid headers pulling in generated |
-// headers). (This is why the Traits::Wrapper type exists; normally, |
-// Traits::Wrapper is just T itself, but that needs to be changed for |
-// forward-declared types.) |
-// |
-// For example, if you want to do this: |
-// |
-// my_class.h |
-// ---------- |
-// #include ".../immutable.h" |
-// |
-// // Forward declaration. |
-// class SomeOtherType; |
-// |
-// class MyClass { |
-// ... |
-// private: |
-// // Doesn't work, as defaults traits class needs SomeOtherType's |
-// // definition to be visible. |
-// Immutable<SomeOtherType> foo_; |
-// }; |
-// |
-// You'll have to do this: |
-// |
-// my_class.h |
-// ---------- |
-// #include ".../immutable.h" |
-// |
-// // Forward declaration. |
-// class SomeOtherType; |
-// |
-// class MyClass { |
-// ... |
-// private: |
-// struct ImmutableSomeOtherTypeTraits { |
-// // scoped_ptr<SomeOtherType> won't work here, either. |
-// typedef SomeOtherType* Wrapper; |
-// |
-// static void InitializeWrapper(Wrapper* wrapper); |
-// |
-// static void DestroyWrapper(Wrapper* wrapper); |
-// ... |
-// }; |
-// |
-// typedef Immutable<SomeOtherType, ImmutableSomeOtherTypeTraits> |
-// ImmutableSomeOtherType; |
-// |
-// ImmutableSomeOtherType foo_; |
-// }; |
-// |
-// my_class.cc |
-// ----------- |
-// #include ".../some_other_type.h" |
-// |
-// void MyClass::ImmutableSomeOtherTypeTraits::InitializeWrapper( |
-// Wrapper* wrapper) { |
-// *wrapper = new SomeOtherType(); |
-// } |
-// |
-// void MyClass::ImmutableSomeOtherTypeTraits::DestroyWrapper( |
-// Wrapper* wrapper) { |
-// delete *wrapper; |
-// } |
-// |
-// ... |
-// |
-// Also note that this incurs an additional memory allocation when you |
-// create an Immutable<SomeOtherType>. |
- |
-template <typename T> |
-struct DefaultImmutableTraits { |
- typedef T Wrapper; |
- |
- static void InitializeWrapper(Wrapper* wrapper) {} |
- |
- static void DestroyWrapper(Wrapper* wrapper) {} |
- |
- static const T& Unwrap(const Wrapper& wrapper) { return wrapper; } |
- |
- static T* UnwrapMutable(Wrapper* wrapper) { return wrapper; } |
- |
- static void Swap(T* t1, T* t2) { |
- // Uses ADL (see |
- // http://en.wikipedia.org/wiki/Argument-dependent_name_lookup). |
- using std::swap; |
- swap(*t1, *t2); |
- } |
-}; |
- |
-// Most STL containers have by-reference swap() member functions, |
-// although they usually already overload std::swap() to use those. |
-template <typename T> |
-struct HasSwapMemFnByRef : public DefaultImmutableTraits<T> { |
- static void Swap(T* t1, T* t2) { |
- t1->swap(*t2); |
- } |
-}; |
- |
-// Most Google-style objects have by-pointer Swap() member functions |
-// (for example, generated protocol buffer classes). |
-template <typename T> |
-struct HasSwapMemFnByPtr : public DefaultImmutableTraits<T> { |
- static void Swap(T* t1, T* t2) { |
- t1->Swap(t2); |
- } |
-}; |
- |
-template <typename T, typename Traits = DefaultImmutableTraits<T> > |
-class Immutable { |
- public: |
- // Puts the underlying object in a default-initialized state. |
- Immutable() : core_(new internal::ImmutableCore<T, Traits>()) {} |
- |
- // Copy constructor and assignment welcome. |
- |
- // Resets |t| to a default-initialized state. |
- explicit Immutable(T* t) |
- : core_(new internal::ImmutableCore<T, Traits>(t)) {} |
- |
- const T& Get() const { |
- return core_->Get(); |
- } |
- |
- private: |
- scoped_refptr<const internal::ImmutableCore<T, Traits> > core_; |
-}; |
- |
-// Helper function to avoid having to write out template arguments. |
-template <typename T> |
-Immutable<T> MakeImmutable(T* t) { |
- return Immutable<T>(t); |
-} |
- |
-} // namespace browser_sync |
- |
-#endif // CHROME_BROWSER_SYNC_UTIL_IMMUTABLE_H_ |