base: Suggest forward declare DefaultSingletonTraits in header files.

base/memory/singleton.h

 // 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.
 
 // PLEASE READ: Do you really need a singleton?
 //
 // Singletons make it hard to determine the lifetime of an object, which can
 // lead to buggy code and spurious crashes.
 //
 // Instead of adding another singleton into the mix, try to identify either:
@@ skipping 131 matching lines @@
 // normal process exit). The Trait::Delete function will not be called on
 // abnormal process exit.
 //
 // DifferentiatingType is used as a key to differentiate two different
 // singletons having the same memory allocation functions but serving a
 // different purpose. This is mainly used for Locks serving different purposes.
 //
 // Example usage:
 //
 // In your header:
-//   #include "base/memory/singleton.h"
+//   template <typename T> struct DefaultSingletonTraits;
 //   class FooClass {
 //    public:
 //     static FooClass* GetInstance();  <-- See comment below on this.
 //     void Bar() { ... }
 //    private:
 //     FooClass() { ... }
 //     friend struct DefaultSingletonTraits<FooClass>;
 //
 //     DISALLOW_COPY_AND_ASSIGN(FooClass);
 //   };
 //
 // In your source file:
+//  #include "base/memory/singleton.h"
 //  FooClass* FooClass::GetInstance() {
 //    return Singleton<FooClass>::get();
 //  }
 //
 // And to call methods on FooClass:
 //   FooClass::GetInstance()->Bar();
 //
 // NOTE: The method accessing Singleton<T>::get() has to be named as GetInstance
 // and it is important that FooClass::GetInstance() is not inlined in the
 // header. This makes sure that when source files from multiple targets include
@@ skipping 100 matching lines @@
     instance_ = 0;
   }
   static base::subtle::AtomicWord instance_;
 };
 
 template <typename Type, typename Traits, typename DifferentiatingType>
 base::subtle::AtomicWord Singleton<Type, Traits, DifferentiatingType>::
     instance_ = 0;
 
 #endif  // BASE_MEMORY_SINGLETON_H_