Require explicit selection of traits for LazyInstance

base/lazy_instance.h

 // Copyright (c) 2012 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 LAZY_INSTANCE_INITIALIZER initializer.
 //
 // 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 = LAZY_INSTANCE_INITIALIZER;
+//   static LazyInstance<MyClass>::Leaky inst = LAZY_INSTANCE_INITIALIZER;
 //   void SomeMethod() {
-//     my_instance.Get().SomeMethod();  // MyClass::SomeMethod()
+//     inst.Get().SomeMethod();  // MyClass::SomeMethod()
 //
-//     MyClass* ptr = my_instance.Pointer();
+//     MyClass* ptr = inst.Pointer();
 //     ptr->DoDoDo();  // MyClass::DoDoDo
 //   }
 
 #ifndef BASE_LAZY_INSTANCE_H_
 #define BASE_LAZY_INSTANCE_H_
 
 #include <new>  // For placement new.
 
 #include "base/atomicops.h"
 #include "base/base_export.h"
 #include "base/debug/leak_annotations.h"
 #include "base/logging.h"
 #include "base/memory/aligned_memory.h"
 #include "base/threading/thread_restrictions.h"
 
 // LazyInstance uses its own struct initializer-list style static
 // initialization, as base's LINKER_INITIALIZED requires a constructor and on
 // some compilers (notably gcc 4.4) this still ends up needing runtime
 // initialization.
 #define LAZY_INSTANCE_INITIALIZER {0}
 
 namespace base {
 
 template <typename Type>
-struct DefaultLazyInstanceTraits {
-  static const bool kRegisterOnExit = true;
-#if DCHECK_IS_ON()
-  static const bool kAllowedToAccessOnNonjoinableThread = false;
-#endif
-
+struct LazyInstanceTraitsBase {
   static Type* New(void* instance) {
-    DCHECK_EQ(reinterpret_cast<uintptr_t>(instance) & (ALIGNOF(Type) - 1), 0u)
-        << ": Bad boy, the buffer passed to placement new is not aligned!\n"
-        "This may break some stuff like SSE-based optimizations assuming the "
-        "<Type> objects are word aligned.";
+    DCHECK_EQ(reinterpret_cast<uintptr_t>(instance) & (ALIGNOF(Type) - 1), 0u);
     // Use placement new to initialize our instance in our preallocated space.
     // The parenthesis is very important here to force POD type initialization.
     return new (instance) Type();
   }
-  static void Delete(Type* instance) {
+
+  static void CallDestructor(Type* instance) {
     // Explicitly call the destructor.
     instance->~Type();
   }
 };
 
 // We pull out some of the functionality into non-templated functions, so we
 // can implement the more complicated pieces out of line in the .cc file.
 namespace internal {
 
+template <typename Type>

[Nico, 2017/03/07 21:43:02]

add comment above this class that it generally shouldn't be used for various
reasons

[scottmg, 2017/03/07 21:56:30]

Done.

+struct DestructorAtExitLazyInstanceTraits {
+  static const bool kRegisterOnExit = true;
+#if DCHECK_IS_ON()
+  static const bool kAllowedToAccessOnNonjoinableThread = false;
+#endif
+
+  static Type* New(void* instance) {
+    return LazyInstanceTraitsBase<Type>::New(instance);
+  }
+
+  static void Delete(Type* instance) {
+    LazyInstanceTraitsBase<Type>::CallDestructor(instance);
+  }
+};
+
 // Use LazyInstance<T>::Leaky for a less-verbose call-site typedef; e.g.:
 // base::LazyInstance<T>::Leaky my_leaky_lazy_instance;
 // instead of:
 // base::LazyInstance<T, base::internal::LeakyLazyInstanceTraits<T> >
 // my_leaky_lazy_instance;
 // (especially when T is MyLongTypeNameImplClientHolderFactory).
 // Only use this internal::-qualified verbose form to extend this traits class
 // (depending on its implementation details).
 template <typename Type>
 struct LeakyLazyInstanceTraits {
   static const bool kRegisterOnExit = false;
 #if DCHECK_IS_ON()
   static const bool kAllowedToAccessOnNonjoinableThread = true;
 #endif
 
   static Type* New(void* instance) {
     ANNOTATE_SCOPED_MEMORY_LEAK;
-    return DefaultLazyInstanceTraits<Type>::New(instance);
+    return LazyInstanceTraitsBase<Type>::New(instance);
   }
   static void Delete(Type* instance) {
   }
 };
 
+template <typename Type>
+struct ErrorMustSelectLazyOrDestructorAtExitForLazyInstance {};
+
 // Our AtomicWord doubles as a spinlock, where a value of
 // kLazyInstanceStateCreating means the spinlock is being held for creation.
 static const subtle::AtomicWord kLazyInstanceStateCreating = 1;
 
 // Check if instance needs to be created. If so return true otherwise
 // if another thread has beat us, wait for instance to be created and
 // return false.
 BASE_EXPORT bool NeedsLazyInstance(subtle::AtomicWord* state);
 
 // After creating an instance, call this to register the dtor to be called
 // at program exit and to update the atomic state to hold the |new_instance|
 BASE_EXPORT void CompleteLazyInstance(subtle::AtomicWord* state,
                                       subtle::AtomicWord new_instance,
                                       void* lazy_instance,
                                       void (*dtor)(void*));
 
 }  // namespace internal
 
-template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> >
+template <
+    typename Type,
+    typename Traits =
+        internal::ErrorMustSelectLazyOrDestructorAtExitForLazyInstance<Type>>

[Nico, 2017/03/07 21:43:02]

could this just not set a default for Traits?

[scottmg, 2017/03/07 21:56:30]

That's what I tried at first, but then slightly odd typedef for
LazyInstance<T>::Leaky can't work.

I also tried putting those into LazyInstance, but everything else in
LazyInstanceImpl, but that didn't quite work either so I ended up here with a
slightly ugly error message.

 class LazyInstance {
  public:
   // Do not define a destructor, as doing so makes LazyInstance a
   // non-POD-struct. We don't want that because then a static initializer will
   // be created to register the (empty) destructor with atexit() under MSVC, for
   // example. We handle destruction of the contained Type class explicitly via
   // the OnExit member function, where needed.
   // ~LazyInstance() {}
 
   // Convenience typedef to avoid having to repeat Type for leaky lazy
   // instances.
-  typedef LazyInstance<Type, internal::LeakyLazyInstanceTraits<Type> > Leaky;
+  typedef LazyInstance<Type, internal::LeakyLazyInstanceTraits<Type>> Leaky;
+  typedef LazyInstance<Type, internal::DestructorAtExitLazyInstanceTraits<Type>>
+      DestructorAtExit;
 
   Type& Get() {
     return *Pointer();
   }
 
   Type* Pointer() {
 #if DCHECK_IS_ON()
     // Avoid making TLS lookup on release builds.
     if (!Traits::kAllowedToAccessOnNonjoinableThread)
       ThreadRestrictions::AssertSingletonAllowed();
@@ skipping 53 matching lines @@
     LazyInstance<Type, Traits>* me =
         reinterpret_cast<LazyInstance<Type, Traits>*>(lazy_instance);
     Traits::Delete(me->instance());
     subtle::NoBarrier_Store(&me->private_instance_, 0);
   }
 };
 
 }  // namespace base
 
 #endif  // BASE_LAZY_INSTANCE_H_