Replace safe-bool idiom with explicit WeakPtr::operator bool()

base/memory/weak_ptr.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.
 
 // Weak pointers are pointers to an object that do not affect its lifetime,
 // and which may be invalidated (i.e. reset to nullptr) by the object, or its
 // owner, at any time, most commonly when the object is about to be deleted.
 
 // Weak pointers are useful when an object needs to be accessed safely by one
 // or more objects other than its owner, and those callers can cope with the
@@ skipping 208 matching lines @@
   T* operator->() const {
     DCHECK(get() != nullptr);
     return get();
   }
 
   void reset() {
     ref_ = internal::WeakReference();
     ptr_ = nullptr;
   }
 
-  // Implement "Safe Bool Idiom"
-  // https://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Safe_bool
-  //
-  // Allow WeakPtr<element_type> to be used in boolean expressions such as
-  //   if (weak_ptr_instance)
-  // But do not become convertible to a real bool (which is dangerous).
-  //   Implementation requires:
-  //     typedef Testable
-  //     operator Testable() const
-  //     operator==
-  //     operator!=
-  //
-  // == and != operators must be declared explicitly or dissallowed, as
-  // otherwise "ptr1 == ptr2" will compile but do the wrong thing (i.e., convert
-  // to Testable and then do the comparison).
-  //
-  // C++11 provides for "explicit operator bool()", however it is currently
-  // banned due to MSVS2013. https://chromium-cpp.appspot.com/#core-blacklist
- private:
-  typedef T* WeakPtr::*Testable;
-
- public:
-  operator Testable() const { return get() ? &WeakPtr::ptr_ : nullptr; }
+  // Allow conditionals to test validity, e.g. if (weak_ptr) {...};
+  explicit operator bool() const { return get() != nullptr; }
 
  private:
-  // Explicitly declare comparison operators as required by the "Safe Bool
-  // Idiom", but keep them private.
-  template <class U> bool operator==(WeakPtr<U> const&) const;
-  template <class U> bool operator!=(WeakPtr<U> const&) const;
-
   friend class internal::SupportsWeakPtrBase;
   template <typename U> friend class WeakPtr;
   friend class SupportsWeakPtr<T>;
   friend class WeakPtrFactory<T>;
 
   WeakPtr(const internal::WeakReference& ref, T* ptr)
       : WeakPtrBase(ref),
         ptr_(ptr) {
   }
 
   // This pointer is only valid when ref_.is_valid() is true.  Otherwise, its
   // value is undefined (as opposed to nullptr).
   T* ptr_;
 };
 
+// Allow callers to compare WeakPtrs against nullptr to test validity.
+template <class T>
+bool operator!=(const WeakPtr<T>& weak_ptr, std::nullptr_t) {
+  return !(weak_ptr == nullptr);
+}
+template <class T>
+bool operator!=(std::nullptr_t, const WeakPtr<T>& weak_ptr) {
+  return weak_ptr != nullptr;
+}
+template <class T>
+bool operator==(const WeakPtr<T>& weak_ptr, std::nullptr_t) {
+  return weak_ptr.get() == nullptr;
+}
+template <class T>
+bool operator==(std::nullptr_t, const WeakPtr<T>& weak_ptr) {
+  return weak_ptr == nullptr;
+}
+
 // A class may be composed of a WeakPtrFactory and thereby
 // control how it exposes weak pointers to itself.  This is helpful if you only
 // need weak pointers within the implementation of a class.  This class is also
 // useful when working with primitive types.  For example, you could have a
 // WeakPtrFactory<bool> that is used to pass around a weak reference to a bool.
 template <class T>
 class WeakPtrFactory {
  public:
   explicit WeakPtrFactory(T* ptr) : ptr_(ptr) {
   }
@@ skipping 64 matching lines @@
 //   base::WeakPtr<Derived> ptr = derived.AsWeakPtr();  // Fails.
 
 template <typename Derived>
 WeakPtr<Derived> AsWeakPtr(Derived* t) {
   return internal::SupportsWeakPtrBase::StaticAsWeakPtr<Derived>(t);
 }
 
 }  // namespace base
 
 #endif  // BASE_MEMORY_WEAK_PTR_H_