Make expressions like "if (weak_ptr)" work

base/memory/scoped_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.
 
 // Scopers help you manage ownership of a pointer, helping you easily manage the
 // a pointer within a scope, and automatically destroying the pointer at the
 // end of a scope.  There are two main classes you will use, which correspond
 // to the operators new/delete and new[]/delete[].
 //
 // Example usage (scoped_ptr<T>):
@@ skipping 366 matching lines @@
     return impl_.get();
   }
   element_type* get() const { return impl_.get(); }
 
   // Access to the deleter.
   deleter_type& get_deleter() { return impl_.get_deleter(); }
   const deleter_type& get_deleter() const { return impl_.get_deleter(); }
 
   // Allow scoped_ptr<element_type> to be used in boolean expressions, but not
   // implicitly convertible to a real bool (which is dangerous).
+  //
+  // Note that this trick is only safe when the == and != operators
+  // are declared explicitly, as otherwise "scoped_ptr1 ==
+  // scoped_ptr2" will compile but do the wrong thing (i.e., convert
+  // to Testable and then do the comparison).
  private:
   typedef base::internal::scoped_ptr_impl<element_type, deleter_type>
       scoped_ptr::*Testable;
 
  public:
   operator Testable() const { return impl_.get() ? &scoped_ptr::impl_ : NULL; }
 
   // Comparison operators.
   // These return whether two scoped_ptr refer to the same object, not just to
   // two different but equal objects.
@@ skipping 295 matching lines @@
 
 // A function to convert T* into scoped_ptr<T>
 // Doing e.g. make_scoped_ptr(new FooBarBaz<type>(arg)) is a shorter notation
 // for scoped_ptr<FooBarBaz<type> >(new FooBarBaz<type>(arg))
 template <typename T>
 scoped_ptr<T> make_scoped_ptr(T* ptr) {
   return scoped_ptr<T>(ptr);
 }
 
 #endif  // BASE_MEMORY_SCOPED_PTR_H_