Make expressions like "if (weak_ptr)" work

base/memory/scoped_ptr.h

Index: base/memory/scoped_ptr.h
diff --git a/base/memory/scoped_ptr.h b/base/memory/scoped_ptr.h
index 5edde8fb32e655f0516f19733c1a8b86c86a59dc..dfa12587242962f5884d1f6246aeeca068d16756 100644
--- a/base/memory/scoped_ptr.h
+++ b/base/memory/scoped_ptr.h
@@ -384,6 +384,11 @@ class scoped_ptr {
 
   // 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;