Eliminate implicit conversion from scoped_refptr<T> to T*

base/memory/ref_counted.h

Index: base/memory/ref_counted.h
diff --git a/base/memory/ref_counted.h b/base/memory/ref_counted.h
index b7cb5e064175466d747e5ae66125262d8f1558cf..e1d958d1a7960deecfdcbdcac4bf01d8dda39816 100644
--- a/base/memory/ref_counted.h
+++ b/base/memory/ref_counted.h
@@ -242,9 +242,30 @@ class scoped_refptr {
   }
 
   T* get() const { return ptr_; }
-  operator T*() const { return ptr_; }
   T* operator->() const { return ptr_; }
 
+  // This operator allows a scoped_refptr to be safely evaluated as bool, using
+  // the "Safe bool" idiom. By "safely", we mean that we are opening up as few
+  // unsafe operations as possible. Simply using operator bool() can allow
+  // silliness like:
+  //   int x = my_scoped_refptr;
+  //   return (my_scoped_refptr < 5);
+  // See, e.g., http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Safe_bool
+  // for more information on this idiom.
+  typedef void (scoped_refptr<T>::*BoolType)() const;
+  void this_type_does_not_support_comparisons() const {}
+  operator BoolType() const {
+    return ptr_ ? &scoped_refptr<T>::this_type_does_not_support_comparisons :
+                  NULL;
+  }
+
+  // Allow scoped_refptr<T> to be used in tree-based containers. Note that
+  // operator< on pointers is technically implementation-defined, but it works
+  // on all our platforms and is a common idiom in Chromium anyway.
+  bool operator<(const scoped_refptr<T>& right) const {
+    return ptr_ < right.ptr_;
+  }
+
   // Release a pointer.
   // The return value is the current pointer held by this object.
   // If this object holds a NULL pointer, the return value is NULL.