Pass bound values as const values to the bound function

base/bind_helpers.h

 // Copyright (c) 2011 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.
 
 // This defines a set of argument wrappers and related factory methods that
 // can be used specify the refcounting and reference semantics of arguments
 // that are bound by the Bind() function in base/bind.h.
 //
 // It also defines a set of simple functions and utilities that people want
 // when using Callback<> and Bind().
@@ skipping 408 matching lines @@
   mutable T scoper_;
 };
 
 // Unwrap the stored parameters for the wrappers above.
 template <typename T>
 const T& Unwrap(const T& o) {
   return o;
 }
 
 template <typename T>
-T* Unwrap(UnretainedWrapper<T> unretained) {
+T* Unwrap(const UnretainedWrapper<T>& unretained) {
   return unretained.get();
 }
 
 template <typename T>
-const T& Unwrap(ConstRefWrapper<T> const_ref) {
+const T& Unwrap(const ConstRefWrapper<T>& const_ref) {
   return const_ref.get();
 }
 
 template <typename T>
 T* Unwrap(const RetainedRefWrapper<T>& o) {
   return o.get();
 }
 
 template <typename T>
 const WeakPtr<T>& Unwrap(const WeakPtr<T>& o) {
   return o;
 }
 
 template <typename T>
 T* Unwrap(const OwnedWrapper<T>& o) {
   return o.get();
 }
 
 template <typename T>
-T Unwrap(PassedWrapper<T>& o) {
+T Unwrap(const PassedWrapper<T>& o) {
   return o.Take();
 }
 
 // IsWeakMethod is a helper that determine if we are binding a WeakPtr<> to a
 // method.  It is used internally by Bind() to select the correct
 // InvokeHelper that will no-op itself in the event the WeakPtr<> for
 // the target object is invalidated.
 //
 // The first argument should be the type of the object that will be received by
 // the method.
@@ skipping 177 matching lines @@
 
 template <typename T>
 struct IsWeakReceiver<internal::ConstRefWrapper<T>> : IsWeakReceiver<T> {};
 
 template <typename T>
 struct IsWeakReceiver<WeakPtr<T>> : std::true_type {};
 
 }  // namespace base
 
 #endif  // BASE_BIND_HELPERS_H_