Disallow redundant Bind calls.

base/bind_internal.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.
 
 #ifndef BASE_BIND_INTERNAL_H_
 #define BASE_BIND_INTERNAL_H_
 
 #include <stddef.h>
 
 #include <tuple>
@@ skipping 404 matching lines @@
 struct CancellationChecker<BindState<Callback<Signature>, BoundArgs...>> {
   static constexpr bool is_cancellable = true;
   static bool Run(const BindStateBase* base) {
     using Functor = Callback<Signature>;
     using BindStateType = BindState<Functor, BoundArgs...>;
     const BindStateType* bind_state = static_cast<const BindStateType*>(base);
     return bind_state->functor_.IsCancelled();
   }
 };
 
+// Template helpers to detect using Bind() on a base::Callback without any
+// additional arguments. In that case, the original base::Callback object should
+// just be directly used.
+template <typename Functor, typename... BoundArgs>
+struct BindingCallbackWithNoArgs {
+  static constexpr bool value = false;
+};
+
+template <typename Signature, typename... BoundArgs>
+struct BindingCallbackWithNoArgs<Callback<Signature>, BoundArgs...> {

[tzik, 2016/09/20 07:33:04]

Could you add a specialization for OnceCallback too?

[dcheng, 2016/09/20 08:22:39]

How come things like CancellationChecker don't currently specialize on
OnceCallback?

[dcheng, 2016/09/24 00:49:17]

Done (though I did this by templatizing on the additional parameters)

+  static constexpr bool value = sizeof...(BoundArgs) == 0;
+};
+
 // BindState<>
 //
 // This stores all the state passed into Bind().
 template <typename Functor, typename... BoundArgs>
 struct BindState final : BindStateBase {
   using IsCancellable = std::integral_constant<
       bool, CancellationChecker<BindState>::is_cancellable>;
 
   template <typename ForwardFunctor, typename... ForwardBoundArgs>
   explicit BindState(BindStateBase::InvokeFuncStorage invoke_func,
                      ForwardFunctor&& functor,
                      ForwardBoundArgs&&... bound_args)
       // IsCancellable is std::false_type if the CancellationChecker<>::Run
       // returns always false. Otherwise, it's std::true_type.
       : BindState(IsCancellable{},
                   invoke_func,
                   std::forward<ForwardFunctor>(functor),
-                  std::forward<ForwardBoundArgs>(bound_args)...) {}
+                  std::forward<ForwardBoundArgs>(bound_args)...) {
+    static_assert(!BindingCallbackWithNoArgs<Functor, BoundArgs...>::value,
+                  "Attempting to bind a base::Callback with no additional "
+                  "arguments: save a heap allocation and use the original "
+                  "base::Callback");
+  }
 
   Functor functor_;
   std::tuple<BoundArgs...> bound_args_;
 
  private:
   template <typename ForwardFunctor, typename... ForwardBoundArgs>
   explicit BindState(std::true_type,
                      BindStateBase::InvokeFuncStorage invoke_func,
                      ForwardFunctor&& functor,
                      ForwardBoundArgs&&... bound_args)
@@ skipping 70 matching lines @@
 
 // Returns a RunType of bound functor.
 // E.g. MakeUnboundRunType<R(A, B, C), A, B> is evaluated to R(C).
 template <typename Functor, typename... BoundArgs>
 using MakeUnboundRunType =
     typename internal::MakeUnboundRunTypeImpl<Functor, BoundArgs...>::Type;
 
 }  // namespace base
 
 #endif  // BASE_BIND_INTERNAL_H_