Introduce OnceClosure and BindOnce

base/callback_internal.cc

Index: base/callback_internal.cc
diff --git a/base/callback_internal.cc b/base/callback_internal.cc
index 24b44272e0b6a8f3cfeb5f2eae0c152868217d7b..6e5a2763931234ddebf342a098a8552a15ee5a94 100644
--- a/base/callback_internal.cc
+++ b/base/callback_internal.cc
@@ -23,6 +23,17 @@ CallbackBase<CopyMode::MoveOnly>::CallbackBase(CallbackBase&& c) = default;
 CallbackBase<CopyMode::MoveOnly>&
 CallbackBase<CopyMode::MoveOnly>::operator=(CallbackBase&& c) = default;
 
+CallbackBase<CopyMode::MoveOnly>::CallbackBase(
+    const CallbackBase<CopyMode::Copyable>& c)
+    : bind_state_(c.bind_state_), polymorphic_invoke_(c.polymorphic_invoke_) {}

[dcheng, 2016/08/26 07:37:57]

If we're creating it from a copyable callback, should we require moving state
out of the original copyable callback? Otherwise, we clone the state, which
defeats the point of a move-only callback, right?

[dcheng, 2016/08/31 06:20:53]

Any thoughts on this comment?

[tzik, 2016/08/31 08:14:10]

Oh, sorry. I missed this comment.

Since the original callback is copyable, the caller can keep a copy of it
anyway.
So I think it's not critical to keep the move-onliness.

+
+CallbackBase<CopyMode::MoveOnly>& CallbackBase<CopyMode::MoveOnly>::operator=(
+    const CallbackBase<CopyMode::Copyable>& c) {
+  bind_state_ = c.bind_state_;
+  polymorphic_invoke_ = c.polymorphic_invoke_;
+  return *this;
+}
+
 void CallbackBase<CopyMode::MoveOnly>::Reset() {
   polymorphic_invoke_ = nullptr;
   // NULL the bind_state_ last, since it may be holding the last ref to whatever