westonpace commented on a change in pull request #8680:
URL: https://github.com/apache/arrow/pull/8680#discussion_r544781160
##########
File path: cpp/src/arrow/util/functional.h
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@@ -79,5 +84,47 @@ struct call_traits {
typename std::enable_if<std::is_same<return_type<F>, T>::value, RT>;
};
+/// A type erased callable object which may only be invoked once.
+/// It can be constructed from any lambda which matches the provided call
signature.
+/// Invoking it results in destruction of the lambda, freeing any
state/references
+/// immediately. Invoking a default constructed FnOnce or one which has
already been
+/// invoked will segfault.
+template <typename Signature>
+class FnOnce;
+
+template <typename R, typename... A>
+class FnOnce<R(A...)> {
+ public:
+ FnOnce() = default;
+
+ template <typename Fn,
+ typename = typename std::enable_if<std::is_convertible<
+ typename std::result_of<Fn && (A...)>::type, R>::value>::type>
+ FnOnce(Fn fn) : impl_(new FnImpl<Fn>(std::move(fn))) { // NOLINT
runtime/explicit
+ }
+
+ explicit operator bool() const { return impl_ != NULLPTR; }
+
+ R operator()(A... a) && {
+ auto bye = std::move(impl_);
+ return bye->invoke(static_cast<A&&>(a)...);
Review comment:
This is mostly true. It can't be perfect forwarding because we need to
capture the values when the FnOnce is constructed (similar to the capture done
by a lambda or std::function). We also then need to move the values once more
to make the actual call to the callback.
However, the static_cast was causing one extra move to happen inside the
FnOnce (from FnOnce to FnOnce::Impl) and that could be avoided with
std::forward. I modified FnOnceTest::MoveOnlyDataType to verify this change
moved us from 3 moves to 2 moves.
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