Function operator()

cpp/src/arrow/util/thread_pool.cc:717–720

    // when calling KeepAlive to keep the OT infrastructure alive.
    struct {
      void operator()() {
        auto scope = ::arrow::internal::tracing::GetTracer()->WithActiveSpan(activeSpan);
        std::move(func)();
      }
      FnOnce<void()> func;
      opentelemetry::nostd::shared_ptr<opentelemetry::trace::Span> activeSpan;
    } wrapper{std::forward<FnOnce<void()>>(task),