C++, ReactiveX - How to call on_next from a declared observable

RxCppCallOnNext.cpp

rxcpp::subjects::subject<int> sub;
auto o = sub.get_subscriber();
auto source = sub.get_observable();

source.subscribe(
    [](int v) {printf("OnNext: %d\n", v); },
    []() {printf("OnCompleted\n"); });

o.on_next(1);
o.on_next(2);
o.on_next(3);

o.on_completed();