tf::Continuation token_; // this is a functor-like object that can be called to signify the task is actually complete.

void StartExpensiveTaskWithToken(tf::Continuation token) {
  token_ = token;
}

tf::Taskflow flow;
auto t1 = flow.emplace([](tf::Continuation token)) {
  std::cout << "T1 running...";
  StartExpensiveTaskWithToken(token);
}

auto t2 = flow.placeholder();
t2.succeed(t1);

// Output
T1 running...
// now t2 is still blocked, but t1 is not scheduled on an executor.  All threads in the executor are free.
// task finishes and somebody calls `token_()`
T2 runs