Class PublisherOptions (2.20.0)

  namespace pubsub = ::google::cloud::pubsub;
  using ::google::cloud::future;
  using ::google::cloud::Options;
  using ::google::cloud::StatusOr;
  [](std::string project_id, std::string topic_id) {
    auto topic = pubsub::Topic(std::move(project_id), std::move(topic_id));
    // By default, the publisher will flush a batch after 10ms, after it
    // contains more than 100 message, or after it contains more than 1MiB of
    // data, whichever comes first. This changes those defaults.
    auto publisher = pubsub::Publisher(pubsub::MakePublisherConnection(
        std::move(topic),
        Options{}
            .set<pubsub::MaxHoldTimeOption>(std::chrono::milliseconds(20))
            .set<pubsub::MaxBatchBytesOption>(4 * 1024 * 1024L)
            .set<pubsub::MaxBatchMessagesOption>(200)));

    std::vector<future<void>> ids;
    for (char const* data : {"1", "2", "3", "go!"}) {
      ids.push_back(
          publisher.Publish(pubsub::MessageBuilder().SetData(data).Build())
              .then([data](future<StatusOr<std::string>> f) {
                auto s = f.get();
                if (!s) return;
                std::cout << "Sent '" << data << "' (" << *s << ")\n";
              }));
    }
    publisher.Flush();
    // Block until they are actually sent.
    for (auto& id : ids) id.get();
  }