How to serve multiple interfaces in Capnproto

Question

In the calculator example, I have added a new interface in the capnp file like below:

interface Main {
  getCalculator @0 () -> (calculator :Calculator);
}

My objective is to make use of multiple interfaces through one implementation Main. How can I achieve this? The extra client code is below (when I try to use calculator it throws a null capability exception):

capnp::EzRpcClient client1(argv[1]);
Main::Client main = client1.getMain<Main>();
auto& waitScope1 = client1.getWaitScope();

auto request1 = main.getCalculatorRequest();
auto evalPromise1 = request1.send();
auto response1 = evalPromise1.wait(waitScope1);
auto calculator = response1.getCalculator();
auto request = calculator.evaluateRequest();
request.getExpression().setLiteral(123);

// Send it, which returns a promise for the result (without blocking).
auto evalPromise = request.send();

// Using the promise, create a pipelined request to call read() on the
// returned object, and then send that.
auto readPromise = evalPromise.getValue().readRequest().send();

// Now that we've sent all the requests, wait for the response.  Until this
// point, we haven't waited at all!
auto response = readPromise.wait(waitScope1);
KJ_ASSERT(response.getValue() == 123);

Changes to the server's main implementation file are given below:

class MainImpl : public Main::Server {
  public:
    kj::Promise<void> getCalculator(GetCalculatorContext context) override {
      context.getResults();
      return kj::READY_NOW;
    }
};

The main function in server serves MainImpl. I do get the calculator object but how can I further invoke methods on that object?

Answer 1

Your getCalculator() function on the server side does not do anything right now. You need to actually set the result if you want the Calculator to get back to the client. That's why you get your "null capability exception".

Something like this:

class MainImpl : public Main::Server {
  public:
    kj::Promise<void> getCalculator(GetCalculatorContext context) override {
      context.getResults().setCalculator(<the calculator capability>);
      return kj::READY_NOW;
    }
};

That will require you to create the Calculator client on the server side (i.e. you won't do Calculator::Client calculator = client.getMain<Calculator>(); on the client side anymore, since calculator will come from getCalculator()).