如何访问 Cap'n'Proto 的序列化数据？

Question

I'm working with Cap'n'Proto and my understanding is there is no need to do serialization as it's already being done. So my question is, how would I access the serialized data and get it's size so that I can pass it in as a byte array to another library.

// person.capnp
struct Person {
    name @0 :Text;
    age @1 :Int16;
}

// ...
::capnp::MallocMessageBuilder message;

Person::Builder person = message.initRoot<Person>();
person.setName("me");
person.setAge(20);

// at this point, how do I get some sort of handle to 
// the serialized data of 'person' as well as it's size?

I've seen the writePackedMessageToFd(fd, message); call, but didn't quite understand what was being passed and couldn't find any API docs on it. I also wasn't trying to write to a file descriptor as I need the serialized data returned as const void* .

Looking in Capnproto's message.h file is this function which is in the base class for MallocMessageBuilder which says it gets the raw data making up the message.

kj::ArrayPtr<const kj::ArrayPtr<const word>> getSegmentsForOutput();
// Get the raw data that makes up the message.

But even then, Im' not sure how to get it as const void* .

Thoughts?

Answer 1

::capnp::MallocMessageBuilder message;

is your binary message, and its size is

message.sizeInWords()

(size in bytes divided by 8).

Answer 2

This appears to be whats needed.

// ...
::capnp::MallocMessageBuilder message;

Person::Builder person = message.initRoot<Person>();
person.setName("me");
person.setAge(20);

kj::Array<capnp::word> dataArr = capnp::messageToFlatArray(message);
kj::ArrayPtr<kj::byte> bytes = dataArr.asBytes();

std::string data(bytes.begin(), bytes.end());

const void* dataPtr = data.c_str();

At this point, I have a const void* dataPtr and size using data.size() .