How to serialize and deserialize in the same application?

[jacek.m...@gmail.com]

Hi
I write simple app. And I want to test efficiency random access Capnpro structures. This is because you claim it is very fast. But I don't know how to create ::capnp::PackedMessageReader mMessageReader;  with  ::capnp::MallocMessageBuilder as input. Can you give me some advices how to do that?

best regards
Jacek

[Kenton Varda]

Hi Jacek,

If you wanted to create a MesasgeReader that reads from a message created by some MessageBuilder, you could use SegmentArrayMessageReader, like:

    capnp::SegmentArrayMessageReader reader(builder.getSegmentsForOutput());

    MyType::Reader root = reader.getRoot<MyType>();

This is a little bit of a funny thing to do. The performance will be almost identical to if you had done:

    MyType::Reader root = builder.getRoot<MyType>().asReader();

If you want to test packing/unpacking, then you will need to write the packed data to a kj::OutputStream and then read it back from a kj::InputStream. You could use kj::ArrayOutputStream and kj::ArrayInputStream with a suitably-sized backing array to do this in-memory.

-Kenton

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[jacek.m...@gmail.com]

Hi Kenton,

Thank you for your reply! But I have further difficulties. I successfully create and load builder:
In my class:

protected: ::capnp::MallocMessageBuilder mBuilder;

Then I Try to get random access to the fields (I need it for my custom QAbstractItemModel).
In order that I use following code:
QModelIndex hCapnpModel::index(int row, int column, const QModelIndex & parent) const
{
    if((column < 0) || (column >= 30)) return QModelIndex();
    capnp::SegmentArrayMessageReader lReader(mBuilder.getSegmentsForOutput());
    //capnp::SegmentArrayMessageReader lReader(((::capnp::MessageBuilder&)mBuilder).getSegmentsForOutput());
    DokmsList::Reader lRoot = lReader.getRoot<DokmsList>();
    ::capnp::List< ::Dokms::Dokm>::Reader lList = lRoot.getDokmsList();
    if((row < 0) || (row >= lList.size())) return QModelIndex();
    return createIndex(row, column);
}
In this function I need just number of rows, but in different function (data()) I access
to the rows. So I really need some solution of the problem.
Problem occurs in line:
capnp::SegmentArrayMessageReader lReader(mBuilder.getSegmentsForOutput());

I got following error from VC++:

..\hqpanelCapnp\hcapnpmodel.cpp(141): error C2662: 'kj::ArrayPtr<const kj::ArrayPtr<const capnp::word>> capnp::MessageBuilder::getSegmentsForOutput(void)': cannot convert 'this' pointer from 'const capnp::MallocMessageBuilder' to 'capnp::MessageBuilder &'

..\hqpanelCapnp\hcapnpmodel.cpp(141): note: Conversion loses qualifiers

..\hqpanelCapnp\hcapnpmodel.cpp(141): error C2512: 'capnp::SegmentArrayMessageReader::SegmentArrayMessageReader': no appropriate default constructor available

I think that (for unknown reason) VC++ thinks that mBuilder is const object so it is 
different from capnp::MessageBuilder.

Now I am wonder how to fix it?

best regards

Jacek

[Kenton Varda]

Hi Jacek,

It looks like `mBuilder` is a member of `hCapnpModel`, and the `index()` method is declared `const`, therefore all its members are const within the method body. You'll either need to declare `mBuilder` as `mutable` or remove `const` from `index()`.

-Kenton

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