You can get the offset and size of a field via the schema API. Check out
schema.h and schema.capnp.
auto fieldDef = KJ_ASSERT_NONNULL(
capnp::Schema::from<MyStruct>().findFieldByName("myField")).getProto().getSlot();
auto offset = fieldDef.getOffset(); // multiply by field size to get
byte offset
switch (fieldDef.getType()) { ... }
-Kenton
On Mon, Jul 29, 2019 at 5:26 AM Dmitry Shmorgun <[email protected]> wrote:
> I tried it, so any message can be reinterpret by a proper generic message
> that has AnyPointer fields in a scheme. Where AnyPointer can be any
> construction like List<List<AnyStruct>> for example, so i can serialize,
> store, manipulate, serialize back any message only knowing a generic
> structure. It works well.
> I saw an example of a list truncation:
> auto orphan = myStruct.disownMyList();
> orphan.truncate(size);
> myStruct.adoptMyList(kj::mv(orphan));
>
> though i haven't found how to do so for AnyList<AnyStruct> structure, so
> it was necessary to calculate m_filledEntries before the creation of the
> list:
>
> auto entriesBuilder =
> AnyPointer::Builder(r1.getEntries()).initAsListOfAnyStruct(m_entrySize,
> m_filledEntries, m_filledEntries);
>
> for (size_t i = 0, j = 0; i < m_entries.size(); ++i) {
> if (!m_entries[i].isBlank) {
> auto dataSection = entriesBuilder[j++].getDataSection();
> std::copy(m_entries[i].buff.begin(),
> m_entries[i].buff.begin() + m_entrySize, dataSection.begin());
> }
> }
>
> Now i'm trying to tell a server that uses generic messages that in a
> struct of a received message there is a key on the byte offset X with the
> size N. The server reads a special message that convey this information. So
> the server can keep a data snapshot in a hash key-value storage and
> manipulate entries by the the byte-key. The problem is how to find the
> offset and size of the specific key field to encode in the message with
> settings. It seems all these things are deeply hidden.
> Any ideas maybe?
>
> Thanks for your help.
>
>
> On Thursday, July 25, 2019 at 6:25:49 PM UTC+3, Dmitry Shmorgun wrote:
>>
>> Hi!
>>
>> According to
>> https://capnproto.org/news/2014-06-17-capnproto-flatbuffers-sbe.html there
>> is a way to extract new unknown fields from a message and then construct
>> the same or slightly modified message using an outdated scheme and place
>> these fields back.
>> But how actually this can be done? I don't find any example.
>> I assume there should be something else than dynamic scheme loading.
>> Can I go further and just reinterpret a message type A as an message
>> containing a header and a one struct that can be specified as a generic
>> message B, modify and pack back as A? So in this way, I can even not know
>> the scheme of A at all.
>>
>> Many thanks
>>
> --
> You received this message because you are subscribed to the Google Groups
> "Cap'n Proto" group.
> To unsubscribe from this group and stop receiving emails from it, send an
> email to [email protected].
> To view this discussion on the web visit
> https://groups.google.com/d/msgid/capnproto/4930b3a4-f153-458c-b445-d7b638d98366%40googlegroups.com
> <https://groups.google.com/d/msgid/capnproto/4930b3a4-f153-458c-b445-d7b638d98366%40googlegroups.com?utm_medium=email&utm_source=footer>
> .
>
--
You received this message because you are subscribed to the Google Groups
"Cap'n Proto" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To view this discussion on the web visit
https://groups.google.com/d/msgid/capnproto/CAJouXQmn07SKjh%2BA90rkscHwHkC0HViqUe%2BmKe7xqA_xoht%2Bzg%40mail.gmail.com.
