QtCS2018 Serialisation

Serialisation

Binary JSON

• Was created for qtjsondb
• fast reading
• mmap()able
• Deprecate soon (Qt 5.12?)
  • Remove from QJsonDocument in Qt 6
  • Provide compat API to read
    • BSJON → JSON → parse again

QJsonDocument

• Limited to 128 MB of RAM footprint (due to Binary JSON)
• Needs to be fixed before Qt 6
• Can use the same backend as QCborValue
• Complaint: updating in-place is not easy
  • If you update an entry in a node, you have to update the chain leading to it
  • QCborValue has the same problem, as it was designed to have the same API

   QJsonValue foo = ...;
   foo["s"] = "value";             // does not compile
   foo.toObject()["s"] = "value";  // updates the temporary!
   
   // must write instead:
   QJsonObject fooObject = foo.toObject();
   fooObject["s"] = "value";
   setFoo(fooObject);

Common DOM API?

• Should we have a common API for manipulating trees of JSON-like values?
• QCborValue is a superset of QJsonValue
  • But it would be confusing to users to use CBOR classes to manipulate JSON
  • Generic name?
• What other uses would this API have?
  • Replace QJSValue (QtQml)?
  • Replace QVariantMap?
  • Entry point for a Protobuf API?
• How to make sure people can't insert combinations not allowed in the output?
  • For example, associative containers using integers as keys in JSON
  • Do they do that? Maybe they won't write such code
    • They know what their content is used for
  • We could use templates, specialising for CBOR, JSON, etc.
  • We could have a wrapper class that has inlines and provides only the possible combinations

• QCborValue integrated last Friday into QtCore 5.12
  • Need to know in the next few weeks if we keep it for 5.12
  • Can yank it out and move to a new module for Tech Preview

Serialising Qt state

• Needed by Qt Remote Objects
• Slightly different from CBOR and JSON purposes
  • Not about a standardised representation of a data model
  • More about transmitting state from two independent processes of the same application
• Currently using QDataStream
  • Has a lot of problems, can't really detect errors and not extensible enough
• Need more exploration, no conclusion

Protobuf

• Need volunteers to write a Proof of Concept
  • plugin to protoc?