Dynamically reading schema const property at runtime (c++)

[Daniel Koohmarey]

Given the following dummy schema test.capnp:

struct Person {

   const type :Text = "person.type.example";

   version @0 :UInt32;

   name @1 :Text;

}

At runtime, how can we read the default const value from a schema struct (i.e the "person.type.example" above? (type is not considered a field within the struct, but it does appear as a node in the proto):

std::string schema_filename = "test.capnp"; kj::ArrayPtr<const kj::StringPtr> import_paths;

capnp::SchemaParser parser;

capnp::ParsedSchema schema = parser.parseDiskFile( schema_filename.c_str(), 

                                                                         schema_filename.c_str(), import_paths);

auto person_maybe = schema.findNested("Person");

auto* person_schema = kj::_::readMaybe(person_maybe);

// how can I read "person.type.example" out of the Person struct?

Any guidance on the matter is appreciated, thanks!

[Kenton Varda]

Hi Daniel,

I think you can do:

    capnp::ConstSchema type = schema.getNested("Person").getNested("type").asConst();

But it's been a long time since I touched this.

By the way, please do not use stuff in kj::_ directly, that is a private namespace. To read a Maybe, use KJ_IF_MAYBE, KJ_ASSERT_NONNULL, KJ_UNWRAP_OR, etc.

-Kenton

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[Daniel Koohmarey]

Appreciate the reply Kenton, that got me to the solution I was looking for:

std::string type = person_schema->getNested("type").asConst().as<capnp::Text>().cStr();

And will do on avoiding kj::_. Thanks again!