JsonCodec should probably automatically initialize itself

[zenhack]

# testcase.capnp
@0x8d8eb9e37e9bbfd9;

using Json = import "/capnp/compat/json.capnp";

struct Foo {
  barBaz @0 :Void $Json.name("bar_baz");
}

// testcase.c++
#include "testcase.capnp.h"
#include <capnp/serialize.h>
#include <capnp/compat/json.h>
#include <iostream>

int main() {
  capnp::MallocMessageBuilder msg;

  auto foo = msg.initRoot<Foo>();
  auto jsonStr = capnp::JsonCodec().encode(foo);
  std::cout << jsonStr.cStr() << std::endl;
  return 0;
}

# build.sh
set -ex
capnp compile -oc++ *.capnp
c++ -o testcase *.c++ -lkj -lcapnp -lcapnp-json

Output:

{"barBaz":null}

...but iiuc the field should be rendered as "bar_baz".

[bretthoerner]

I think you just need to call handleByAnnotation<Foo>(); on your JsonCodec first:

capnproto/c++/src/capnp/compat/json.h

Lines 209 to 217 in ee3aac9

	void handleByAnnotation(Schema schema);
	template <typename T> void handleByAnnotation();
	// Inspects the given type (as specified by type parameter or dynamic schema) and all its
	// dependencies looking for JSON annotations (see json.capnp), building and registering Handlers
	// based on these annotations.
	//
	// If you'd like to use annotations to control JSON, you must call these functions before you
	// start using the codec. They are not loaded "on demand" because that would require mutex
	// locking.

[zenhack]

Aha! thanks, closing.

[kentonv]

Yeah this has proven to be a source of confusion. :/

Since JsonCodec is designed to be callable from multiple threads, we either need an explicit initialization method, or we need to use thread-safe lazy init, and thread-safe lazy init just feels so hacky, but maybe it's the right thing to do after all.