C++ codegen with __declspec(dllexport)

[compilerintrinsics]

Currently, generated C++ source code from capnp schema files use CAPNP_DECLARE_SCHEMA macro, which expands to extern <schema global vars>. As a result, I am unable to have users of my shared library link to these schema variables since they are not exported.

Is there a way for me to control the codegen to generate __declspec(dllexport) in place of extern? Better yet, would it be possible for it to generate MY_API macro in place of exern so that MY_API can generate platform specific export attributes?

[kentonv]

This is not supported, no. Even the Cap'n Proto runtime library itself lacks export declarations. Somewhat surprisingly, I don't think anyone has ever brought up the issue until now, suggesting that demand for dynamically linking capnproto on Windows is pretty low.

I'd accept a PR to support it, but it's a lot of work and I don't expect to have the bandwidth to work on it myself. If you want to make a PR, please target the master branch, which is the 1.x release branch. Be sure to define a separate export macro for each library (libkj, libkj-async, libcapnp, libcapnp-rpc, etc.).

[jonahbeckford]

I had a need to add Windows export declarations for capnproto-c (not C++) a while back.

If needed, you may want to start with the CMake bits at https://gitlab.com/dkml/ext/c-capnproto/-/blob/1089ab8441b10765579d17ca519278a5c31dea0c/CMakeLists.txt#L55-136 and then work your way into the code.

[compilerintrinsics]

Thanks for the suggestions! However, in the end, we went with the cheapest solution and hacked our build system to do a regex replace of CAPNP_DECLARE_SCHEMA(...) with relevant exports.