foo.nim

import plugin_manager

type
  FooPlugin* {.exportc.} = ref object of Plugin

{.pragma: rtl, exportc, dynlib, cdecl.}

proc create*(): FooPlugin {.rtl.} = 
  result = new(FooPlugin)

main.nim

import dynlib, plugin_manager

type
  FooPlugin {.importc.} = ref object of Plugin
  creator = proc(): FooPlugin {.gcsafe, cdecl.}
  
var lib = dynlib.loadLib("libfoo.dylib")

let create = cast[creator](lib.symAddr("create"))

let fooPlugin = create()

echo repr fooPlugin

lib.unloadLib()

plugin_manager.nim

type
  Plugin* = ref object of RootObj