Seems to work, but suffers from JuliaLang/julia#54832

src/interface-implementation.jl

@@ -279,13 +279,13 @@ macro implement(interfaces, struct_def)
  begin
  #@show fname, farg_types, freturn
  # Expr(:macrocall, Symbol("@cfunction"), fname, freturn, :((Ptr{Cvoid}, $(farg_types...)),))
- evaled_arg_types = (Core.eval(__module__, arg_type) for arg_type in farg_types)
+ evaled_arg_types = Tuple(Core.eval(__module__, arg_type) for arg_type in farg_types)
  carg_types = to_C_type.(evaled_arg_types)
  wrapper_name = wrapper_callback[1]
  (Symbol("construct_$wrapper_name"), :(
  # $(@__MODULE__()).cfunction(Val($wrapper_name), $freturn, Tuple{Ptr{Cvoid}, $(farg_types...)})
  #carg_types = Tuple($((:(to_C_type(t)) for t in farg_types)...));
- @cfunction($wrapper_name, $freturn, (Ptr{Cvoid}, $(carg_types)...))
+ @cfunction($wrapper_name, $freturn, (Ptr{Cvoid}, $(carg_types...)))
  ))
  end
  for (wrapper_callback, farg_types, freturn) in
@@ -298,14 +298,33 @@ macro implement(interfaces, struct_def)
  for (constructor_name, f) in wrapper_function_defs
  ]
 
+ init_func = Symbol("init_$(ImplStructName)!")
+
+ reinit_func = Symbol("reinit_$(ImplStructName)!")
+ reinit_func_def = :(function $(reinit_func)()
+ #if $(ImplInstanceName).$(first(function_var_names)) === C_NULL
+ # $(init_func)($(ImplInstanceName),
+ # $((
+ # begin
+ # constructor_name = wrapper_def[1]
+ # :($(__module__).$(constructor_name)())
+ # end
+ # for wrapper_def in wrapper_function_defs
+ # )...))
+ #end
+ end)
+
+
  convert_def = if ismutable
  if !has_params
  :(function Base.convert(::Type{$interface_name}, obj::$T_name)::$interface_name
+ $reinit_func()
  $interface_name($ImplInstanceName, pointer_from_objref(obj), obj)
  end)
  else
  # For structs with type parameters, we need to pass the concrete type through.
  :(function Base.convert(::Type{$interface_name}, obj::$T_name)::$interface_name
+ $reinit_func()
  $interface_name($ImplInstanceName, pointer_from_objref(obj), obj)
  end)
  end
@@ -314,8 +333,6 @@ macro implement(interfaces, struct_def)
  error("Only supports mutable structs for now")
  end
 
- init_func = Symbol("init_$(ImplStructName)!")
-
  esc(Base.remove_linenums!(quote
  $struct_def
 
@@ -325,28 +342,25 @@ macro implement(interfaces, struct_def)
 
  # TODO: Check function types against type check const from interface
 
+ $reinit_func_def
 
  $(toplevel_cfunction_constructors...)
 
  # We eval this const, so that the functions above are defined earlier.
  const $(ImplInstanceName) = eval($(QuoteNode(:($ImplStructName(
- $((:($f()) for f in first.(wrapper_function_defs))...)
+ $((:(Ptr{Cvoid}(C_NULL)) for f in first.(wrapper_function_defs))...)
  )))))
+ # # We eval this const, so that the functions above are defined earlier.
+ # const $(ImplInstanceName) = eval($(QuoteNode(:($ImplStructName(
+ # $((:($f()) for f in first.(wrapper_function_defs))...)
+ # )))))
+
+ #eval($(QuoteNode(:(module $(Symbol("___vtable_init_$(ImplInstanceName)"))
+ # using ..$(nameof(__module__)): $(ImplInstanceName), $(init_func)
 
- eval($(QuoteNode(:(module $(Symbol("___vtable_init_$(ImplInstanceName)"))
- using ..$(nameof(__module__)): $(ImplInstanceName), $(init_func)
-
- function __init__()
- $(init_func)($(ImplInstanceName),
- $((
- begin
- constructor_name = wrapper_def[1]
- :($(__module__).$(constructor_name)())
- end
- for wrapper_def in wrapper_function_defs
- )...))
- end
- end))))
+ # function __init__()
+ # end
+ #end))))
 
  $convert_def