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Rory O'Connell
Rory O'Connell

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Receiving Data types as arugments with mruby

It's funny when searching for examples on how to do something you end up getting results for posts you wrote a few days ago and not much else. I suppose that's a sign that I'm doing something bleeding edge and esoteric. Also props on for good SEO.

A quick hit for today, since apparently I not only accidentally started writing a book and accidentally started this mruby project, I also accidentally became the primary documenter for using mruby in practice. We learned how to create a Ruby class in C with mruby which stores arbitrary C data using the Data class type. What I want to do now is pass a Data instance as a parameter into another method. For example I'm pulling Win32 window management and Vulkan state into Ruby classes. Creating a VulkanState depends on a native window, in this case a Win32 window. The Ruby side is easy enough

@native_window = 'Foo', width: 1024, height: 768
@vulkan_state =

Getting started with creating a VulkanState class is also typical

RClass *VulkanState_C = mrb_define_class(state, "VulkanState", state->object_class);
mrb_define_method(state, VulkanState_C, "initialize",
                VulkanState_initialize, MRB_ARGS_REQ(1));

It's extracting the Data type with mrb_get_args which had me stumped for a while as I could not figure out how to get the right data type out. The documentation for mrb_get_args states how to get a Data type argument

| `d`  | data           | void *, {mrb_data_type} const | 2nd argument will be used to check data type so it won't be modified; when `!` follows, the value may be `nil` |

First try is a normal mrb_value

VulkanState_initialize(mrb_state* state, mrb_value self) {
  mrb_value gui_window;
  mrb_get_args(state, "d", &gui_window, &W32Window_data_type);

Which creates some wild looking data pointers in gui_window

Next attempt is try pulling the data out with DATA_PTR from the mrb_value

W32Window* window = reinterpret_cast<W32Window *>(DATA_PTR(gui_window));

Still wild data

Digging into the code, mrb_get_args automatically pulls out and type casts the void * for us using mrb_data_get_ptr. Therefore pulling a Data type out doesn't require any extra work from our side.

W32Window_data* gui_window;
mrb_get_args(state, "d", &gui_window, &W32Window_data_type);

Now it's correct

That's all! A complete abstract example on receiving a Data type as a method argument

struct Foo_data {
  uint32_t baz;

static const Foo_data_type = { "Foo", mrb_free };

Bar_get_foo(mrb_state* state, mrb_value* self) {
  Foo_data *foo_data;
  mrb_get_args(state, "d", &foo_data, &Foo_data_type);
  return mrb_nil_value();

RClass *Foo = mrb_define_class(state, "Foo", state->object_class);
RClass *Bar = mrb_define_class(state, "Bar", state->object_class);
mrb_define_method(state, Bar, "get_foo", Bar_get_foo, MRB_ARGS_REQ(1));

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