Function mrb_class_defined_under 

pub unsafe extern "C" fn mrb_class_defined_under(
    mrb: *mut mrb_state,
    outer: *mut RClass,
    name: *const c_char,
) -> mrb_bool

Returns an mrb_bool. True if inner class was defined, and false if the inner class was not defined.

Example: void mrb_example_gem_init(mrb_state* mrb) { struct RClass *example_outer, *example_inner; mrb_bool cd;

  example_outer = mrb_define_module(mrb, "ExampleOuter");

  example_inner = mrb_define_class_under(mrb, example_outer, "ExampleInner", mrb->object_class);
  cd = mrb_class_defined_under(mrb, example_outer, "ExampleInner");

  // If mrb_class_defined_under returns TRUE then puts "True"
  // If mrb_class_defined_under returns FALSE then puts "False"
  if (cd) {
    puts("True");
  }
  else {
    puts("False");
  }
 }

@param mrb The current mruby state. @param outer The name of the outer class. @param name A string representing the name of the inner class. @return mrb_bool A boolean value.