use namespace & not fix return

arena_guard.h

@@ -0,0 +1,29 @@
+//
+// Created by ZekeXiao on 2025/10/17.
+//
+
+#ifndef MRUBYPP_MRUBYPP_UTILS_H
+#define MRUBYPP_MRUBYPP_UTILS_H
+
+#include <mruby.h>
+
+namespace mrubypp {
+// gc arena
+class arena_guard {
+public:
+  explicit arena_guard(mrb_state *mrb) : mrb(mrb) {
+    ai = mrb_gc_arena_save(mrb);
+  }
+  arena_guard(arena_guard &&other) = delete;
+  arena_guard(const arena_guard &other) = delete;
+  ~arena_guard() { mrb_gc_arena_restore(mrb, ai); }
+
+  // arena_idx
+  [[nodiscard]] int get_ai() const { return ai; }
+
+private:
+  mrb_state *mrb;
+  int ai;
+};
+} // namespace mrubypp
+#endif // MRUBYPP_MRUBYPP_UTILS_H