Let us start here, from square one. No, from Zero!

external/layeredfs/modpath_handler.h

@@ -0,0 +1,29 @@
+#pragma once
+
+#include <string>
+#include <vector>
+
+#if 0
+#include <experimental/optional>
+using std::experimental::optional;
+#else
+#include <optional>
+using std::optional;
+#endif
+
+using std::string;
+using std::vector;
+
+#define MOD_FOLDER "./data_mods"
+#define CACHE_FOLDER MOD_FOLDER "/_cache"
+
+namespace layeredfs {
+
+    void cache_mods(void);
+    vector<string> available_mods();
+    optional<string> normalise_path(const string &path);
+    optional<string> find_first_modfile(const string &norm_path);
+    optional<string> find_first_modfolder(const string &norm_path);
+    vector<string> find_all_modfile(const string &norm_path);
+    bool mkdir_p(string &path);
+}