diff --git a/include/bout/bout.hxx b/include/bout/bout.hxx
index 8588fe56d9..a08696ec66 100644
--- a/include/bout/bout.hxx
+++ b/include/bout/bout.hxx
@@ -34,6 +34,8 @@
 #ifndef BOUT_H
 #define BOUT_H
 
+#include <filesystem> // std::filesystem (C++17)
+
 // IWYU pragma: begin_keep, begin_export
 #include "bout/build_defines.hxx"
 
@@ -106,10 +108,10 @@ void setupGetText();
 struct CommandLineArgs {
   int verbosity{4};
   bool color_output{false};
-  std::string data_dir{"data"};          ///< Directory for data input/output
-  std::string opt_file{"BOUT.inp"};      ///< Filename for the options file
-  std::string set_file{"BOUT.settings"}; ///< Filename for the options file
-  std::string log_file{"BOUT.log"};      ///< File name for the log file
+  std::filesystem::path data_dir{"data"};          ///< Directory for data input/output
+  std::filesystem::path opt_file{"BOUT.inp"};      ///< Filename for the options file
+  std::filesystem::path set_file{"BOUT.settings"}; ///< Filename for the options file
+  std::filesystem::path log_file{"BOUT.log"};      ///< File name for the log file
   /// The original set of command line arguments
   std::vector<std::string> original_argv;
   /// The "canonicalised" command line arguments, with single-letter
diff --git a/src/bout++.cxx b/src/bout++.cxx
index 7f23cf5f91..6de1c5713b 100644
--- a/src/bout++.cxx
+++ b/src/bout++.cxx
@@ -4,7 +4,7 @@
  * Adapted from the BOUT code by B.Dudson, University of York, Oct 2007
  *
  **************************************************************************
- * Copyright 2010-2023 BOUT++ contributors
+ * Copyright 2010-2025 BOUT++ contributors
  *
  * Contact Ben Dudson, dudson2@llnl.gov
  *
@@ -84,7 +84,7 @@ const char DEFAULT_DIR[] = "data";
 // Define S_ISDIR if not defined by system headers (that is, MSVC)
 // Taken from https://github.com/curl/curl/blob/e59540139a398dc70fde6aec487b19c5085105af/lib/curl_setup.h#L748-L751
 #if !defined(S_ISDIR) && defined(S_IFMT) && defined(S_IFDIR)
-#define S_ISDIR(m) (((m)&S_IFMT) == S_IFDIR)
+#define S_ISDIR(m) (((m) & S_IFMT) == S_IFDIR)
 #endif
 
 #ifdef _MSC_VER
@@ -180,7 +180,7 @@ int BoutInitialise(int& argc, char**& argv) {
     // `optionfile` here, but we'd need to call parseCommandLine
     // _first_ in order to do that and set the source, etc., but we
     // need to call that _second_ in order to override the input file
-    reader->read(Options::getRoot(), "{}/{}", args.data_dir, args.opt_file);
+    reader->read(Options::getRoot(), "{}", (args.data_dir / args.opt_file).string());
 
     // Get options override from command-line
     reader->parseCommandLine(Options::getRoot(), args.argv);