New utility for pattern-matching style over std::variant

cpp/core/match_variant.h

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+/* Copyright (c) 2008-2025 the MRtrix3 contributors.
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ *
+ * Covered Software is provided under this License on an "as is"
+ * basis, without warranty of any kind, either expressed, implied, or
+ * statutory, including, without limitation, warranties that the
+ * Covered Software is free of defects, merchantable, fit for a
+ * particular purpose or non-infringing.
+ * See the Mozilla Public License v. 2.0 for more details.
+ *
+ * For more details, see http://www.mrtrix.org/.
+ */
+
+#pragma once
+
+#include <variant>
+
+// Utility for pattern-matching style over std::variant.
+// - `overload` builds an overload set from multiple lambdas (or function objects).
+// - `match_v` wraps std::visit, forwarding the variant and visitors.
+//   It returns whatever the selected lambda returns.
+// Example:
+//   std::variant<int, std::string> v = 42;
+//   auto s = MR::match_v(v,
+//       [](int i) { return std::to_string(i); },
+//       [](const std::string& s) { return s; }); // check_syntax off
+//   // s == "42"
+
+namespace MR {
+template <class... Ts> struct overload : Ts... {
+  using Ts::operator()...;
+};
+template <class... Ts> overload(Ts...) -> overload<Ts...>;
+template <class var_t, class... Func> decltype(auto) match_v(var_t &&variant, Func &&...funcs) {
+  return std::visit(overload{std::forward<Func>(funcs)...}, std::forward<var_t>(variant));
+}
+} // namespace MR