variadic: add filter call

variadic.hpp

@@ -20,6 +20,7 @@
 #include <cstddef>
 #include <type_traits>
 #include <utility>
+#include <tuple>
 
 
 namespace util::variadic {
@@ -53,7 +54,32 @@ namespace util::variadic {
         else
             return get<IndexV-1> (std::forward<TailT> (tail)...);
     }
-}
+
+
+    ///////////////////////////////////////////////////////////////////////////
+    /// returns a tuple of all arguments that satisfy the trait QueryT
+    template <template <typename> class QueryT>
+    auto filter () { return std::tuple {}; }
+
+
+    /// returns a tuple of all arguments that satisfy the trait QueryT
+    template <
+        template <typename> class QueryT,
+        typename HeadT,
+        typename ...ArgsT
+    >
+    auto
+    filter (HeadT &&head, ArgsT &&...args)
+    {
+        if constexpr (QueryT<HeadT>::value)
+            return std::tuple_cat (
+                std::tuple (std::forward<HeadT> (head)),
+                filter<QueryT> (std::forward<ArgsT> (args)...)
+            );
+        else
+            return filter<QueryT> (std::forward<ArgsT> (args)...);
+    }
+};
 
 
 #endif