typeidx: add variant query

typeidx.hpp

@@ -9,6 +9,7 @@
 #pragma once
 
 #include <atomic>
+#include <variant>
 
 
 namespace cruft {
@@ -49,4 +50,21 @@ namespace cruft {
         static auto id = detail::typeidx_next<TagT> ();
         return id;
     }
+
+
+    /// Returns the typeidx of the contained value within a variant.
+    ///
+    /// May throw std::bad_variant_access if the variant is
+    /// valueless_by_exception.
+    template <typename TagT=void, typename ...ComponentT>
+    int
+    value_typeidx (std::variant<ComponentT...> const &val)
+    {
+        return std::visit (
+            [] <typename ValueT> (ValueT const &) {
+                return typeidx<ValueT> ();
+            },
+            val
+        );
+    }
 }