From ff5bff1aa58945afdcebc408e983a2e428511e08 Mon Sep 17 00:00:00 2001
From: Roland Conybeare <rconybeare@gmail.com>
Date: Fri, 13 Feb 2026 15:15:08 -0500
Subject: [PATCH] xo-gc stack: streamline object pointer forwarding

---
 include/xo/facet/FacetRegistry.hpp | 39 ++++++++++++++++++++++++------
 include/xo/facet/obj.hpp           |  6 +++++
 2 files changed, 38 insertions(+), 7 deletions(-)

diff --git a/include/xo/facet/FacetRegistry.hpp b/include/xo/facet/FacetRegistry.hpp
index d4b1518..9b7e7ad 100644
--- a/include/xo/facet/FacetRegistry.hpp
+++ b/include/xo/facet/FacetRegistry.hpp
@@ -159,7 +159,7 @@ namespace xo {
              *    }
              **/
             template <typename ATo, typename AFrom>
-            obj<ATo> try_variant(obj<AFrom> from) {
+            obj<ATo> try_variant(obj<AFrom> from) noexcept {
                 return try_variant<ATo>(from._typeseq(), from.data());
             }
 
@@ -173,7 +173,7 @@ namespace xo {
              *      = FacetRegistry::variant<ABar>(foo._typeseq(), foo.opaque_data());
              **/
             template <typename AFacet>
-            obj<AFacet> try_variant(typeseq repr_id, void * data) {
+            obj<AFacet> try_variant(typeseq repr_id, void * data) noexcept {
                 const AFacet * iface = this->lookup<AFacet>(repr_id);
 
                 if (iface)
@@ -246,12 +246,37 @@ namespace xo {
         obj<AOther,DRepr>
         obj<AFacet,DRepr>::to_facet()
         {
-            if constexpr (std::is_same_v<DRepr, DVariantPlaceholder>) {
-                // return type has type-erased data
-                return FacetRegistry::instance().variant<AOther,AFacet>(*this);
+            if (this->data()) {
+                if constexpr (std::is_same_v<DRepr, DVariantPlaceholder>) {
+                    // return type has type-erased data
+                    return FacetRegistry::instance().variant<AOther,AFacet>(*this);
+                } else {
+                    // return type has known data
+                    return obj<AOther,DRepr>(this->data());
+                }
             } else {
-                // return type has known data
-                return obj<AOther,DRepr>(this->data());
+                return obj<AOther,DRepr>();
+            }
+        }
+
+        // Deferred definitioon of obj<AFacet,DRepr>::to_facet(),
+        // since implementation requires FacetRegistry
+        //
+        template <typename AFacet, typename DRepr>
+        template <typename AOther>
+        obj<AOther,DRepr>
+        obj<AFacet,DRepr>::try_to_facet() noexcept
+        {
+            if (this->data()) {
+                if constexpr (std::is_same_v<DRepr, DVariantPlaceholder>) {
+                    // return type has type-erased data
+                    return FacetRegistry::instance().try_variant<AOther,AFacet>(*this);
+                } else {
+                    // return type has known data
+                    return obj<AOther,DRepr>(this->data());
+                }
+            } else {
+                return obj<AOther,DRepr>();
             }
         }
 
diff --git a/include/xo/facet/obj.hpp b/include/xo/facet/obj.hpp
index 3619ae3..ada5c83 100644
--- a/include/xo/facet/obj.hpp
+++ b/include/xo/facet/obj.hpp
@@ -137,6 +137,12 @@ namespace xo {
             template <typename AOther>
             obj<AOther,DRepr> to_facet();
 
+            /** like to_facet<AOther>(),
+             *  but on failure return empty obj instead of throwing exception 
+             **/
+            template <typename AOther>
+            obj<AOther,DRepr> try_to_facet() noexcept;
+
             /** enabled when RRouter<AFacet> provides _preincrement.
              *  Note we don't need this trick for comparison operators,
              *  since return type is fixed.