diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0b2b026..9c10270 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -35,6 +35,15 @@ xo_add_genfacet(
 
 # ----------------------------------------------------------------
 
+# note: manual target; generated code committed to git
+xo_add_genfacet(
+    TARGET xo-alloc2-facet-gcobjectvisitor
+    FACET GCObjectVisitor
+    INPUT idl/GCObjectVisitor.json5
+)
+
+# ----------------------------------------------------------------
+
 # note: manual target; generated code committed to git
 xo_add_genfacet(
     TARGET xo-alloc2-facet-resourcevisitor
diff --git a/idl/GCObjectVisitor.json5 b/idl/GCObjectVisitor.json5
new file mode 100644
index 0000000..b465842
--- /dev/null
+++ b/idl/GCObjectVisitor.json5
@@ -0,0 +1,75 @@
+{
+    mode: "facet",
+    output_cpp_dir: "src/alloc2/facet",
+    output_hpp_dir: "include/xo/alloc2",
+    output_impl_subdir: "gc",
+    includes: [
+//        "<xo/alloc2/Allocator.hpp>",
+//        "<xo/alloc2/Collector.hpp>",
+//        "<cstdint>",
+//        "<cstddef>",
+    ],
+    // extra includes in GCObject.hpp, if any
+    user_hpp_includes: [
+//        "\"gc/RCollector_aux.hpp\"",
+    ],
+    namespace1: "xo",
+    namespace2: "mm",
+    pretext: [
+        "// see GCObject.hpp, also in xo-alloc2/",
+        "namespace xo { namespace mm { class AGCObject; }}",
+    ],
+    facet: "GCObjectVisitor",
+    detail_subdir: "gc",
+    brief: "gc-aware object visitor",
+    using_doxygen: true,
+    doc: [
+        "Visit a gc-aware object. Visitor can traverse and update child pointers in-place."
+    ],
+    types: [
+        // using size_type = std::size_t
+//        {
+//            name: "size_type",
+//            doc: ["type for an amount of memory"],
+//            definition: "std::size_t",
+//        },
+//        {
+//            name: "AAllocator",
+//            doc: ["fomo allocator type"],
+//            definition: "xo::mm::AAllocator",
+//        },
+//        {
+//            name: "ACollector",
+//            doc: ["fomo collector type"],
+//            definition: "xo::mm::ACollector",
+//        },
+    ],
+    const_methods: [
+        // size_type shallow_size() const noexcept
+//        {
+//            name: "shallow_size",
+//            doc: ["memory consumption for this instance"],
+//            return_type: "size_type",
+//            args: [],
+//            const: true,
+//            noexcept: true,
+//            attributes: [],
+//        },
+    ],
+    nonconst_methods: [
+        // void visit_child(AGCObject * iface, void ** pp_data) noexcept;
+        {
+            name: "visit_child",
+            doc: ["visit child of a gc-aware object. May update child in-place!"],
+            return_type: "void",
+            args:[
+                {type: "AGCObject *", name: "iface"},
+                {type: "void **", name: "pp_data"},
+            ],
+            const: true,  // technical const. I/face not modified
+            noexcept: true,
+            attributes: [],
+        },
+    ],
+    router_facet_explicit_content: []
+}
diff --git a/include/xo/alloc2/GCObjectVisitor.hpp b/include/xo/alloc2/GCObjectVisitor.hpp
new file mode 100644
index 0000000..7805cc9
--- /dev/null
+++ b/include/xo/alloc2/GCObjectVisitor.hpp
@@ -0,0 +1,22 @@
+/** @file GCObjectVisitor.hpp
+ *
+ *  Generated automagically from ingredients:
+ *  1. code generator:
+ *       [xo-facet/codegen/genfacet]
+ *     arguments:
+ *       --input [idl/GCObjectVisitor.json5]
+ *  2. jinja2 template for facet .hpp file:
+ *       [facet.hpp.j2]
+ *  3. idl for facet methods
+ *       [idl/GCObjectVisitor.json5]
+ **/
+
+#pragma once
+
+#include "gc/AGCObjectVisitor.hpp"
+#include "gc/IGCObjectVisitor_Any.hpp"
+#include "gc/IGCObjectVisitor_Xfer.hpp"
+#include "gc/RGCObjectVisitor.hpp"
+
+
+/* end GCObjectVisitor.hpp */
diff --git a/include/xo/alloc2/gc/AGCObjectVisitor.hpp b/include/xo/alloc2/gc/AGCObjectVisitor.hpp
new file mode 100644
index 0000000..b533076
--- /dev/null
+++ b/include/xo/alloc2/gc/AGCObjectVisitor.hpp
@@ -0,0 +1,79 @@
+/** @file AGCObjectVisitor.hpp
+ *
+ *  Generated automagically from ingredients:
+ *  1. code generator:
+ *       [xo-facet/codegen/genfacet]
+ *     arguments:
+ *       --input [idl/GCObjectVisitor.json5]
+ *  2. jinja2 template for abstract facet .hpp file:
+ *       [abstract_facet.hpp.j2]
+ *  3. idl for facet methods
+ *       [idl/GCObjectVisitor.json5]
+ **/
+
+#pragma once
+
+// includes (via {facet_includes})
+#include <xo/facet/obj.hpp>
+#include <xo/facet/facet_implementation.hpp>
+#include <xo/facet/typeseq.hpp>
+
+// see GCObject.hpp, also in xo-alloc2/
+namespace xo { namespace mm { class AGCObject; }}
+
+namespace xo {
+namespace mm {
+
+using Copaque = const void *;
+using Opaque = void *;
+
+/**
+Visit a gc-aware object. Visitor can traverse and update child pointers in-place.
+**/
+class AGCObjectVisitor {
+public:
+    /** @defgroup mm-gcobjectvisitor-type-traits **/
+    ///@{
+    // types
+    /** integer identifying a type **/
+    using typeseq = xo::facet::typeseq;
+    using Copaque = const void *;
+    using Opaque = void *;
+    ///@}
+
+    /** @defgroup mm-gcobjectvisitor-methods **/
+    ///@{
+    // const methods
+    /** An uninitialized AGCObjectVisitor instance will have zero vtable pointer (per {linux,osx} abi).
+     *  Use case for this is narrow. We go to some lengths to avoid null vtable pointers. For example
+     *  obj<AFacet> will have non-null vtable (via IFacet_Any) with all methods terminating.
+     **/
+    bool _has_null_vptr() const noexcept { return *reinterpret_cast<const void * const *>(this) == nullptr; }
+    /** RTTI: unique id# for actual runtime data representation **/
+    virtual typeseq _typeseq() const noexcept = 0;
+    /** destroy instance @p d; calls c++ dtor only for actual runtime type; does not recover memory **/
+    virtual void _drop(Opaque d) const noexcept = 0;
+
+    // nonconst methods
+    /** visit child of a gc-aware object. May update child in-place! **/
+    virtual void visit_child(Opaque data, AGCObject * iface, void ** pp_data)  const  noexcept = 0;
+    ///@}
+}; /*AGCObjectVisitor*/
+
+/** Implementation IGCObjectVisitor_DRepr of AGCObjectVisitor for state DRepr
+ *  should provide a specialization:
+ *
+ *    template <>
+ *    struct xo::facet::FacetImplementation<AGCObjectVisitor, DRepr> {
+ *        using Impltype = IGCObjectVisitor_DRepr;
+ *    };
+ *
+ *  then IGCObjectVisitor_ImplType<DRepr> --> IGCObjectVisitor_DRepr
+ **/
+template <typename DRepr>
+using IGCObjectVisitor_ImplType = xo::facet::FacetImplType<AGCObjectVisitor, DRepr>;
+
+} /*namespace mm*/
+} /*namespace xo*/
+
+/* AGCObjectVisitor.hpp */
diff --git a/include/xo/alloc2/gc/IGCObjectVisitor_Any.hpp b/include/xo/alloc2/gc/IGCObjectVisitor_Any.hpp
new file mode 100644
index 0000000..287252c
--- /dev/null
+++ b/include/xo/alloc2/gc/IGCObjectVisitor_Any.hpp
@@ -0,0 +1,88 @@
+/** @file IGCObjectVisitor_Any.hpp
+ *
+ *  Generated automagically from ingredients:
+ *  1. code generator:
+ *       [xo-facet/codegen/genfacet]
+ *     arguments:
+ *       --input [idl/GCObjectVisitor.json5]
+ *  2. jinja2 template for abstract facet .hpp file:
+ *       [iface_facet_any.hpp.j2]
+ *  3. idl for facet methods
+ *       [idl/GCObjectVisitor.json5]
+ **/
+
+#pragma once
+
+#include "AGCObjectVisitor.hpp"
+#include <xo/facet/obj.hpp>
+
+namespace xo { namespace mm { class IGCObjectVisitor_Any; } }
+
+namespace xo {
+namespace facet {
+
+template <>
+struct FacetImplementation<xo::mm::AGCObjectVisitor,
+                           DVariantPlaceholder>
+{
+    using ImplType = xo::mm::IGCObjectVisitor_Any;
+};
+
+}
+}
+
+namespace xo {
+namespace mm {
+
+    /** @class IGCObjectVisitor_Any
+     *  @brief AGCObjectVisitor implementation for empty variant instance
+     **/
+    class IGCObjectVisitor_Any : public AGCObjectVisitor {
+    public:
+        /** @defgroup mm-gcobjectvisitor-any-type-traits **/
+        ///@{
+
+        /** integer identifying a type **/
+        using typeseq = xo::facet::typeseq;
+
+        ///@}
+        /** @defgroup mm-gcobjectvisitor-any-methods **/
+        ///@{
+
+        const AGCObjectVisitor * iface() const { return std::launder(this); }
+
+        // from AGCObjectVisitor
+
+        // builtin methods
+        typeseq _typeseq() const noexcept override { return s_typeseq; }
+        [[noreturn]] void _drop(Opaque) const noexcept override { _fatal(); }
+
+        // const methods
+
+        // nonconst methods
+        [[noreturn]] void visit_child(Opaque, AGCObject *, void **)  const  noexcept override;
+
+        ///@}
+
+    private:
+        /** @defgraoup mm-gcobjectvisitor-any-private-methods **/
+        ///@{
+
+        [[noreturn]] static void _fatal();
+
+        ///@}
+
+    public:
+        /** @defgroup mm-gcobjectvisitor-any-member-vars **/
+        ///@{
+
+        static typeseq s_typeseq;
+        static bool _valid;
+
+        ///@}
+    };
+
+} /*namespace mm */
+} /*namespace xo */
+
+/* IGCObjectVisitor_Any.hpp */
diff --git a/include/xo/alloc2/gc/IGCObjectVisitor_Xfer.hpp b/include/xo/alloc2/gc/IGCObjectVisitor_Xfer.hpp
new file mode 100644
index 0000000..aa9b476
--- /dev/null
+++ b/include/xo/alloc2/gc/IGCObjectVisitor_Xfer.hpp
@@ -0,0 +1,82 @@
+/** @file IGCObjectVisitor_Xfer.hpp
+ *
+ *  Generated automagically from ingredients:
+ *  1. code generator:
+ *       [xo-facet/codegen/genfacet]
+ *     arguments:
+ *       --input [idl/GCObjectVisitor.json5]
+ *  2. jinja2 template for abstract facet .hpp file:
+ *       [iface_facet_any.hpp.j2]
+ *  3. idl for facet methods
+ *       [idl/GCObjectVisitor.json5]
+ **/
+
+#pragma once
+
+
+namespace xo {
+namespace mm {
+    /** @class IGCObjectVisitor_Xfer
+     **/
+    template <typename DRepr, typename IGCObjectVisitor_DRepr>
+    class IGCObjectVisitor_Xfer : public AGCObjectVisitor {
+    public:
+        /** @defgroup mm-gcobjectvisitor-xfer-type-traits **/
+        ///@{
+        /** actual implementation (not generated; often delegates to DRepr) **/
+        using Impl = IGCObjectVisitor_DRepr;
+        /** integer identifying a type **/
+        using typeseq = AGCObjectVisitor::typeseq;
+        ///@}
+
+        /** @defgroup mm-gcobjectvisitor-xfer-methods **/
+        ///@{
+
+        static const DRepr & _dcast(Copaque d) { return *(const DRepr *)d; }
+        static DRepr & _dcast(Opaque d) { return *(DRepr *)d; }
+
+        // from AGCObjectVisitor
+
+        // builtin methods
+        typeseq _typeseq() const noexcept override { return s_typeseq; }
+        void _drop(Opaque d) const noexcept override { _dcast(d).~DRepr(); }
+
+        // const methods
+
+        // non-const methods
+        void visit_child(Opaque data, AGCObject * iface, void ** pp_data)  const  noexcept override {
+            return I::visit_child(_dcast(data), iface, pp_data);
+        }
+
+        ///@}
+
+    private:
+        using I = Impl;
+
+    public:
+        /** @defgroup mm-gcobjectvisitor-xfer-member-vars **/
+        ///@{
+
+        /** typeseq for template parameter DRepr **/
+        static typeseq s_typeseq;
+        /** true iff satisfies facet implementation **/
+        static bool _valid;
+
+        ///@}
+    };
+
+    template <typename DRepr, typename IGCObjectVisitor_DRepr>
+    xo::facet::typeseq
+    IGCObjectVisitor_Xfer<DRepr, IGCObjectVisitor_DRepr>::s_typeseq
+      = xo::facet::typeseq::id<DRepr>();
+
+    template <typename DRepr, typename IGCObjectVisitor_DRepr>
+    bool
+    IGCObjectVisitor_Xfer<DRepr, IGCObjectVisitor_DRepr>::_valid
+      = xo::facet::valid_facet_implementation<AGCObjectVisitor,
+                                              IGCObjectVisitor_Xfer>();
+
+} /*namespace mm */
+} /*namespace xo*/
+
+/* end IGCObjectVisitor_Xfer.hpp */
diff --git a/include/xo/alloc2/gc/RGCObjectVisitor.hpp b/include/xo/alloc2/gc/RGCObjectVisitor.hpp
new file mode 100644
index 0000000..800d197
--- /dev/null
+++ b/include/xo/alloc2/gc/RGCObjectVisitor.hpp
@@ -0,0 +1,84 @@
+/** @file RGCObjectVisitor.hpp
+ *
+ *  Generated automagically from ingredients:
+ *  1. code generator:
+ *       [xo-facet/codegen/genfacet]
+ *     arguments:
+ *       --input [idl/GCObjectVisitor.json5]
+ *  2. jinja2 template for abstract facet .hpp file:
+ *       [iface_facet_any.hpp.j2]
+ *  3. idl for facet methods
+ *       [idl/GCObjectVisitor.json5]
+ **/
+
+#pragma once
+
+#include "AGCObjectVisitor.hpp"
+
+namespace xo {
+namespace mm {
+
+/** @class RGCObjectVisitor
+ **/
+template <typename Object>
+class RGCObjectVisitor : public Object {
+private:
+    using O = Object;
+
+public:
+    /** @defgroup mm-gcobjectvisitor-router-type-traits **/
+    ///@{
+    using ObjectType = Object;
+    using DataPtr = Object::DataPtr;
+    using typeseq = xo::reflect::typeseq;
+    ///@}
+
+    /** @defgroup mm-gcobjectvisitor-router-ctors **/
+    ///@{
+    RGCObjectVisitor() {}
+    RGCObjectVisitor(Object::DataPtr data) : Object{std::move(data)} {}
+    RGCObjectVisitor(const AGCObjectVisitor * iface, void * data)
+      requires std::is_same_v<typename Object::DataType, xo::facet::DVariantPlaceholder>
+      : Object(iface, data) {}
+
+    ///@}
+    /** @defgroup mm-gcobjectvisitor-router-methods **/
+    ///@{
+
+    // explicit injected content
+
+    // builtin methods
+    typeseq _typeseq() const noexcept { return O::iface()->_typeseq(); }
+    void _drop() const noexcept { O::iface()->_drop(O::data()); }
+
+    // const methods
+
+    // non-const methods (still const in router!)
+    void visit_child(AGCObject * iface, void ** pp_data)  noexcept {
+        return O::iface()->visit_child(O::data(), iface, pp_data);
+    }
+
+    ///@}
+    /** @defgroup mm-gcobjectvisitor-member-vars **/
+    ///@{
+
+    static bool _valid;
+
+    ///@}
+};
+
+template <typename Object>
+bool
+RGCObjectVisitor<Object>::_valid = xo::facet::valid_object_router<Object>();
+
+} /*namespace mm*/
+} /*namespace xo*/
+
+namespace xo { namespace facet {
+    template <typename Object>
+    struct RoutingFor<xo::mm::AGCObjectVisitor, Object> {
+        using RoutingType = xo::mm::RGCObjectVisitor<Object>;
+    };
+} }
+
+/* end RGCObjectVisitor.hpp */
diff --git a/src/alloc2/CMakeLists.txt b/src/alloc2/CMakeLists.txt
index 8399d1b..7929371 100644
--- a/src/alloc2/CMakeLists.txt
+++ b/src/alloc2/CMakeLists.txt
@@ -9,7 +9,9 @@ set(SELF_SRCS
     CollectorTypeRegistry.cpp
 
     facet/ICollector_Any.cpp
+
     IGCObject_Any.cpp
+    facet/IGCObjectVisitor_Any.cpp
 
     AAllocator.cpp
     IAllocator_Any.cpp
diff --git a/src/alloc2/facet/IGCObjectVisitor_Any.cpp b/src/alloc2/facet/IGCObjectVisitor_Any.cpp
new file mode 100644
index 0000000..53d13ca
--- /dev/null
+++ b/src/alloc2/facet/IGCObjectVisitor_Any.cpp
@@ -0,0 +1,48 @@
+/** @file IGCObjectVisitor_Any.cpp
+ *
+ **/
+
+#include "gc/IGCObjectVisitor_Any.hpp"
+#include <iostream>
+#include <exception>
+
+namespace xo {
+namespace mm {
+
+using xo::facet::DVariantPlaceholder;
+using xo::facet::typeseq;
+using xo::facet::valid_facet_implementation;
+
+void
+IGCObjectVisitor_Any::_fatal()
+{
+    /* control here on uninitialized IAllocator_Any.
+     * Initialized instance will have specific implementation type
+     */
+    std::cerr << "fatal"
+              << ": attempt to call uninitialized"
+              << " IGCObjectVisitor_Any method"
+              << std::endl;
+    std::terminate();
+}
+
+typeseq
+IGCObjectVisitor_Any::s_typeseq = typeseq::id<DVariantPlaceholder>();
+
+bool
+IGCObjectVisitor_Any::_valid
+  = valid_facet_implementation<AGCObjectVisitor, IGCObjectVisitor_Any>();
+
+// nonconst methods
+
+auto
+IGCObjectVisitor_Any::visit_child(Opaque, AGCObject *, void **)  const  noexcept -> void
+{
+    _fatal();
+}
+
+
+} /*namespace mm*/
+} /*namespace xo*/
+
+/* end IGCObjectVisitor_Any.cpp */