diff --git a/src/user/user_api.cc b/src/user/user_api.cc
index 74ec08f0..05f03cc6 100644
--- a/src/user/user_api.cc
+++ b/src/user/user_api.cc
@@ -48,8 +48,8 @@ MJAPI void mjm_finalize(mjElement object) {
 
 
 // add child body to body, return child spec
-mjmBody* mjm_addBody(mjmBody* bodyspec, void* defspec) {
-  mjCDef* def = static_cast<mjCDef*>(defspec);
+mjmBody* mjm_addBody(mjmBody* bodyspec, mjmDefault* defspec) {
+  mjCDef* def = defspec ? reinterpret_cast<mjCDef*>(defspec->element) : 0;
   mjCBody* body = reinterpret_cast<mjCBody*>(bodyspec->element)->AddBody(def);
   return &body->spec;
 }
@@ -57,8 +57,8 @@ mjmBody* mjm_addBody(mjmBody* bodyspec, void* defspec) {
 
 
 // add site to body, return site spec
-mjmSite* mjm_addSite(mjmBody* bodyspec, void* defspec) {
-  mjCDef* def = static_cast<mjCDef*>(defspec);
+mjmSite* mjm_addSite(mjmBody* bodyspec, mjmDefault* defspec) {
+  mjCDef* def = defspec ? reinterpret_cast<mjCDef*>(defspec->element) : 0;
   mjCBody* body = reinterpret_cast<mjCBody*>(bodyspec->element);
   mjCSite* site = body->AddSite(def);
   return &site->spec;
@@ -67,8 +67,8 @@ mjmSite* mjm_addSite(mjmBody* bodyspec, void* defspec) {
 
 
 // add joint to body
-mjmJoint* mjm_addJoint(mjmBody* bodyspec, void* defspec) {
-  mjCDef* def = static_cast<mjCDef*>(defspec);
+mjmJoint* mjm_addJoint(mjmBody* bodyspec, mjmDefault* defspec) {
+  mjCDef* def = defspec ? reinterpret_cast<mjCDef*>(defspec->element) : 0;
   mjCBody* body = reinterpret_cast<mjCBody*>(bodyspec->element);
   mjCJoint* joint = body->AddJoint(def);
   return &joint->spec;
@@ -86,8 +86,8 @@ mjmJoint* mjm_addFreeJoint(mjmBody* bodyspec) {
 
 
 // add geom to body
-mjmGeom* mjm_addGeom(mjmBody* bodyspec, void* defspec) {
-  mjCDef* def = static_cast<mjCDef*>(defspec);
+mjmGeom* mjm_addGeom(mjmBody* bodyspec, mjmDefault* defspec) {
+  mjCDef* def = defspec ? reinterpret_cast<mjCDef*>(defspec->element) : 0;
   mjCBody* body = reinterpret_cast<mjCBody*>(bodyspec->element);
   mjCGeom* geom = body->AddGeom(def);
   return &geom->spec;
@@ -96,8 +96,8 @@ mjmGeom* mjm_addGeom(mjmBody* bodyspec, void* defspec) {
 
 
 // add camera to body
-mjmCamera* mjm_addCamera(mjmBody* bodyspec, void* defspec) {
-  mjCDef* def = static_cast<mjCDef*>(defspec);
+mjmCamera* mjm_addCamera(mjmBody* bodyspec, mjmDefault* defspec) {
+  mjCDef* def = defspec ? reinterpret_cast<mjCDef*>(defspec->element) : 0;
   mjCBody* body = reinterpret_cast<mjCBody*>(bodyspec->element);
   mjCCamera* camera = body->AddCamera(def);
   return &camera->spec;
@@ -106,8 +106,8 @@ mjmCamera* mjm_addCamera(mjmBody* bodyspec, void* defspec) {
 
 
 // add light to body
-mjmLight* mjm_addLight(mjmBody* bodyspec, void* defspec) {
-  mjCDef* def = static_cast<mjCDef*>(defspec);
+mjmLight* mjm_addLight(mjmBody* bodyspec, mjmDefault* defspec) {
+  mjCDef* def = defspec ? reinterpret_cast<mjCDef*>(defspec->element) : 0;
   mjCBody* body = reinterpret_cast<mjCBody*>(bodyspec->element);
   mjCLight* light = body->AddLight(def);
   return &light->spec;
@@ -138,8 +138,8 @@ mjmFrame* mjm_addFrame(mjmBody* bodyspec, mjmFrame* parentframe) {
 
 
 // add mesh to model
-mjmMesh* mjm_addMesh(void* model, void* defspec) {
-  mjCDef* def = defspec ? reinterpret_cast<mjCDef*>(defspec) : 0;
+mjmMesh* mjm_addMesh(void* model, mjmDefault* defspec) {
+  mjCDef* def = defspec ? reinterpret_cast<mjCDef*>(defspec->element) : 0;
   mjCModel* modelC = static_cast<mjCModel*>(model);
   mjCMesh* mesh = modelC->AddMesh(def);
   return &mesh->spec;
@@ -175,9 +175,9 @@ mjmTexture* mjm_addTexture(void* model) {
 
 
 // add material to model
-mjmMaterial* mjm_addMaterial(void* model, void* defspec) {
+mjmMaterial* mjm_addMaterial(void* model, mjmDefault* defspec) {
   mjCModel* modelC = static_cast<mjCModel*>(model);
-  mjCDef* def = static_cast<mjCDef*>(defspec);
+  mjCDef* def = defspec ? reinterpret_cast<mjCDef*>(defspec->element) : 0;
   mjCMaterial* material = modelC->AddMaterial(def);
   return &material->spec;
 }
@@ -185,9 +185,9 @@ mjmMaterial* mjm_addMaterial(void* model, void* defspec) {
 
 
 // add pair to model
-mjmPair* mjm_addPair(void* model, void* defspec) {
+mjmPair* mjm_addPair(void* model, mjmDefault* defspec) {
   mjCModel* modelC = static_cast<mjCModel*>(model);
-  mjCDef* def = static_cast<mjCDef*>(defspec);
+  mjCDef* def = defspec ? reinterpret_cast<mjCDef*>(defspec->element) : 0;
   mjCPair* pair = modelC->AddPair(def);
   return &pair->spec;
 }
@@ -204,9 +204,9 @@ mjmExclude* mjm_addExclude(void* model) {
 
 
 // add equality to model
-mjmEquality* mjm_addEquality(void* model, void* defspec) {
+mjmEquality* mjm_addEquality(void* model, mjmDefault* defspec) {
   mjCModel* modelC = static_cast<mjCModel*>(model);
-  mjCDef* def = static_cast<mjCDef*>(defspec);
+  mjCDef* def = defspec ? reinterpret_cast<mjCDef*>(defspec->element) : 0;
   mjCEquality* equality = modelC->AddEquality(def);
   return &equality->spec;
 }
@@ -214,9 +214,9 @@ mjmEquality* mjm_addEquality(void* model, void* defspec) {
 
 
 // add tendon to model
-mjmTendon* mjm_addTendon(void* model, void* defspec) {
+mjmTendon* mjm_addTendon(void* model, mjmDefault* defspec) {
   mjCModel* modelC = static_cast<mjCModel*>(model);
-  mjCDef* def = static_cast<mjCDef*>(defspec);
+  mjCDef* def = defspec ? reinterpret_cast<mjCDef*>(defspec->element) : 0;
   mjCTendon* tendon = modelC->AddTendon(def);
   return &tendon->spec;
 }
@@ -260,9 +260,9 @@ mjmWrap* mjm_wrapPulley(mjmTendon* tendonspec, double divisor) {
 
 
 // add actuator to model
-mjmActuator* mjm_addActuator(void* model, void* defspec) {
+mjmActuator* mjm_addActuator(void* model, mjmDefault* defspec) {
   mjCModel* modelC = static_cast<mjCModel*>(model);
-  mjCDef* def = static_cast<mjCDef*>(defspec);
+  mjCDef* def = defspec ? reinterpret_cast<mjCDef*>(defspec->element) : 0;
   mjCActuator* actuator = modelC->AddActuator(def);
   return &actuator->spec;
 }
@@ -314,7 +314,7 @@ mjmKey* mjm_addKey(void* model) {
 
 
 
-// Add plugin to model.
+// add plugin to model
 mjElement mjm_addPlugin(void* model) {
   mjCModel* modelC = static_cast<mjCModel*>(model);
   mjCPlugin* plugin = modelC->AddPlugin();
@@ -323,6 +323,19 @@ mjElement mjm_addPlugin(void* model) {
 
 
 
+// add default to model
+mjmDefault* mjm_addDefault(void* model, const char* classname, int parentid) {
+  mjCModel* modelC = static_cast<mjCModel*>(model);
+  mjCDef* def = modelC->AddDef(classname, parentid);
+  if (def) {
+    return &def->spec;
+  } else {
+    return nullptr;
+  }
+}
+
+
+
 // get objects
 void* mjm_getModel(mjmBody* bodyspec) {
   return reinterpret_cast<mjCBody*>(bodyspec->element)->model;
@@ -331,8 +344,8 @@ void* mjm_getModel(mjmBody* bodyspec) {
 
 
 // get default
-void* mjm_getDefault(mjElement element) {
-  return reinterpret_cast<mjCBase*>(element)->def;
+mjmDefault* mjm_getDefault(mjElement element) {
+  return &(reinterpret_cast<mjCBase*>(element)->def->spec);
 }
 
 
@@ -381,9 +394,10 @@ int mjm_getId(mjElement element) {
 
 
 // set default
-void mjm_setDefault(mjElement element, void* defspec) {
+void mjm_setDefault(mjElement element, mjmDefault* defspec) {
   mjCBase* baseC = reinterpret_cast<mjCBase*>(element);
-  baseC->def = static_cast<mjCDef*>(defspec);
+  baseC->def = reinterpret_cast<mjCDef*>(defspec->element);
+  baseC->def->PointToLocal();
 }
 
 
diff --git a/src/user/user_api.h b/src/user/user_api.h
index fb9be03f..05ca79d2 100644
--- a/src/user/user_api.h
+++ b/src/user/user_api.h
@@ -641,6 +641,24 @@ typedef struct _mjmKey {           // keyframe specification
 } mjmKey;
 
 
+typedef struct _mjmDefault {       // default specification
+  mjString name;                   // name
+  mjElement element;               // internal, do not modify
+  mjmJoint* joint;                 // joint defaults
+  mjmGeom* geom;                   // geom defaults
+  mjmSite* site;                   // site defaults
+  mjmCamera* camera;               // camera defaults
+  mjmLight* light;                 // light defaults
+  mjmFlex* flex;                   // flex defaults
+  mjmMesh* mesh;                   // mesh defaults
+  mjmMaterial* material;           // material defaults
+  mjmPair* pair;                   // pair defaults
+  mjmEquality* equality;           // equality defaults
+  mjmTendon* tendon;               // tendon defaults
+  mjmActuator* actuator;           // actuator defaults
+} mjmDefault;
+
+
 //---------------------------------- API functions -------------------------------------------------
 
 // Create model.
@@ -653,25 +671,25 @@ MJAPI void mjm_deleteModel(void* modelspec);
 MJAPI void mjm_finalize(mjElement object);
 
 // Add child body to body, return child spec.
-MJAPI mjmBody* mjm_addBody(mjmBody* body, void* defspec);
+MJAPI mjmBody* mjm_addBody(mjmBody* body, mjmDefault* def);
 
 // Add site to body, return site spec.
-MJAPI mjmSite* mjm_addSite(mjmBody* body, void* defspec);
+MJAPI mjmSite* mjm_addSite(mjmBody* body, mjmDefault* def);
 
 // Add joint to body.
-MJAPI mjmJoint* mjm_addJoint(mjmBody* body, void* defspec);
+MJAPI mjmJoint* mjm_addJoint(mjmBody* body, mjmDefault* def);
 
 // Add freejoint to body.
 MJAPI mjmJoint* mjm_addFreeJoint(mjmBody* body);
 
 // Add geom to body.
-MJAPI mjmGeom* mjm_addGeom(mjmBody* body, void* defspec);
+MJAPI mjmGeom* mjm_addGeom(mjmBody* body, mjmDefault* def);
 
 // Add camera to body.
-MJAPI mjmCamera* mjm_addCamera(mjmBody* body, void* defspec);
+MJAPI mjmCamera* mjm_addCamera(mjmBody* body, mjmDefault* def);
 
 // Add light to body.
-MJAPI mjmLight* mjm_addLight(mjmBody* body, void* defspec);
+MJAPI mjmLight* mjm_addLight(mjmBody* body, mjmDefault* def);
 
 // Add frame to body.
 MJAPI mjmFrame* mjm_addFrame(mjmBody* body, mjmFrame* parentframe);
@@ -680,7 +698,7 @@ MJAPI mjmFrame* mjm_addFrame(mjmBody* body, mjmFrame* parentframe);
 MJAPI mjmFlex* mjm_addFlex(void* model);
 
 // Add mesh to model.
-MJAPI mjmMesh* mjm_addMesh(void* model, void* defspec);
+MJAPI mjmMesh* mjm_addMesh(void* model, mjmDefault* def);
 
 // Add height field to model.
 MJAPI mjmHField* mjm_addHField(void* model);
@@ -692,19 +710,19 @@ MJAPI mjmSkin* mjm_addSkin(void* model);
 MJAPI mjmTexture* mjm_addTexture(void* model);
 
 // Add material to model.
-MJAPI mjmMaterial* mjm_addMaterial(void* model, void* defspec);
+MJAPI mjmMaterial* mjm_addMaterial(void* model, mjmDefault* def);
 
 // Add pair to model.
-MJAPI mjmPair* mjm_addPair(void* model, void* defspec);
+MJAPI mjmPair* mjm_addPair(void* model, mjmDefault* def);
 
 // Add excluded body pair to model.
 MJAPI mjmExclude* mjm_addExclude(void *model);
 
 // Add equality to model.
-MJAPI mjmEquality* mjm_addEquality(void* model, void* defspec);
+MJAPI mjmEquality* mjm_addEquality(void* model, mjmDefault* def);
 
 // Add tendon to model.
-MJAPI mjmTendon* mjm_addTendon(void* model, void* defspec);
+MJAPI mjmTendon* mjm_addTendon(void* model, mjmDefault* def);
 
 // Wrap site using tendon.
 MJAPI mjmWrap* mjm_wrapSite(mjmTendon* tendon, const char* name);
@@ -719,7 +737,7 @@ MJAPI mjmWrap* mjm_wrapJoint(mjmTendon* tendon, const char* name, double coef);
 MJAPI mjmWrap* mjm_wrapPulley(mjmTendon* tendon, double divisor);
 
 // Add actuator to model.
-MJAPI mjmActuator* mjm_addActuator(void* model, void* defspec);
+MJAPI mjmActuator* mjm_addActuator(void* model, mjmDefault* def);
 
 // Add sensor to model.
 MJAPI mjmSensor* mjm_addSensor(void* model);
@@ -739,11 +757,14 @@ MJAPI mjmKey* mjm_addKey(void* model);
 // Add plugin to model.
 MJAPI mjElement mjm_addPlugin(void* model);
 
+// Add default to model.
+MJAPI mjmDefault* mjm_addDefault(void* model, const char* classname, int parentid);
+
 // Get model from body.
 MJAPI void* mjm_getModel(mjmBody* body);
 
 // Get default corresponding to an mjElement.
-MJAPI void* mjm_getDefault(mjElement element);
+MJAPI mjmDefault* mjm_getDefault(mjElement element);
 
 // Find body in model by name.
 MJAPI mjmBody* mjm_findBody(void* modelspec, const char* name);
@@ -788,7 +809,7 @@ MJAPI const char* mjm_getString(mjString source);
 MJAPI const double* mjm_getDouble(mjDoubleVec source, int* size);
 
 // Set default.
-MJAPI void mjm_setDefault(mjElement element, void* defspec);
+MJAPI void mjm_setDefault(mjElement element, mjmDefault* def);
 
 // Set frame.
 MJAPI void mjm_setFrame(mjElement dest, mjmFrame* frame);
diff --git a/src/user/user_composite.cc b/src/user/user_composite.cc
index ddb7e88a..f30e9fd9 100644
--- a/src/user/user_composite.cc
+++ b/src/user/user_composite.cc
@@ -116,9 +116,16 @@ bool mjCComposite::AddDefaultJoint(char* error, int error_sz) {
       return false;
     } else {
       mjCDef jnt;
-      jnt.joint.spec.group = 3;
+      jnt.spec.joint->group = 3;
       defjoint[(mjtCompKind)i].push_back(jnt);
     }
+
+    // TODO: push_back might invoke a copy constructor, we undo its effect on pointers here
+    for (int i=0; i<mjNCOMPKINDS; i++) {
+      for (int j=0; j<defjoint[(mjtCompKind)i].size(); j++) {
+        defjoint[(mjtCompKind)i][j].PointToLocal();
+      }
+    }
   }
   return true;
 }
@@ -137,9 +144,9 @@ void mjCComposite::SetDefault(void) {
 
   // set all deafult groups to 3
   for (int i=0; i<mjNCOMPKINDS; i++) {
-    def[i].geom.spec.group = 3;
-    def[i].site.spec.group = 3;
-    def[i].tendon.spec.group = 3;
+    def[i].spec.geom->group = 3;
+    def[i].spec.site->group = 3;
+    def[i].spec.tendon->group = 3;
   }
 
   // set default joint
@@ -153,8 +160,8 @@ void mjCComposite::SetDefault(void) {
       type==mjCOMPTYPE_CABLE      ||
       (type==mjCOMPTYPE_GRID && tmpdim==1)) {
     for (int i=0; i<mjNCOMPKINDS; i++) {
-      def[i].geom.spec.group = 0;
-      def[i].tendon.spec.group = 0;
+      def[i].spec.geom->group = 0;
+      def[i].spec.tendon->group = 0;
     }
   }
 
@@ -163,15 +170,15 @@ void mjCComposite::SetDefault(void) {
   case mjCOMPTYPE_PARTICLE:       // particle
 
     // no friction with anything
-    def[0].geom.spec.condim = 1;
-    def[0].geom.spec.priority = 1;
+    def[0].spec.geom->condim = 1;
+    def[0].spec.geom->priority = 1;
     break;
 
   case mjCOMPTYPE_GRID:           // grid
 
     // hard main tendon fix
-    AdjustSoft(def[mjCOMPKIND_TENDON].equality.spec.solref,
-               def[mjCOMPKIND_TENDON].equality.spec.solimp, 0);
+    AdjustSoft(def[mjCOMPKIND_TENDON].spec.equality->solref,
+               def[mjCOMPKIND_TENDON].spec.equality->solimp, 0);
 
     break;
 
@@ -193,19 +200,19 @@ void mjCComposite::SetDefault(void) {
   case mjCOMPTYPE_ELLIPSOID:
 
     // no self-collisions
-    def[0].geom.spec.contype = 0;
+    def[0].spec.geom->contype = 0;
 
     // soft smoothing
     AdjustSoft(solrefsmooth, solimpsmooth, 1);
 
     // soft fix everywhere
     for (int i=0; i<mjNCOMPKINDS; i++) {
-      AdjustSoft(def[i].equality.spec.solref, def[i].equality.spec.solimp, 1);
+      AdjustSoft(def[i].spec.equality->solref, def[i].spec.equality->solimp, 1);
     }
 
     // hard main tendon fix
-    AdjustSoft(def[mjCOMPKIND_TENDON].equality.spec.solref,
-               def[mjCOMPKIND_TENDON].equality.spec.solimp, 0);
+    AdjustSoft(def[mjCOMPKIND_TENDON].spec.equality->solref,
+               def[mjCOMPKIND_TENDON].spec.equality->solimp, 0);
     break;
   default:
     // SHOULD NOT OCCUR
@@ -219,9 +226,9 @@ void mjCComposite::SetDefault(void) {
 // make composite object
 bool mjCComposite::Make(mjCModel* model, mjmBody* body, char* error, int error_sz) {
   // check geom type
-  if ((def[0].geom.spec.type!=mjGEOM_SPHERE &&
-       def[0].geom.spec.type!=mjGEOM_CAPSULE &&
-       def[0].geom.spec.type!=mjGEOM_ELLIPSOID) &&
+  if ((def[0].spec.geom->type!=mjGEOM_SPHERE &&
+       def[0].spec.geom->type!=mjGEOM_CAPSULE &&
+       def[0].spec.geom->type!=mjGEOM_ELLIPSOID) &&
       type!=mjCOMPTYPE_PARTICLE && type!=mjCOMPTYPE_CABLE) {
     return comperr(error, "Composite geom type must be sphere, capsule or ellipsoid", error_sz);
   }
@@ -240,8 +247,8 @@ bool mjCComposite::Make(mjCModel* model, mjmBody* body, char* error, int error_s
 
   // check spacing
   if (type==mjCOMPTYPE_GRID || (type==mjCOMPTYPE_PARTICLE && uservert.empty())) {
-    if (spacing < mju_max(def[0].geom.spec.size[0],
-                  mju_max(def[0].geom.spec.size[1], def[0].geom.spec.size[2]))) {
+    if (spacing < mju_max(def[0].spec.geom->size[0],
+                  mju_max(def[0].spec.geom->size[1], def[0].spec.geom->size[2]))) {
       return comperr(error, "Spacing must be larger than geometry size",
                      error_sz);
     }
@@ -342,8 +349,8 @@ bool mjCComposite::MakeParticle(mjCModel* model, mjmBody* body, char* error, int
 
   // populate vertices and names
   if (uservert.empty()) {
-    if (spacing < mju_max(def[0].geom.spec.size[0],
-                  mju_max(def[0].geom.spec.size[1], def[0].geom.spec.size[2])))
+    if (spacing < mju_max(def[0].spec.geom->size[0],
+                  mju_max(def[0].spec.geom->size[1], def[0].spec.geom->size[2])))
       return comperr(error, "Spacing must be larger than geometry size", error_sz);
 
     for (int ix=0; ix<count[0]; ix++) {
@@ -466,7 +473,7 @@ bool mjCComposite::MakeParticle(mjCModel* model, mjmBody* body, char* error, int
     // add slider joints if none defined
     if (!add[mjCOMPKIND_PARTICLE]) {
       for (int i=0; i<3; i++) {
-        mjmJoint* jnt = mjm_addJoint(b, &defjoint[mjCOMPKIND_JOINT][0]);
+        mjmJoint* jnt = mjm_addJoint(b, &defjoint[mjCOMPKIND_JOINT][0].spec);
         mjm_setDefault(jnt->element, mjm_getDefault(body->element));
         jnt->type = mjJNT_SLIDE;
         mjuu_setvec(jnt->pos, 0, 0, 0);
@@ -478,17 +485,17 @@ bool mjCComposite::MakeParticle(mjCModel* model, mjmBody* body, char* error, int
     // add user-specified joints
     else {
       for (auto defjnt : defjoint[mjCOMPKIND_PARTICLE]) {
-        mjmJoint* jnt = mjm_addJoint(b, &defjnt);
+        mjmJoint* jnt = mjm_addJoint(b, &defjnt.spec);
         mjm_setDefault(jnt->element, mjm_getDefault(body->element));
       }
     }
 
     // add geom
-    mjmGeom* g = mjm_addGeom(b, def);
+    mjmGeom* g = mjm_addGeom(b, &def[0].spec);
     mjm_setDefault(g->element, mjm_getDefault(body->element));
 
     // add site
-    mjmSite* s = mjm_addSite(b, def);
+    mjmSite* s = mjm_addSite(b, &def[0].spec);
     mjm_setDefault(s->element, mjm_getDefault(body->element));
     s->type = mjGEOM_SPHERE;
     mju::sprintf_arr(txt, "%sS%d", prefix.c_str(), i);
@@ -544,16 +551,16 @@ bool mjCComposite::MakeParticle(mjCModel* model, mjmBody* body, char* error, int
       mju::sprintf_arr(txt2, "%sS%d", prefix.c_str(), v1);
 
       // create tendon
-      mjmTendon* ten = mjm_addTendon(model, def + mjCOMPKIND_TENDON);
-      mjm_setDefault(ten->element, model->defaults[0]);
+      mjmTendon* ten = mjm_addTendon(model, &def[mjCOMPKIND_TENDON].spec);
+      mjm_setDefault(ten->element, &model->defaults[0]->spec);
       mjm_setString(ten->name, txt0);
       ten->group = 4;
       mjm_wrapSite(ten, txt1);
       mjm_wrapSite(ten, txt2);
 
       // add equality constraint
-      mjmEquality* eq = mjm_addEquality(model, def + mjCOMPKIND_TENDON);
-      mjm_setDefault(eq->element, model->defaults[0]);
+      mjmEquality* eq = mjm_addEquality(model, &def[mjCOMPKIND_TENDON].spec);
+      mjm_setDefault(eq->element, &model->defaults[0]->spec);
       eq->type = mjEQ_TENDON;
       mjm_setString(eq->name1, mjm_getString(ten->name));
     }
@@ -609,14 +616,14 @@ bool mjCComposite::MakeGrid(mjCModel* model, mjmBody* body, char* error, int err
       b->pos[2] = offset[2];
 
       // add geom
-      mjmGeom* g = mjm_addGeom(b, def);
+      mjmGeom* g = mjm_addGeom(b, &def[0].spec);
       mjm_setDefault(g->element, mjm_getDefault(body->element));
       g->type = mjGEOM_SPHERE;
       mju::sprintf_arr(txt, "%sG%d_%d", prefix.c_str(), ix, iy);
       mjm_setString(g->name, txt);
 
       // add site
-      mjmSite* s = mjm_addSite(b, def);
+      mjmSite* s = mjm_addSite(b, &def[0].spec);
       mjm_setDefault(s->element, mjm_getDefault(body->element));
       s->type = mjGEOM_SPHERE;
       mju::sprintf_arr(txt, "%sS%d_%d", prefix.c_str(), ix, iy);
@@ -637,7 +644,7 @@ bool mjCComposite::MakeGrid(mjCModel* model, mjmBody* body, char* error, int err
       // add slider joint
       mjmJoint* jnt[3];
       for (int i=0; i<3; i++) {
-        jnt[i] = mjm_addJoint(b, &defjoint[mjCOMPKIND_JOINT][0]);
+        jnt[i] = mjm_addJoint(b, &defjoint[mjCOMPKIND_JOINT][0].spec);
         mjm_setDefault(jnt[i]->element, mjm_getDefault(body->element));
         mju::sprintf_arr(txt, "%sJ%d_%d_%d", prefix.c_str(), i, ix, iy);
         mjm_setString(jnt[i]->name, txt);
@@ -666,8 +673,8 @@ bool mjCComposite::MakeGrid(mjCModel* model, mjmBody* body, char* error, int err
         ten->WrapSite(txt2);
 
         // add equality constraint
-        mjmEquality* eq = mjm_addEquality(model, def + mjCOMPKIND_TENDON);
-        mjm_setDefault(eq->element, model->defaults[0]);
+        mjmEquality* eq = mjm_addEquality(model, &def[mjCOMPKIND_TENDON].spec);
+        mjm_setDefault(eq->element, &model->defaults[0]->spec);
         eq->type = mjEQ_TENDON;
         mjm_setString(eq->name1, ten->name.c_str());
       }
@@ -700,9 +707,9 @@ bool mjCComposite::MakeCable(mjCModel* model, mjmBody* body, char* error, int er
   }
 
   // check geom type
-  if (def[0].geom.spec.type!=mjGEOM_CYLINDER &&
-      def[0].geom.spec.type!=mjGEOM_CAPSULE &&
-      def[0].geom.spec.type!=mjGEOM_BOX) {
+  if (def[0].spec.geom->type!=mjGEOM_CYLINDER &&
+      def[0].spec.geom->type!=mjGEOM_CAPSULE &&
+      def[0].spec.geom->type!=mjGEOM_BOX) {
     return comperr(error, "Cable geom type must be sphere, capsule or box", error_sz);
   }
 
@@ -748,14 +755,14 @@ bool mjCComposite::MakeCable(mjCModel* model, mjmBody* body, char* error, int er
   }
 
   // add skin
-  if (def[0].geom.spec.type==mjGEOM_BOX) {
+  if (def[0].spec.geom->type==mjGEOM_BOX) {
     if (skinsubgrid>0) {
       count[1]+=2;
-      MakeSkin2Subgrid(model, 2*def[0].geom.spec.size[2]);
+      MakeSkin2Subgrid(model, 2*def[0].spec.geom->size[2]);
       count[1]-=2;
     } else {
       count[1]++;
-      MakeSkin2(model, 2*def[0].geom.spec.size[2]);
+      MakeSkin2(model, 2*def[0].spec.geom->size[2]);
       count[1]--;
     }
   }
@@ -835,14 +842,14 @@ mjmBody* mjCComposite::AddCableBody(mjCModel* model, mjmBody* body, int ix, mjtN
   }
 
   // add geom
-  mjmGeom* geom = mjm_addGeom(body, def);
+  mjmGeom* geom = mjm_addGeom(body, &def[0].spec);
   mjm_setDefault(geom->element, mjm_getDefault(body->element));
   mjm_setString(geom->name, txt_geom);
-  if (def[0].geom.spec.type==mjGEOM_CYLINDER ||
-      def[0].geom.spec.type==mjGEOM_CAPSULE) {
+  if (def[0].spec.geom->type==mjGEOM_CYLINDER ||
+      def[0].spec.geom->type==mjGEOM_CAPSULE) {
     mjuu_zerovec(geom->fromto, 6);
     geom->fromto[3] = length;
-  } else if (def[0].geom.spec.type==mjGEOM_BOX) {
+  } else if (def[0].spec.geom->type==mjGEOM_BOX) {
     mjuu_zerovec(geom->pos, 3);
     geom->pos[0] = length/2;
     geom->size[0] = length/2;
@@ -862,7 +869,7 @@ mjmBody* mjCComposite::AddCableBody(mjCModel* model, mjmBody* body, int ix, mjtN
 
   // add curvature joint
   if (!first || strcmp(initial.c_str(), "none")) {
-    mjmJoint* jnt = mjm_addJoint(body, &defjoint[mjCOMPKIND_JOINT][0]);
+    mjmJoint* jnt = mjm_addJoint(body, &defjoint[mjCOMPKIND_JOINT][0].spec);
     mjm_setDefault(jnt->element, mjm_getDefault(body->element));
     jnt->type = (first && strcmp(initial.c_str(), "free")==0) ? mjJNT_FREE : mjJNT_BALL;
     jnt->damping = jnt->type==mjJNT_FREE ? 0 : jnt->damping;
@@ -880,7 +887,7 @@ mjmBody* mjCComposite::AddCableBody(mjCModel* model, mjmBody* body, int ix, mjtN
 
   // add site at the boundary
   if (last || first) {
-    mjmSite* site = mjm_addSite(body, def);
+    mjmSite* site = mjm_addSite(body, &def[0].spec);
     mjm_setDefault(site->element, mjm_getDefault(body->element));
     mjm_setString(site->name, txt_site);
     mjuu_setvec(site->pos, last ? length : 0, 0, 0);
@@ -991,7 +998,7 @@ mjmBody* mjCComposite::AddRopeBody(mjCModel* model, mjmBody* body, int ix, int i
   }
 
   // add geom
-  mjmGeom* geom = mjm_addGeom(body, def);
+  mjmGeom* geom = mjm_addGeom(body, &def[0].spec);
   mjm_setDefault(geom->element, mjm_getDefault(body->element));
   mju::sprintf_arr(txt, "%sG%d", prefix.c_str(), ix1);
   mjm_setString(geom->name, txt);
@@ -1006,7 +1013,7 @@ mjmBody* mjCComposite::AddRopeBody(mjCModel* model, mjmBody* body, int ix, int i
   // add main joint
   for (int i=0; i<2; i++) {
     // add joint
-    mjmJoint* jnt = mjm_addJoint(body, &defjoint[mjCOMPKIND_JOINT][0]);
+    mjmJoint* jnt = mjm_addJoint(body, &defjoint[mjCOMPKIND_JOINT][0].spec);
     mjm_setDefault(jnt->element, mjm_getDefault(body->element));
     mju::sprintf_arr(txt, "%sJ%d_%d", prefix.c_str(), i, ix1);
     mjm_setString(jnt->name, txt);
@@ -1019,7 +1026,7 @@ mjmBody* mjCComposite::AddRopeBody(mjCModel* model, mjmBody* body, int ix, int i
   // add twist joint
   if (add[mjCOMPKIND_TWIST]) {
     // add joint
-    mjmJoint* jnt = mjm_addJoint(body, &defjoint[mjCOMPKIND_TWIST][0]);
+    mjmJoint* jnt = mjm_addJoint(body, &defjoint[mjCOMPKIND_TWIST][0].spec);
     mjm_setDefault(jnt->element, mjm_getDefault(body->element));
     mju::sprintf_arr(txt, "%sJT%d", prefix.c_str(), ix1);
     mjm_setString(jnt->name, txt);
@@ -1028,8 +1035,8 @@ mjmBody* mjCComposite::AddRopeBody(mjCModel* model, mjmBody* body, int ix, int i
     mjuu_setvec(jnt->axis, 1, 0, 0);
 
     // add constraint
-    mjmEquality* eq = mjm_addEquality(model, def + mjCOMPKIND_TWIST);
-    mjm_setDefault(eq->element, model->defaults[0]);
+    mjmEquality* eq = mjm_addEquality(model, &def[mjCOMPKIND_TWIST].spec);
+    mjm_setDefault(eq->element, &model->defaults[0]->spec);
     eq->type = mjEQ_JOINT;
     mjm_setString(eq->name1, mjm_getString(jnt->name));
   }
@@ -1037,7 +1044,7 @@ mjmBody* mjCComposite::AddRopeBody(mjCModel* model, mjmBody* body, int ix, int i
   // add stretch joint
   if (add[mjCOMPKIND_STRETCH]) {
     // add joint
-    mjmJoint* jnt = mjm_addJoint(body, &defjoint[mjCOMPKIND_STRETCH][0]);
+    mjmJoint* jnt = mjm_addJoint(body, &defjoint[mjCOMPKIND_STRETCH][0].spec);
     mjm_setDefault(jnt->element, mjm_getDefault(body->element));
     mju::sprintf_arr(txt, "%sJS%d", prefix.c_str(), ix1);
     mjm_setString(jnt->name, txt);
@@ -1046,8 +1053,8 @@ mjmBody* mjCComposite::AddRopeBody(mjCModel* model, mjmBody* body, int ix, int i
     mjuu_setvec(jnt->axis, 1, 0, 0);
 
     // add constraint
-    mjmEquality* eq = mjm_addEquality(model, def + mjCOMPKIND_STRETCH);
-    mjm_setDefault(eq->element, model->defaults[0]);
+    mjmEquality* eq = mjm_addEquality(model, &def[mjCOMPKIND_STRETCH].spec);
+    mjm_setDefault(eq->element,  &model->defaults[0]->spec);
     eq->type = mjEQ_JOINT;
     mjm_setString(eq->name1, mjm_getString(jnt->name));
   }
@@ -1103,7 +1110,7 @@ bool mjCComposite::MakeBox(mjCModel* model, mjmBody* body, char* error, int erro
   }
 
   // center geom: two times bigger
-  mjmGeom* geom = mjm_addGeom(body, def);
+  mjmGeom* geom = mjm_addGeom(body, &def[0].spec);
   mjm_setDefault(geom->element, mjm_getDefault(body->element));
   geom->type = mjGEOM_SPHERE;
   mju::sprintf_arr(txt, "%sGcenter", prefix.c_str());
@@ -1144,7 +1151,7 @@ bool mjCComposite::MakeBox(mjCModel* model, mjmBody* body, char* error, int erro
           mjuu_normvec(b->alt.zaxis, 3);
 
           // add geom
-          mjmGeom* g = mjm_addGeom(b, def);
+          mjmGeom* g = mjm_addGeom(b, &def[0].spec);
           mjm_setDefault(g->element, mjm_getDefault(body->element));
           mju::sprintf_arr(txt, "%sG%d_%d_%d", prefix.c_str(), ix, iy, iz);
           mjm_setString(g->name, txt);
@@ -1158,7 +1165,7 @@ bool mjCComposite::MakeBox(mjCModel* model, mjmBody* body, char* error, int erro
           }
 
           // add slider joint
-          mjmJoint* jnt = mjm_addJoint(b, &defjoint[mjCOMPKIND_JOINT][0]);
+          mjmJoint* jnt = mjm_addJoint(b, &defjoint[mjCOMPKIND_JOINT][0].spec);
           mjm_setDefault(jnt->element, mjm_getDefault(body->element));
           mju::sprintf_arr(txt, "%sJ%d_%d_%d", prefix.c_str(), ix, iy, iz);
           mjm_setString(jnt->name, txt);
@@ -1167,8 +1174,8 @@ bool mjCComposite::MakeBox(mjCModel* model, mjmBody* body, char* error, int erro
           mjuu_setvec(jnt->axis, 0, 0, 1);
 
           // add fix constraint
-          mjmEquality* eq = mjm_addEquality(model, def + mjCOMPKIND_JOINT);
-          mjm_setDefault(eq->element, model->defaults[0]);
+          mjmEquality* eq = mjm_addEquality(model, &def[mjCOMPKIND_JOINT].spec);
+          mjm_setDefault(eq->element,  &model->defaults[0]->spec);
           eq->type = mjEQ_JOINT;
           mjm_setString(eq->name1, mjm_getString(jnt->name));
 
@@ -1201,8 +1208,8 @@ bool mjCComposite::MakeBox(mjCModel* model, mjmBody* body, char* error, int erro
   }
 
   // finalize fixed tendon
-  mjmEquality* eqt = mjm_addEquality(model, def + mjCOMPKIND_TENDON);
-  mjm_setDefault(eqt->element, model->defaults[0]);
+  mjmEquality* eqt = mjm_addEquality(model, &def[mjCOMPKIND_TENDON].spec);
+  mjm_setDefault(eqt->element, &model->defaults[0]->spec);
   eqt->type = mjEQ_TENDON;
   mjm_setString(eqt->name1, ten->name.c_str());
 
@@ -1237,8 +1244,8 @@ void mjCComposite::MakeShear(mjCModel* model) {
       ten->name = txt;
 
       // equality constraint
-      mjmEquality* eq = mjm_addEquality(model, def + mjCOMPKIND_SHEAR);
-      mjm_setDefault(eq->element, model->defaults[0]);
+      mjmEquality* eq = mjm_addEquality(model, &def[mjCOMPKIND_SHEAR].spec);
+      mjm_setDefault(eq->element, &model->defaults[0]->spec);
       eq->type = mjEQ_TENDON;
       mjm_setString(eq->name1, txt);
     }
@@ -1495,15 +1502,15 @@ void mjCComposite::MakeCableBones(mjCModel* model, mjmSkin* skin) {
       // bind pose
       if (iy==0) {
         mjm_appendString(skin->bodyname, this_body);
-        bindpos.push_back((ix==count[0]-1) ? -2*def[0].geom.spec.size[0] : 0);
-        bindpos.push_back(-def[0].geom.spec.size[1]);
+        bindpos.push_back((ix==count[0]-1) ? -2*def[0].spec.geom->size[0] : 0);
+        bindpos.push_back(-def[0].spec.geom->size[1]);
         bindpos.push_back(0);
         bindquat.push_back(1); bindquat.push_back(0);
         bindquat.push_back(0); bindquat.push_back(0);
       } else {
         mjm_appendString(skin->bodyname, this_body);
-        bindpos.push_back((ix==count[0]-1) ? -2*def[0].geom.spec.size[0] : 0);
-        bindpos.push_back(def[0].geom.spec.size[1]);
+        bindpos.push_back((ix==count[0]-1) ? -2*def[0].spec.geom->size[0] : 0);
+        bindpos.push_back(def[0].spec.geom->size[1]);
         bindpos.push_back(0);
         bindquat.push_back(1); bindquat.push_back(0);
         bindquat.push_back(0); bindquat.push_back(0);
@@ -1535,15 +1542,15 @@ void mjCComposite::MakeCableBonesSubgrid(mjCModel* model, mjmSkin* skin) {
 
       // bind pose
       if (iy==0) {
-        bindpos.push_back((ix==count[0]-1) ? -2*def[0].geom.spec.size[0] : 0);
+        bindpos.push_back((ix==count[0]-1) ? -2*def[0].spec.geom->size[0] : 0);
         bindpos.push_back(-def[0].geom.spec.size[1]);
         bindpos.push_back(0);
       } else if (iy==2) {
-        bindpos.push_back((ix==count[0]-1) ? -2*def[0].geom.spec.size[0] : 0);
+        bindpos.push_back((ix==count[0]-1) ? -2*def[0].spec.geom->size[0] : 0);
         bindpos.push_back(def[0].geom.spec.size[1]);
         bindpos.push_back(0);
       } else {
-        bindpos.push_back((ix==count[0]-1) ? -2*def[0].geom.spec.size[0] : 0);
+        bindpos.push_back((ix==count[0]-1) ? -2*def[0].spec.geom->size[0] : 0);
         bindpos.push_back(0);
         bindpos.push_back(0);
       }
diff --git a/src/user/user_flexcomp.cc b/src/user/user_flexcomp.cc
index 06bda286..ef298c05 100644
--- a/src/user/user_flexcomp.cc
+++ b/src/user/user_flexcomp.cc
@@ -78,8 +78,7 @@ mjCFlexcomp::mjCFlexcomp(void) {
 
 // make flexcomp object
 bool mjCFlexcomp::Make(mjCModel* model, mjmBody* body, char* error, int error_sz) {
-  def.flex.CopyFromSpec();
-
+  mjmFlex* dflex = def.spec.flex;
   bool radial = (type==mjFCOMPTYPE_BOX ||
                  type==mjFCOMPTYPE_CYLINDER ||
                  type==mjFCOMPTYPE_ELLIPSOID);
@@ -100,7 +99,7 @@ bool mjCFlexcomp::Make(mjCModel* model, mjmBody* body, char* error, int error_sz
   }
 
   // check spacing
-  double minspace = 2*def.flex.spec.radius + def.flex.spec.margin;
+  double minspace = 2*dflex->radius + dflex->margin;
   if (!direct) {
     if (spacing[0]<minspace ||
         spacing[1]<minspace ||
@@ -158,7 +157,7 @@ bool mjCFlexcomp::Make(mjCModel* model, mjmBody* body, char* error, int error_sz
   }
 
   // get dim and check
-  int dim = def.flex.spec.dim;
+  int dim = dflex->dim;
   if (dim<1 || dim>3) {
     return comperr(error, "Invalid dim, must be between 1 and 3", error_sz);
   }
@@ -166,7 +165,7 @@ bool mjCFlexcomp::Make(mjCModel* model, mjmBody* body, char* error, int error_sz
   // force flatskin shading for box, cylinder and 3D grid
   if (type==mjFCOMPTYPE_BOX || type==mjFCOMPTYPE_CYLINDER ||
       (type==mjFCOMPTYPE_GRID && dim==3)) {
-    def.flex.spec.flatskin = true;
+    dflex->flatskin = true;
   }
 
   // check pin sizes
@@ -485,8 +484,8 @@ bool mjCFlexcomp::Make(mjCModel* model, mjmBody* body, char* error, int error_sz
 
   // create edge equality constraint
   if (equality) {
-    mjmEquality* pe = mjm_addEquality(model, &def);
-    mjm_setDefault(pe->element, model->defaults[0]);
+    mjmEquality* pe = mjm_addEquality(model, &def.spec);
+    mjm_setDefault(pe->element, &model->defaults[0]->spec);
     pe->type = mjEQ_FLEX;
     pe->active = true;
     mjm_setString(pe->name1, name.c_str());
@@ -690,7 +689,7 @@ bool mjCFlexcomp::MakeBox(char* error, int error_sz) {
   double pos[3];
 
   // set 3D
-  def.flex.spec.dim = 3;
+  def.spec.flex->dim = 3;
 
   // add center point
   point.push_back(0);
@@ -822,7 +821,7 @@ bool mjCFlexcomp::MakeMesh(mjCModel* model, char* error, int error_sz) {
   }
 
   // check dim
-  if (def.flex.spec.dim!=2) {
+  if (def.spec.flex->dim!=2) {
     return comperr(error, "Flex dim must be 2 in for mesh", error_sz);
   }
 
@@ -1034,7 +1033,7 @@ void mjCFlexcomp::LoadGMSH(mjCModel* model, mjResource* resource) {
     if (entityDim<1 || entityDim>3) {
       throw mjCError(NULL, "Entity must be 1D, 2D or 3D");
     }
-    def.flex.spec.dim = entityDim;
+    def.spec.flex->dim = entityDim;
 
     // read and discard node tags; require range from minNodeTag to maxNodeTag
     for (size_t i=0; i<numNodes; i++) {
@@ -1086,7 +1085,7 @@ void mjCFlexcomp::LoadGMSH(mjCModel* model, mjResource* resource) {
     if (entityDim<1 || entityDim>3) {
       throw mjCError(NULL, "Entity must be 1D, 2D or 3D");
     }
-    def.flex.spec.dim = entityDim;
+    def.spec.flex->dim = entityDim;
 
     // check section byte size
     if (nodeend-nodebegin < 52+numNodes*4*8) {
@@ -1134,7 +1133,7 @@ void mjCFlexcomp::LoadGMSH(mjCModel* model, mjResource* resource) {
     }
 
     // dimensionality must be same as nodes
-    if (entityDim!=def.flex.spec.dim) {
+    if (entityDim!=def.spec.flex->dim) {
       throw mjCError(NULL, "Inconsistent dimensionality in Elements");
     }
 
@@ -1183,7 +1182,7 @@ void mjCFlexcomp::LoadGMSH(mjCModel* model, mjResource* resource) {
     }
 
     // dimensionality must be same as nodes
-    if (entityDim!=def.flex.spec.dim) {
+    if (entityDim!=def.spec.flex->dim) {
       throw mjCError(NULL, "Inconsistent dimensionality in Elements");
     }
 
diff --git a/src/user/user_objects.cc b/src/user/user_objects.cc
index cf19599d..f017995c 100644
--- a/src/user/user_objects.cc
+++ b/src/user/user_objects.cc
@@ -471,6 +471,19 @@ mjCDef::mjCDef(void) {
   name.clear();
   parentid = -1;
   childid.clear();
+  mjm_defaultJoint(joint.spec);
+  mjm_defaultGeom(geom.spec);
+  mjm_defaultSite(site.spec);
+  mjm_defaultCamera(camera.spec);
+  mjm_defaultLight(light.spec);
+  mjm_defaultFlex(flex.spec);
+  mjm_defaultMesh(mesh.spec);
+  mjm_defaultMaterial(material.spec);
+  mjm_defaultPair(pair.spec);
+  mjm_defaultEquality(equality.spec);
+  mjm_defaultTendon(tendon.spec);
+  mjm_defaultActuator(actuator.spec);
+  PointToLocal();
 }
 
 
@@ -494,6 +507,28 @@ void mjCDef::PointToLocal() {
   geom.PointToLocal();
   site.PointToLocal();
   camera.PointToLocal();
+  light.PointToLocal();
+  flex.PointToLocal();
+  mesh.PointToLocal();
+  material.PointToLocal();
+  pair.PointToLocal();
+  equality.PointToLocal();
+  tendon.PointToLocal();
+  actuator.PointToLocal();
+  spec.element = (mjElement)this;
+  spec.name = (mjString)&name;
+  spec.joint = &joint.spec;
+  spec.geom = &geom.spec;
+  spec.site = &site.spec;
+  spec.camera = &camera.spec;
+  spec.light = &light.spec;
+  spec.flex = &flex.spec;
+  spec.mesh = &mesh.spec;
+  spec.material = &material.spec;
+  spec.pair = &pair.spec;
+  spec.equality = &equality.spec;
+  spec.tendon = &tendon.spec;
+  spec.actuator = &actuator.spec;
 }
 
 
@@ -1248,7 +1283,6 @@ mjCJoint::mjCJoint(mjCModel* _model, mjCDef* _def) {
 
   // reset to default if given
   if (_def) {
-    _def->joint.CopyFromSpec();
     *this = _def->joint;
   }
 
@@ -1442,7 +1476,6 @@ mjCGeom::mjCGeom(mjCModel* _model, mjCDef* _def) {
 
   // reset to default if given
   if (_def) {
-    _def->geom.CopyFromSpec();
     *this = _def->geom;
   }
 
@@ -2043,7 +2076,6 @@ mjCSite::mjCSite(mjCModel* _model, mjCDef* _def) {
 
   // reset to default if given
   if (_def) {
-    _def->site.CopyFromSpec();
     *this = _def->site;
   }
 
@@ -2185,7 +2217,6 @@ mjCCamera::mjCCamera(mjCModel* _model, mjCDef* _def) {
 
   // reset to default if given
   if (_def) {
-    _def->camera.CopyFromSpec();
     *this = _def->camera;
   }
 
@@ -2322,7 +2353,6 @@ mjCLight::mjCLight(mjCModel* _model, mjCDef* _def) {
 
   // reset to default if given
   if (_def) {
-    _def->light.CopyFromSpec();
     *this = _def->light;
   }
 
@@ -2330,14 +2360,18 @@ mjCLight::mjCLight(mjCModel* _model, mjCDef* _def) {
   model = _model;
   def = (_def ? _def : (_model ? _model->defaults[0] : 0));
 
-  // point to local
+  PointToLocal();
+  CopyFromSpec();
+}
+
+
+
+void mjCLight::PointToLocal() {
   spec.element = (mjElement)this;
   spec.name = (mjString)&name;
   spec.classname = (mjString)&classname;
   spec.targetbody = (mjString)&spec_targetbody_;
   spec.info = (mjString)&info;
-
-  CopyFromSpec();
 }
 
 
@@ -3350,7 +3384,6 @@ mjCMaterial::mjCMaterial(mjCModel* _model, mjCDef* _def) {
 
   // reset to default if given
   if (_def) {
-    _def->material.CopyFromSpec();
     *this = _def->material;
   }
 
@@ -3409,7 +3442,6 @@ mjCPair::mjCPair(mjCModel* _model, mjCDef* _def) {
 
   // reset to default if given
   if (_def) {
-    _def->pair.CopyFromSpec();
     *this = _def->pair;
   }
 
@@ -3669,7 +3701,6 @@ mjCEquality::mjCEquality(mjCModel* _model, mjCDef* _def) {
 
   // reset to default if given
   if (_def) {
-    _def->equality.CopyFromSpec();
     *this = _def->equality;
   }
 
@@ -4185,7 +4216,7 @@ mjCActuator::mjCActuator(mjCModel* _model, mjCDef* _def) {
   CopyFromSpec();
 
   // point to local (needs to be after defaults)
-  MakePointerLocal();
+  PointToLocal();
 }
 
 
@@ -4196,7 +4227,7 @@ bool mjCActuator::is_actlimited() const { return islimited(actlimited, actrange)
 
 
 
-void mjCActuator::MakePointerLocal() {
+void mjCActuator::PointToLocal() {
   spec.element = (mjElement)this;
   spec.name = (mjString)&name;
   spec.classname = (mjString)&classname;
diff --git a/src/user/user_objects.h b/src/user/user_objects.h
index feaec000..804ec564 100644
--- a/src/user/user_objects.h
+++ b/src/user/user_objects.h
@@ -534,6 +534,7 @@ class mjCLight : public mjCBase, private mjmLight {
   mjCLight(mjCModel* = 0, mjCDef* = 0);   // constructor
   void Compile(void);                     // compiler
   void CopyFromSpec(void);
+  void PointToLocal(void);
 
   mjCBody* body;                  // light's body
   int targetbodyid;               // id of target body; -1: none
@@ -1203,7 +1204,7 @@ class mjCActuator : public mjCBase, private mjmActuator {
   mjCActuator(mjCModel* = 0, mjCDef* = 0);  // constructor
   void Compile(void);                       // compiler
   void CopyFromSpec();
-  void MakePointerLocal();
+  void PointToLocal();
 
   int trnid[2];                   // id of transmission target
 
@@ -1385,6 +1386,8 @@ class mjCKey : public mjCBase, private mjmKey {
 // Describes one set of defaults
 
 class mjCDef {
+  friend class mjXWriter;
+
  public:
   mjCDef(void);                           // constructor
   void Compile(const mjCModel* model);    // compiler
@@ -1395,7 +1398,9 @@ class mjCDef {
   int parentid;                   // id of parent class
   std::vector<int> childid;       // ids of child classes
 
-  // default objects
+  mjmDefault spec;
+
+  // default objects (TODO: they should become private)
   mjCJoint    joint;
   mjCGeom     geom;
   mjCSite     site;
diff --git a/src/xml/xml_native_reader.cc b/src/xml/xml_native_reader.cc
index 3557a295..13834b72 100644
--- a/src/xml/xml_native_reader.cc
+++ b/src/xml/xml_native_reader.cc
@@ -1665,7 +1665,7 @@ void mjXReader::OneGeom(XMLElement* elem, mjmGeom* pgeom) {
 
 
 // site element parser
-void mjXReader::OneSite(XMLElement* elem, mjmSite& site) {
+void mjXReader::OneSite(XMLElement* elem, mjmSite* site) {
   int n;
   string text, name, classname;
   std::vector<double> userdata;
@@ -1673,30 +1673,30 @@ void mjXReader::OneSite(XMLElement* elem, mjmSite& site) {
 
   // read attributes
   if (ReadAttrTxt(elem, "name", name)) {
-    mjm_setString(site.name, name.c_str());
+    mjm_setString(site->name, name.c_str());
   }
   if (ReadAttrTxt(elem, "class", classname)) {
-    mjm_setString(site.classname, classname.c_str());
+    mjm_setString(site->classname, classname.c_str());
   }
   if (MapValue(elem, "type", &n, geom_map, mjNGEOMTYPES)) {
-    site.type = (mjtGeom)n;
+    site->type = (mjtGeom)n;
   }
-  ReadAttr(elem, "size", 3, site.size, text, false, false);
-  ReadAttrInt(elem, "group", &site.group);
-  ReadAttr(elem, "pos", 3, site.pos, text);
-  ReadQuat(elem, "quat", site.quat, text);
+  ReadAttr(elem, "size", 3, site->size, text, false, false);
+  ReadAttrInt(elem, "group", &site->group);
+  ReadAttr(elem, "pos", 3, site->pos, text);
+  ReadQuat(elem, "quat", site->quat, text);
   if (ReadAttrTxt(elem, "material", material)) {
-    mjm_setString(site.material, material.c_str());
+    mjm_setString(site->material, material.c_str());
   }
-  ReadAttr(elem, "rgba", 4, site.rgba, text);
-  ReadAttr(elem, "fromto", 6, site.fromto, text);
-  ReadAlternative(elem, site.alt);
+  ReadAttr(elem, "rgba", 4, site->rgba, text);
+  ReadAttr(elem, "fromto", 6, site->fromto, text);
+  ReadAlternative(elem, site->alt);
   if (ReadVector(elem, "user", userdata, text)) {
-    mjm_setDouble(site.userdata, userdata.data(), userdata.size());
+    mjm_setDouble(site->userdata, userdata.data(), userdata.size());
   }
 
   // write error info
-  mjm_setString(site.info,
+  mjm_setString(site->info,
       std::string("line = " + std::to_string(elem->GetLineNum()) + ", column = -1").c_str());
 }
 
@@ -2240,7 +2240,7 @@ void mjXReader::OneActuator(XMLElement* elem, mjmActuator* pact) {
 
 
 // make composite
-void mjXReader::OneComposite(XMLElement* elem, mjmBody* pbody, mjCDef* def) {
+void mjXReader::OneComposite(XMLElement* elem, mjmBody* pbody, mjmDefault* def) {
   string text;
   int n;
 
@@ -2326,7 +2326,7 @@ void mjXReader::OneComposite(XMLElement* elem, mjmBody* pbody, mjCDef* def) {
   XMLElement* egeom = FirstChildElement(elem, "geom");
   if (egeom) {
     std::string material;
-    mjmGeom& dgeom = comp.def[0].geom.spec;
+    mjmGeom& dgeom = *comp.def[0].spec.geom;
     if (MapValue(egeom, "type", &n, geom_map, mjNGEOMTYPES)) {
       dgeom.type = (mjtGeom)n;
     }
@@ -2354,7 +2354,7 @@ void mjXReader::OneComposite(XMLElement* elem, mjmBody* pbody, mjCDef* def) {
   XMLElement* esite = FirstChildElement(elem, "site");
   if (esite) {
     std::string material;
-    mjmSite& dsite = comp.def[0].site.spec;
+    mjmSite& dsite = *comp.def[0].spec.site;
     ReadAttr(esite, "size", 3, dsite.size, text, false, false);
     ReadAttrInt(esite, "group", &dsite.group);
     ReadAttrTxt(esite, "material", material);
@@ -2379,33 +2379,35 @@ void mjXReader::OneComposite(XMLElement* elem, mjmBody* pbody, mjCDef* def) {
     comp.add[kind] = true;
 
     // get element
-    mjCDef *el = &comp.defjoint[(mjtCompKind)kind].back();
+    mjmDefault* dspec = &comp.defjoint[(mjtCompKind)kind].back().spec;
+    mjmJoint& djoint = *dspec->joint;
+    mjmEquality& dequality = *dspec->equality;
 
     // particle joint
     if (MapValue(ejnt, "type", &n, joint_map, joint_sz)) {
-      el->joint.spec.type = (mjtJoint)n;
+      djoint.type = (mjtJoint)n;
     }
-    ReadAttr(ejnt, "axis", 3, el->joint.spec.axis, text);
+    ReadAttr(ejnt, "axis", 3, djoint.axis, text);
 
     // solreffix, solimpfix
-    ReadAttr(ejnt, "solreffix", mjNREF, el->equality.spec.solref, text, false, false);
-    ReadAttr(ejnt, "solimpfix", mjNIMP, el->equality.spec.solimp, text, false, false);
+    ReadAttr(ejnt, "solreffix", mjNREF, dequality.solref, text, false, false);
+    ReadAttr(ejnt, "solimpfix", mjNIMP, dequality.solimp, text, false, false);
 
     // joint attributes
-    MapValue(elem, "limited", &el->joint.spec.limited, TFAuto_map, 3);
-    ReadAttrInt(ejnt, "group", &el->joint.spec.group);
-    ReadAttr(ejnt, "solreflimit", mjNREF, el->joint.spec.solref_limit, text, false, false);
-    ReadAttr(ejnt, "solimplimit", mjNIMP, el->joint.spec.solimp_limit, text, false, false);
+    MapValue(elem, "limited", &djoint.limited, TFAuto_map, 3);
+    ReadAttrInt(ejnt, "group", &djoint.group);
+    ReadAttr(ejnt, "solreflimit", mjNREF, djoint.solref_limit, text, false, false);
+    ReadAttr(ejnt, "solimplimit", mjNIMP, djoint.solimp_limit, text, false, false);
     ReadAttr(ejnt,
-             "solreffriction", mjNREF, el->joint.spec.solref_friction, text, false, false);
+             "solreffriction", mjNREF, djoint.solref_friction, text, false, false);
     ReadAttr(ejnt,
-             "solimpfriction", mjNIMP, el->joint.spec.solimp_friction, text, false, false);
-    ReadAttr(ejnt, "stiffness", 1, &el->joint.spec.stiffness, text);
-    ReadAttr(ejnt, "range", 2, el->joint.spec.range, text);
-    ReadAttr(ejnt, "margin", 1, &el->joint.spec.margin, text);
-    ReadAttr(ejnt, "armature", 1, &el->joint.spec.armature, text);
-    ReadAttr(ejnt, "damping", 1, &el->joint.spec.damping, text);
-    ReadAttr(ejnt, "frictionloss", 1, &el->joint.spec.frictionloss, text);
+             "solimpfriction", mjNIMP, djoint.solimp_friction, text, false, false);
+    ReadAttr(ejnt, "stiffness", 1, &djoint.stiffness, text);
+    ReadAttr(ejnt, "range", 2, djoint.range, text);
+    ReadAttr(ejnt, "margin", 1, &djoint.margin, text);
+    ReadAttr(ejnt, "armature", 1, &djoint.armature, text);
+    ReadAttr(ejnt, "damping", 1, &djoint.damping, text);
+    ReadAttr(ejnt, "frictionloss", 1, &djoint.frictionloss, text);
 
     // advance
     ejnt = NextSiblingElement(ejnt, "joint");
@@ -2419,29 +2421,33 @@ void mjXReader::OneComposite(XMLElement* elem, mjmBody* pbody, mjCDef* def) {
     MapValue(eten, "kind", &kind, tkind_map, 2, true);
     comp.add[kind] = true;
 
+    // get default structs
+    mjmTendon& dtendon = *comp.def[kind].spec.tendon;
+    mjmEquality& dequality = *comp.def[kind].spec.equality;
+
     // solreffix, solimpfix
-    ReadAttr(eten, "solreffix", mjNREF, comp.def[kind].equality.spec.solref, text, false, false);
-    ReadAttr(eten, "solimpfix", mjNIMP, comp.def[kind].equality.spec.solimp, text, false, false);
+    ReadAttr(eten, "solreffix", mjNREF, dequality.solref, text, false, false);
+    ReadAttr(eten, "solimpfix", mjNIMP, dequality.solimp, text, false, false);
 
     // tendon attributes
     std::string material;
-    MapValue(elem, "limited", &comp.def[kind].tendon.spec.limited, TFAuto_map, 3);
-    ReadAttrInt(eten, "group", &comp.def[kind].tendon.spec.group);
-    ReadAttr(eten, "solreflimit", mjNREF, comp.def[kind].tendon.spec.solref_limit, text, false, false);
-    ReadAttr(eten, "solimplimit", mjNIMP, comp.def[kind].tendon.spec.solimp_limit, text, false, false);
+    MapValue(elem, "limited", &dtendon.limited, TFAuto_map, 3);
+    ReadAttrInt(eten, "group", &dtendon.group);
+    ReadAttr(eten, "solreflimit", mjNREF, dtendon.solref_limit, text, false, false);
+    ReadAttr(eten, "solimplimit", mjNIMP, dtendon.solimp_limit, text, false, false);
     ReadAttr(eten,
-             "solreffriction", mjNREF, comp.def[kind].tendon.spec.solref_friction, text, false, false);
+             "solreffriction", mjNREF, dtendon.solref_friction, text, false, false);
     ReadAttr(eten,
-             "solimpfriction", mjNIMP, comp.def[kind].tendon.spec.solimp_friction, text, false, false);
-    ReadAttr(eten, "range", 2, comp.def[kind].tendon.spec.range, text);
-    ReadAttr(eten, "margin", 1, &comp.def[kind].tendon.spec.margin, text);
-    ReadAttr(eten, "stiffness", 1, &comp.def[kind].tendon.spec.stiffness, text);
-    ReadAttr(eten, "damping", 1, &comp.def[kind].tendon.spec.damping, text);
-    ReadAttr(eten, "frictionloss", 1, &comp.def[kind].tendon.spec.frictionloss, text);
+             "solimpfriction", mjNIMP, dtendon.solimp_friction, text, false, false);
+    ReadAttr(eten, "range", 2, dtendon.range, text);
+    ReadAttr(eten, "margin", 1, &dtendon.margin, text);
+    ReadAttr(eten, "stiffness", 1, &dtendon.stiffness, text);
+    ReadAttr(eten, "damping", 1, &dtendon.damping, text);
+    ReadAttr(eten, "frictionloss", 1, &dtendon.frictionloss, text);
     ReadAttrTxt(eten, "material", material);
-    mjm_setString(comp.def[kind].tendon.spec.material, material.c_str());
-    ReadAttr(eten, "rgba", 4, comp.def[kind].tendon.spec.rgba, text);
-    ReadAttr(eten, "width", 1, &comp.def[kind].tendon.spec.width, text);
+    mjm_setString(dtendon.material, material.c_str());
+    ReadAttr(eten, "rgba", 4, dtendon.rgba, text);
+    ReadAttr(eten, "width", 1, &dtendon.width, text);
 
     // advance
     eten = NextSiblingElement(eten, "tendon");
@@ -2481,6 +2487,7 @@ void mjXReader::OneFlexcomp(XMLElement* elem, mjmBody* pbody) {
 
   // create out-of-DOM element
   mjCFlexcomp fcomp;
+  mjmFlex& dflex = *fcomp.def.spec.flex;
 
   // common properties
   ReadAttrTxt(elem, "name", fcomp.name, true);
@@ -2494,15 +2501,15 @@ void mjXReader::OneFlexcomp(XMLElement* elem, mjmBody* pbody) {
   ReadAttr(elem, "inertiabox", 1, &fcomp.inertiabox, text);
   ReadAttrTxt(elem, "file", fcomp.file);
   if (ReadAttrTxt(elem, "material", material)) {
-    mjm_setString(fcomp.def.flex.spec.material, material.c_str());
+    mjm_setString(dflex.material, material.c_str());
   }
-  ReadAttr(elem, "rgba", 4, fcomp.def.flex.spec.rgba, text);
+  ReadAttr(elem, "rgba", 4, dflex.rgba, text);
   if (MapValue(elem, "flatskin", &n, bool_map, 2)) {
-    fcomp.def.flex.spec.flatskin = (n==1);
+    dflex.flatskin = (n==1);
   }
-  ReadAttrInt(elem, "dim", &fcomp.def.flex.spec.dim);
-  ReadAttr(elem, "radius", 1, &fcomp.def.flex.spec.radius, text);
-  ReadAttrInt(elem, "group", &fcomp.def.flex.spec.group);
+  ReadAttrInt(elem, "dim", &dflex.dim);
+  ReadAttr(elem, "radius", 1, &dflex.radius, text);
+  ReadAttrInt(elem, "group", &dflex.group);
 
   // pose
   ReadAttr(elem, "pos", 3, fcomp.pos, text);
@@ -2529,30 +2536,30 @@ void mjXReader::OneFlexcomp(XMLElement* elem, mjmBody* pbody) {
     if (MapValue(edge, "equality", &n, bool_map, 2)) {
       fcomp.equality = (n==1);
     }
-    ReadAttr(edge, "solref", mjNREF, fcomp.def.equality.spec.solref, text, false, false);
-    ReadAttr(edge, "solimp", mjNIMP, fcomp.def.equality.spec.solimp, text, false, false);
-    ReadAttr(edge, "stiffness", 1, &fcomp.def.flex.spec.edgestiffness, text);
-    ReadAttr(edge, "damping", 1, &fcomp.def.flex.spec.edgedamping, text);
+    ReadAttr(edge, "solref", mjNREF, fcomp.def.spec.equality->solref, text, false, false);
+    ReadAttr(edge, "solimp", mjNIMP, fcomp.def.spec.equality->solimp, text, false, false);
+    ReadAttr(edge, "stiffness", 1, &dflex.edgestiffness, text);
+    ReadAttr(edge, "damping", 1, &dflex.edgedamping, text);
   }
 
   // contact
   XMLElement* cont = FirstChildElement(elem, "contact");
   if (cont) {
-    ReadAttrInt(cont, "contype", &fcomp.def.flex.spec.contype);
-    ReadAttrInt(cont, "conaffinity", &fcomp.def.flex.spec.conaffinity);
-    ReadAttrInt(cont, "condim", &fcomp.def.flex.spec.condim);
-    ReadAttrInt(cont, "priority", &fcomp.def.flex.spec.priority);
-    ReadAttr(cont, "friction", 3, fcomp.def.flex.spec.friction, text, false, false);
-    ReadAttr(cont, "solmix", 1, &fcomp.def.flex.spec.solmix, text);
-    ReadAttr(cont, "solref", mjNREF, fcomp.def.flex.spec.solref, text, false, false);
-    ReadAttr(cont, "solimp", mjNIMP, fcomp.def.flex.spec.solimp, text, false, false);
-    ReadAttr(cont, "margin", 1, &fcomp.def.flex.spec.margin, text);
-    ReadAttr(cont, "gap", 1, &fcomp.def.flex.spec.gap, text);
+    ReadAttrInt(cont, "contype", &dflex.contype);
+    ReadAttrInt(cont, "conaffinity", &dflex.conaffinity);
+    ReadAttrInt(cont, "condim", &dflex.condim);
+    ReadAttrInt(cont, "priority", &dflex.priority);
+    ReadAttr(cont, "friction", 3, dflex.friction, text, false, false);
+    ReadAttr(cont, "solmix", 1, &dflex.solmix, text);
+    ReadAttr(cont, "solref", mjNREF, dflex.solref, text, false, false);
+    ReadAttr(cont, "solimp", mjNIMP, dflex.solimp, text, false, false);
+    ReadAttr(cont, "margin", 1, &dflex.margin, text);
+    ReadAttr(cont, "gap", 1, &dflex.gap, text);
     if (MapValue(cont, "internal", &n, bool_map, 2)) {
-      fcomp.def.flex.spec.internal = (n==1);
+      dflex.internal = (n==1);
     }
-    MapValue(cont, "selfcollide", &fcomp.def.flex.spec.selfcollide, flexself_map, 5);
-    ReadAttrInt(cont, "activelayers", &fcomp.def.flex.spec.activelayers);
+    MapValue(cont, "selfcollide", &dflex.selfcollide, flexself_map, 5);
+    ReadAttrInt(cont, "activelayers", &dflex.activelayers);
   }
 
   // pin
@@ -2633,7 +2640,7 @@ void mjXReader::OnePlugin(XMLElement* elem, mjmPlugin* plugin) {
 void mjXReader::Default(XMLElement* section, int parentid) {
   XMLElement* elem;
   string text, name;
-  mjCDef* def;
+  mjmDefault* def;
   int thisid;
 
   // create new default, except at top level (already added in mjCModel ctor)
@@ -2648,14 +2655,14 @@ void mjXReader::Default(XMLElement* section, int parentid) {
   }
   if (parentid>=0) {
     thisid = (int)model->defaults.size();
-    def = model->AddDef(text, parentid);
+    def = mjm_addDefault(model, text.c_str(), parentid);
     if (!def) {
       throw mjXError(section, "repeated default class name");
     }
   } else {
     thisid = 0;
-    def = model->defaults[0];
-    def->name = text;
+    def = &model->defaults[0]->spec;
+    mjm_setString(def->name, text.c_str());
   }
 
   // iterate over elements other than nested defaults
@@ -2665,34 +2672,34 @@ void mjXReader::Default(XMLElement* section, int parentid) {
     name = elem->Value();
 
     // read mesh
-    if (name=="mesh") OneMesh(elem, &def->mesh.spec);
+    if (name=="mesh") OneMesh(elem, def->mesh);
 
     // read material
-    else if (name=="material") OneMaterial(elem, &def->material.spec);
+    else if (name=="material") OneMaterial(elem, def->material);
 
     // read joint
-    else if (name=="joint") OneJoint(elem, &def->joint.spec);
+    else if (name=="joint") OneJoint(elem, def->joint);
 
     // read geom
-    else if (name=="geom") OneGeom(elem, &def->geom.spec);
+    else if (name=="geom") OneGeom(elem, def->geom);
 
     // read site
-    else if (name=="site") OneSite(elem, def->site.spec);
+    else if (name=="site") OneSite(elem, def->site);
 
     // read camera
-    else if (name=="camera") OneCamera(elem, &def->camera.spec);
+    else if (name=="camera") OneCamera(elem, def->camera);
 
     // read light
-    else if (name=="light") OneLight(elem, &def->light.spec);
+    else if (name=="light") OneLight(elem, def->light);
 
     // read pair
-    else if (name=="pair") OnePair(elem, &def->pair.spec);
+    else if (name=="pair") OnePair(elem, def->pair);
 
     // read equality
-    else if (name=="equality") OneEquality(elem, &def->equality.spec);
+    else if (name=="equality") OneEquality(elem, def->equality);
 
     // read tendon
-    else if (name=="tendon") OneTendon(elem, &def->tendon.spec);
+    else if (name=="tendon") OneTendon(elem, def->tendon);
 
     // read actuator
     else if (name=="general"     ||
@@ -2704,21 +2711,22 @@ void mjXReader::Default(XMLElement* section, int parentid) {
              name=="cylinder"    ||
              name=="muscle"      ||
              name=="adhesion") {
-      OneActuator(elem, &def->actuator.spec);
+      OneActuator(elem, def->actuator);
     }
 
     // copy into private attributes
-    mjm_finalize(def->geom.spec.element);
-    mjm_finalize(def->joint.spec.element);
-    mjm_finalize(def->site.spec.element);
-    mjm_finalize(def->camera.spec.element);
-    mjm_finalize(def->light.spec.element);
-    mjm_finalize(def->actuator.spec.element);
-    mjm_finalize(def->material.spec.element);
-    mjm_finalize(def->equality.spec.element);
-    mjm_finalize(def->tendon.spec.element);
-    mjm_finalize(def->flex.spec.element);
-    mjm_finalize(def->pair.spec.element);
+    mjm_finalize(def->joint->element);
+    mjm_finalize(def->geom->element);
+    mjm_finalize(def->site->element);
+    mjm_finalize(def->camera->element);
+    mjm_finalize(def->light->element);
+    mjm_finalize(def->flex->element);
+    mjm_finalize(def->mesh->element);
+    mjm_finalize(def->material->element);
+    mjm_finalize(def->pair->element);
+    mjm_finalize(def->equality->element);
+    mjm_finalize(def->tendon->element);
+    mjm_finalize(def->actuator->element);
 
     // advance
     elem = NextSiblingElement(elem);
@@ -3066,9 +3074,9 @@ void mjXReader::Asset(XMLElement* section) {
     name = elem->Value();
 
     // get class if specified, otherwise use default0
-    mjCDef* def = GetClass(elem);
+    mjmDefault* def = GetClass(elem);
     if (!def) {
-      def = model->defaults[0];
+      def = &model->defaults[0]->spec;
     }
 
     // texture sub-element
@@ -3248,9 +3256,9 @@ void mjXReader::Body(XMLElement* section, mjmBody* pbody, mjmFrame* frame) {
     name = elem->Value();
 
     // get class if specified, otherwise use body
-    mjCDef* def = GetClass(elem);
+    mjmDefault* def = GetClass(elem);
     if (!def) {
-      def = (mjCDef*)mjm_getDefault(pbody->element);
+      def = (mjmDefault*)mjm_getDefault(pbody->element);
     }
 
     // inertial sub-element
@@ -3318,7 +3326,7 @@ void mjXReader::Body(XMLElement* section, mjmBody* pbody, mjmFrame* frame) {
     else if (name=="site") {
       // create site and parse
       mjmSite* site = mjm_addSite(pbody,  def);
-      OneSite(elem, *site);
+      OneSite(elem, site);
       mjm_setFrame(site->element, frame);
     }
 
@@ -3370,9 +3378,9 @@ void mjXReader::Body(XMLElement* section, mjmBody* pbody, mjmFrame* frame) {
     // body sub-element
     else if (name=="body") {
       // read childdef
-      mjCDef* childdef = 0;
+      mjmDefault* childdef = 0;
       if (ReadAttrTxt(elem, "childclass", text)) {
-        childdef = model->FindDef(text);
+        childdef = &model->FindDef(text)->spec;
         if (!childdef) {
           throw mjXError(elem, "unknown default childclass");
         }
@@ -3437,9 +3445,9 @@ void mjXReader::Contact(XMLElement* section) {
     name = elem->Value();
 
     // get class if specified, otherwise use default0
-    mjCDef* def = GetClass(elem);
+    mjmDefault* def = GetClass(elem);
     if (!def) {
-      def = model->defaults[0];
+      def = &model->defaults[0]->spec;
     }
 
     // geom pair to include
@@ -3482,9 +3490,9 @@ void mjXReader::Equality(XMLElement* section) {
   elem = FirstChildElement(section);
   while (elem) {
     // get class if specified, otherwise use default0
-    mjCDef* def = GetClass(elem);
+    mjmDefault* def = GetClass(elem);
     if (!def) {
-      def = model->defaults[0];
+      def = &model->defaults[0]->spec;
     }
 
     // create equality constraint and parse
@@ -3510,9 +3518,9 @@ void mjXReader::Deformable(XMLElement* section) {
     name = elem->Value();
 
     // get class if specified, otherwise use default0
-    mjCDef* def = GetClass(elem);
+    mjmDefault* def = GetClass(elem);
     if (!def) {
-      def = model->defaults[0];
+      def = &model->defaults[0]->spec;
     }
 
     // flex sub-element
@@ -3546,9 +3554,9 @@ void mjXReader::Tendon(XMLElement* section) {
   elem = FirstChildElement(section);
   while (elem) {
     // get class if specified, otherwise use default0
-    mjCDef* def = GetClass(elem);
+    mjmDefault* def = GetClass(elem);
     if (!def) {
-      def = model->defaults[0];
+      def = &model->defaults[0]->spec;
     }
 
     // create equality constraint and parse
@@ -3612,9 +3620,9 @@ void mjXReader::Actuator(XMLElement* section) {
   elem = FirstChildElement(section);
   while (elem) {
     // get class if specified, otherwise use default0
-    mjCDef* def = GetClass(elem);
+    mjmDefault* def = GetClass(elem);
     if (!def) {
-      def = model->defaults[0];
+      def = &model->defaults[0]->spec;
     }
 
     // create actuator and parse
@@ -4003,12 +4011,12 @@ void mjXReader::Keyframe(XMLElement* section) {
 
 
 // get defaults class
-mjCDef* mjXReader::GetClass(XMLElement* section) {
+mjmDefault* mjXReader::GetClass(XMLElement* section) {
   string text;
-  mjCDef* def = 0;
+  mjmDefault* def = 0;
 
   if (ReadAttrTxt(section, "class", text)) {
-    def = model->FindDef(text);
+    def = &model->FindDef(text)->spec;
     if (!def) {
       throw mjXError(section, "unknown default class");
     }
diff --git a/src/xml/xml_native_reader.h b/src/xml/xml_native_reader.h
index f9bf0fde..cd8d8208 100644
--- a/src/xml/xml_native_reader.h
+++ b/src/xml/xml_native_reader.h
@@ -64,19 +64,19 @@ class mjXReader : public mjXBase {
   void OneMaterial(tinyxml2::XMLElement* elem, mjmMaterial* pmaterial);
   void OneJoint(tinyxml2::XMLElement* elem, mjmJoint* pjoint);
   void OneGeom(tinyxml2::XMLElement* elem, mjmGeom* pgeom);
-  void OneSite(tinyxml2::XMLElement* elem, mjmSite& site);
+  void OneSite(tinyxml2::XMLElement* elem, mjmSite* site);
   void OneCamera(tinyxml2::XMLElement* elem, mjmCamera* pcamera);
   void OneLight(tinyxml2::XMLElement* elem, mjmLight* plight);
   void OnePair(tinyxml2::XMLElement* elem, mjmPair* ppair);
   void OneEquality(tinyxml2::XMLElement* elem, mjmEquality* pequality);
   void OneTendon(tinyxml2::XMLElement* elem, mjmTendon* ptendon);
   void OneActuator(tinyxml2::XMLElement* elem, mjmActuator* pactuator);
-  void OneComposite(tinyxml2::XMLElement* elem, mjmBody* pbody, mjCDef* def);
+  void OneComposite(tinyxml2::XMLElement* elem, mjmBody* pbody, mjmDefault* def);
   void OneFlexcomp(tinyxml2::XMLElement* elem, mjmBody* pbody);
   void OnePlugin(tinyxml2::XMLElement* elem, mjmPlugin* plugin);
 
   mjXSchema schema;                                                   // schema used for validation
-  mjCDef* GetClass(tinyxml2::XMLElement* section);                    // get default class name
+  mjmDefault* GetClass(tinyxml2::XMLElement* section);                    // get default class name
   static void GetXMLPos(tinyxml2::XMLElement* elem, mjCBase* obj);    // get xml position
 
   bool readingdefaults;  // true while reading defaults
diff --git a/test/xml/xml_native_reader_test.cc b/test/xml/xml_native_reader_test.cc
index b85aa8db..8be293fb 100644
--- a/test/xml/xml_native_reader_test.cc
+++ b/test/xml/xml_native_reader_test.cc
@@ -437,6 +437,31 @@ TEST_F(XMLReaderTest, InvalidDoubleOrientation) {
     }
   }
 }
+
+TEST_F(XMLReaderTest, RepeatedDefaultName) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <default>
+      <default class="sphere">
+        <geom type="sphere" size="1"/>
+      </default>
+      <default class="sphere">
+        <geom type="capsule" size="1 1"/>
+      </default>
+    </default>
+    <worldbody>
+      <body>
+        <geom class="sphere"/>
+      </body>
+    </worldbody>
+  </mujoco>
+  )";
+  std::array<char, 1024> error;
+  mjModel* model = LoadModelFromString(xml, error.data(), error.size());
+  ASSERT_THAT(model, IsNull()) << error.data();
+  EXPECT_THAT(error.data(), HasSubstr("repeated default class name"));
+}
+
 // ------------------------ test including -------------------------------------
 
 TEST_F(XMLReaderTest, IncludeTest) {
diff --git a/test/xml/xml_native_writer_test.cc b/test/xml/xml_native_writer_test.cc
index d3e000cd..53147f0f 100644
--- a/test/xml/xml_native_writer_test.cc
+++ b/test/xml/xml_native_writer_test.cc
@@ -694,6 +694,38 @@ TEST_F(XMLWriterTest, WritesDefaults) {
   mj_deleteModel(model);
 }
 
+TEST_F(XMLWriterTest, WritesActuatorDefaults) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <default>
+      <default class="position">
+        <position kp="3" kv="4" />
+      </default>
+      <default class="intvelocity">
+        <intvelocity kp="5" kv="6" />
+      </default>
+    </default>
+    <worldbody>
+      <body>
+        <geom size="1"/>
+        <joint name="jnt" type="slide" axis="1 0 0" range="0 2"/>
+      </body>
+    </worldbody>
+    <actuator>
+      <position joint="jnt" class="position"/>
+      <intvelocity joint="jnt" actrange="-1 1"/>
+    </actuator>
+  </mujoco>
+  )";
+  mjModel* model = LoadModelFromString(xml);
+  ASSERT_THAT(model, NotNull());
+  std::string saved_xml = SaveAndReadXml(model);
+  EXPECT_THAT(saved_xml, Not(HasSubstr("mass")));
+  EXPECT_THAT(saved_xml, HasSubstr(
+      "<general biastype=\"affine\" gainprm=\"3 0 0 0 0 0 0 0 0 0\""));
+  mj_deleteModel(model);
+}
+
 TEST_F(XMLWriterTest, WritesDensity) {
   static constexpr char xml[] = R"(
   <mujoco>