Add mjmBody and mjmPlugin and their compiler C APIs.

PiperOrigin-RevId: 604937231
Change-Id: Ib8a2105ce6c0500d6a610258b46f192d9e73169d

src/user/user_api.cc

@@ -1,4 +1,4 @@
-// Copyright 2021 DeepMind Technologies Limited
+// Copyright 2024 DeepMind Technologies Limited
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -31,48 +31,159 @@ void* mjm_createModel() {
 
 
 // delete model
-void mjm_deleteModel(void* model) {
+void mjm_deleteModel(void* modelspec) {
+  mjCModel* model = static_cast<mjCModel*>(modelspec);
+  delete model;
+}
+
+
+
+// add child body to body, return child spec
+mjmBody* mjm_addBody(mjmBody* bodyspec, void* defspec) {
+  mjCDef* def = static_cast<mjCDef*>(defspec);
+  mjCBody* body = reinterpret_cast<mjCBody*>(bodyspec->element)->AddBody(def);
+  return &body->spec;
+}
+
+
+
+// add site to body, return site spec
+mjmSite* mjm_addSite(mjmBody* bodyspec, void* defspec) {
+  mjCDef* def = static_cast<mjCDef*>(defspec);
+  mjCBody* body = reinterpret_cast<mjCBody*>(bodyspec->element);
+  mjCSite* site = body->AddSite(def);
+  return &site->spec;
+}
+
+
+
+// add joint to body
+void* mjm_addJoint(mjmBody* bodyspec, void* defspec) {
+  mjCDef* def = static_cast<mjCDef*>(defspec);
+  mjCBody* body = reinterpret_cast<mjCBody*>(bodyspec->element);
+  mjCJoint* joint = body->AddJoint(def);
+  return joint;
+}
+
+
+
+// add free joint to body
+void* mjm_addFreeJoint(mjmBody* bodyspec) {
+  mjCBody* body = reinterpret_cast<mjCBody*>(bodyspec->element);
+  mjCJoint* joint = body->AddFreeJoint();
+  return joint;
+}
+
+
+
+// add geom to body
+void* mjm_addGeom(mjmBody* bodyspec, void* defspec) {
+  mjCDef* def = static_cast<mjCDef*>(defspec);
+  mjCBody* body = reinterpret_cast<mjCBody*>(bodyspec->element);
+  mjCGeom* geom = body->AddGeom(def);
+  return geom;
+}
+
+
+
+// add camera to body
+void* mjm_addCamera(mjmBody* bodyspec, void* defspec) {
+  mjCDef* def = static_cast<mjCDef*>(defspec);
+  mjCBody* body = reinterpret_cast<mjCBody*>(bodyspec->element);
+  mjCCamera* camera = body->AddCamera(def);
+  return camera;
+}
+
+
+
+// add light to body
+void* mjm_addLight(mjmBody* bodyspec, void* defspec) {
+  mjCDef* def = static_cast<mjCDef*>(defspec);
+  mjCBody* body = reinterpret_cast<mjCBody*>(bodyspec->element);
+  mjCLight* light = body->AddLight(def);
+  return light;
+}
+
+
+
+// add frame to body
+void* mjm_addFrame(mjmBody* bodyspec, void* parentframe) {
+  mjCFrame* parentframeC = static_cast<mjCFrame*>(parentframe);
+  mjCFrame* frameC = reinterpret_cast<mjCBody*>(bodyspec->element)->AddFrame(parentframeC);
+  return frameC;
+}
+
+
+
+// Add plugin to model.
+mjElement mjm_addPlugin(void* model) {
   mjCModel* modelC = static_cast<mjCModel*>(model);
-  delete modelC;
+  mjCPlugin* plugin = modelC->AddPlugin();
+  return (mjElement)plugin;
 }
 
 
 
-// add body to body
+// get objects
-void* mjm_addBody(void* body, void* def) {
+void* mjm_getModel(mjmBody* bodyspec) {
-  mjCDef* defC = static_cast<mjCDef*>(def);
+  return reinterpret_cast<mjCBody*>(bodyspec->element)->model;
-  mjCBody* bodyC = static_cast<mjCBody*>(body);
-  return bodyC->AddBody(defC);
 }
 
 
 
-// add site to body
+// get default
-mjmSite* mjm_addSite(void* body, void* def) {
+void* mjm_getDefault(mjElement element) {
-  mjCDef* defC = static_cast<mjCDef*>(def);
+  return reinterpret_cast<mjCBase*>(element)->def;
-  mjCSite* siteC = static_cast<mjCBody*>(body)->AddSite(defC);
-  return &siteC->spec;
 }
 
 
 
-// get object of given type
+// find body in model by name
-void* mjm_findObject(void* model, mjtObj type, const char* name) {
+mjmBody* mjm_findBody(void* modelspec, const char* name) {
-  mjCModel* modelC = static_cast<mjCModel*>(model);
+  mjCModel* model = static_cast<mjCModel*>(modelspec);
-  return modelC->FindObject(type, std::string(name));
+  mjCBase* body = model->FindObject(mjOBJ_BODY, std::string(name));
+  if (!body) {
+    return 0;
+  }
+  return &(static_cast<mjCBody*>(body)->spec);
 }
 
 
 
-// set parent frame of dest
+// find child of a body by name
-void mjm_setFrame(void* dest, void* frame) {
+mjmBody* mjm_findChild(mjmBody* bodyspec, const char* name) {
+  mjCBody* body = reinterpret_cast<mjCBody*>(bodyspec->element);
+  mjCBase* child = body->FindObject(mjOBJ_BODY, std::string(name));
+  if (!child) {
+    return 0;
+  }
+  return &(static_cast<mjCBody*>(child)->spec);
+}
+
+
+
+// set frame
+void mjm_setFrame(mjElement dest, void* frame) {
   mjCFrame* frameC = static_cast<mjCFrame*>(frame);
-  mjCBase* baseC = static_cast<mjCBase*>(dest);
+  mjCBase* baseC = reinterpret_cast<mjCBase*>(dest);
   baseC->SetFrame(frameC);
 }
 
 
 
+// get id
+int mjm_getId(mjElement element) {
+  return reinterpret_cast<mjCBase*>(element)->id;
+}
+
+
+// set default
+void mjm_setDefault(mjElement element, void* defspec) {
+  mjCBase* baseC = reinterpret_cast<mjCBase*>(element);
+  baseC->def = static_cast<mjCDef*>(defspec);
+}
+
+
 // set string
 void mjm_setString(mjString dest, const char* text) {
   std::string* str = reinterpret_cast<std::string*>(dest);
@@ -108,3 +219,13 @@ const double* mjm_getDouble(const mjDouble source, int* size) {
   }
   return v->data();
 }
+
+
+
+// compute full inertia
+const char* mjm_setFullInertia(mjmBody* bodyspec, double quat[4], double inertia[3]) {
+  mjCBody* body = reinterpret_cast<mjCBody*>(bodyspec->element);
+  return body->FullInertia(quat, inertia);
+}
+
+

src/user/user_api.h

@@ -1,4 +1,4 @@
-// Copyright 2021 DeepMind Technologies Limited
+// Copyright 2024 DeepMind Technologies Limited
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -17,14 +17,20 @@
 
 #include <mujoco/mjexport.h>
 #include <mujoco/mjmodel.h>
+#include <mujoco/mjtnum.h>
 
 typedef struct _mjString* mjString;
 typedef struct _mjDouble* mjDouble;
 typedef struct _mjElement* mjElement;
 
 
+// this is a C-API
+#ifdef __cplusplus
+extern "C" {
+#endif
 
 //---------------------------------- Public structs ------------------------------------------------
+
 typedef struct _mjmOrientation {
   double axisangle[4];            // rotation axis and angle
   double xyaxes[6];               // x and y axes
@@ -32,42 +38,104 @@ typedef struct _mjmOrientation {
   double euler[3];                // euler rotations
 } mjmOrientation;
 
-typedef struct _mjmSite {
+
-  mjElement element;              // only used internally, do not modify
+typedef struct _mjmPlugin {
+  bool active;
+  mjString name;
+  mjString instance_name;
+  mjElement instance;
+} mjmPlugin;
+
+
+typedef struct _mjmBody {
+  mjElement element;              // compiler only, do not modify
   mjString name;                  // name
   mjString classname;             // class name
+  double pos[3];                  // frame position
+  double quat[4];                 // frame orientation
+  mjtByte mocap;                  // is this a mocap body
+  mjmOrientation alt;             // frame alternative orientation
+  double gravcomp;                // gravity compensation
+  mjDouble userdata;              // user data
+  double ipos[3];                 // inertial frame position
+  double iquat[4];                // inertial frame orientation
+  double mass;                    // mass
+  double inertia[3];              // diagonal inertia (in i-frame)
+  mjmOrientation ialt;            // inertial frame alternative orientation
+  double fullinertia[6];          // non-axis-aligned inertia matrix
+  mjtByte explicitinertial;       // whether to save the body with an explicit inertial clause
+  mjmPlugin plugin;               // passive force plugin
   mjString info;                  // message appended to errors
+} mjmBody;
+
+
+typedef struct _mjmSite {
+  mjElement element;              // compiler only, do not modify
+  mjString name;                  // name
+  mjString classname;             // class name
   mjtGeom type;                   // geom type for rendering
   int group;                      // group id, used for visualization
-  double size[3];                 // geom size for rendering
   double pos[3];                  // position
   double quat[4];                 // orientation
   mjString material;              // name of material for rendering
-  mjDouble userdata;              // user data
+  double size[3];                 // geom size for rendering
-  float rgba[4];                  // rgba when material is omitted
   double fromto[6];               // alternative for capsule, cylinder, box, ellipsoid
   mjmOrientation alt;             // alternative orientation specification
+  float rgba[4];                  // rgba when material is omitted
+  mjDouble userdata;              // user data
+  mjString info;                  // message appended to errors
 } mjmSite;
 
+
 //---------------------------------- Public API ----------------------------------------------------
-#ifdef __cplusplus
-extern "C" {
-#endif
 
 // Create model.
 MJAPI void* mjm_createModel();
 
 // Delete model.
-MJAPI void mjm_deleteModel(void* model);
+MJAPI void mjm_deleteModel(void* modelspec);
 
-// Add body to body.
+// Add child body to body, return child spec.
-MJAPI void* mjm_addBody(void* body, void* def);
+MJAPI mjmBody* mjm_addBody(mjmBody* body, void* defspec);
 
-// Add site to body.
+// Add site to body, return site spec.
-MJAPI mjmSite* mjm_addSite(void* body, void* def);
+MJAPI mjmSite* mjm_addSite(mjmBody* body, void* defspec);
 
-// Find object of given type.
+// Add joint to body.
-MJAPI void* mjm_findObject(void* model, mjtObj type, const char* name);
+MJAPI void* mjm_addJoint(mjmBody* body, void* defspec);
+
+// Add freejoint to body.
+MJAPI void* mjm_addFreeJoint(mjmBody* body);
+
+// Add geom to body.
+MJAPI void* mjm_addGeom(mjmBody* body, void* defspec);
+
+// Add camera to body.
+MJAPI void* mjm_addCamera(mjmBody* body, void* defspec);
+
+// Add light to body.
+MJAPI void* mjm_addLight(mjmBody* body, void* defspec);
+
+// Add frame to body.
+MJAPI void* mjm_addFrame(mjmBody* body, void* parentframe);
+
+// Add plugin to model.
+MJAPI mjElement mjm_addPlugin(void* model);
+
+// Get model from body.
+MJAPI void* mjm_getModel(mjmBody* body);
+
+// Get default corresponding to an mjElement.
+MJAPI void* mjm_getDefault(mjElement element);
+
+// Finding body with given name in model.
+MJAPI mjmBody* mjm_findBody(void* modelspec, const char* name);
+
+// Finding body with given name in body.
+MJAPI mjmBody* mjm_findChild(mjmBody* body, const char* name);
+
+// Get element id.
+MJAPI int mjm_getId(mjElement element);
 
 // Copy input text to destination string.
 MJAPI void mjm_setString(mjString dest, const char* text);
@@ -75,14 +143,20 @@ MJAPI void mjm_setString(mjString dest, const char* text);
 // Copy input array to destination vector.
 MJAPI void mjm_setDouble(mjDouble dest, const double* array, int size);
 
-// Get const pointer to mjString data.
+// Get string contents.
 MJAPI const char* mjm_getString(mjString source);
 
-// Get const pointer to mjDouble data and its size.
+// Get double array contents and optionally its size.
 MJAPI const double* mjm_getDouble(mjDouble source, int* size);
 
+// Set default.
+MJAPI void mjm_setDefault(mjElement element, void* defspec);
+
 // Set frame.
-MJAPI void mjm_setFrame(void* dest, void* frame);
+MJAPI void mjm_setFrame(mjElement dest, void* frame);
+
+// Compute quat and inertia from body->fullinertia..
+MJAPI const char* mjm_setFullInertia(mjmBody* body, double quat[4], double inertia[3]);
 
 #ifdef __cplusplus
 }

src/user/user_composite.cc

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#include "user/user_api.h"
 #include "user/user_composite.h"
 
 #include <algorithm>
@@ -216,7 +217,7 @@ void mjCComposite::SetDefault(void) {
 
 
 // make composite object
-bool mjCComposite::Make(mjCModel* model, mjCBody* body, char* error, int error_sz) {
+bool mjCComposite::Make(mjCModel* model, mjmBody* body, char* error, int error_sz) {
   // check geom type
   if ((def[0].geom.type!=mjGEOM_SPHERE &&
        def[0].geom.type!=mjGEOM_CAPSULE &&
@@ -326,7 +327,7 @@ bool mjCComposite::Make(mjCModel* model, mjCBody* body, char* error, int error_s
 
 
 
-bool mjCComposite::MakeParticle(mjCModel* model, mjCBody* body, char* error, int error_sz) {
+bool mjCComposite::MakeParticle(mjCModel* model, mjmBody* body, char* error, int error_sz) {
   char txt[100];
   std::vector<int> face;
 
@@ -439,13 +440,13 @@ bool mjCComposite::MakeParticle(mjCModel* model, mjCBody* body, char* error, int
   // create bodies and geoms
   for (int i=0; i<uservert.size()/3; i++) {
     // create body
-    mjCBody* b = body->AddBody(NULL);
+    mjmBody* b = mjm_addBody(body, NULL);
 
     if (!username.empty()) {
-      b->name = username[i];
+      mjm_setString(b->name, username[i].c_str());
     } else {
       mju::sprintf_arr(txt, "%sB%d", prefix.c_str(), i);
-      b->name = txt;
+      mjm_setString(b->name, txt);
     }
 
     // set body position
@@ -456,8 +457,8 @@ bool mjCComposite::MakeParticle(mjCModel* model, mjCBody* body, char* error, int
     // add slider joints if none defined
     if (!add[mjCOMPKIND_PARTICLE]) {
       for (int i=0; i<3; i++) {
-        mjCJoint* jnt = b->AddJoint(&defjoint[mjCOMPKIND_JOINT][0]);
+        mjCJoint* jnt = (mjCJoint*)mjm_addJoint(b, &defjoint[mjCOMPKIND_JOINT][0]);
-        jnt->def = body->def;
+        jnt->def = (mjCDef*)mjm_getDefault(body->element);
         jnt->type = mjJNT_SLIDE;
         mjuu_setvec(jnt->pos, 0, 0, 0);
         mjuu_setvec(jnt->axis, 0, 0, 0);
@@ -468,35 +469,36 @@ bool mjCComposite::MakeParticle(mjCModel* model, mjCBody* body, char* error, int
     // add user-specified joints
     else {
       for (auto defjnt : defjoint[mjCOMPKIND_PARTICLE]) {
-        mjCJoint* jnt = b->AddJoint(&defjnt);
+        mjCJoint* jnt = (mjCJoint*)mjm_addJoint(b, &defjnt);
-        jnt->def = body->def;
+        jnt->def = (mjCDef*)mjm_getDefault(body->element);
       }
     }
 
     // add geom
-    mjCGeom* g = b->AddGeom(def);
+    mjCGeom* g = (mjCGeom*)mjm_addGeom(b, def);
-    g->def = body->def;
+    g->def = (mjCDef*)mjm_getDefault(body->element);
 
     // add site
-    mjCSite* s = b->AddSite(def);
+    mjmSite* s = mjm_addSite(b, def);
-    s->def = body->def;
+    mjm_setDefault(s->element, mjm_getDefault(body->element));
-    s->spec.type = mjGEOM_SPHERE;
+    s->type = mjGEOM_SPHERE;
     mju::sprintf_arr(txt, "%sS%d", prefix.c_str(), i);
-    s->name = txt;
+    mjm_setString(s->name, txt);
 
     // add plugin
     if (plugin_instance) {
-      b->is_plugin = true;
+      mjmPlugin* plugin = &b->plugin;
-      b->plugin_name = plugin_name;
+      plugin->active = true;
-      b->plugin_instance = plugin_instance;
+      plugin->instance = (mjElement)plugin_instance;
-      b->plugin_instance_name = plugin_instance_name;
+      mjm_setString(plugin->instance_name, plugin_instance_name.c_str());
+      mjm_setString(plugin->name, plugin_name.c_str());
 
       if (i==0 && !plugin_instance->config_attribs["face"].empty()) {
         return comperr(error, "Face attribute already exists in plugin", error_sz);
       }
 
-      b->plugin_instance->config_attribs["face"] = userface;
+      plugin_instance->config_attribs["face"] = userface;
-      b->plugin_instance->config_attribs["edge"] = "";
+      plugin_instance->config_attribs["edge"] = "";
 
       // update density
       if (dim == 2) {
@@ -566,7 +568,7 @@ bool mjCComposite::MakeParticle(mjCModel* model, mjCBody* body, char* error, int
 
 
 // make grid connected with tendons
-bool mjCComposite::MakeGrid(mjCModel* model, mjCBody* body, char* error, int error_sz) {
+bool mjCComposite::MakeGrid(mjCModel* model, mjmBody* body, char* error, int error_sz) {
   char txt[100], txt1[100], txt2[100];
 
   // check dimensionality
@@ -588,9 +590,9 @@ bool mjCComposite::MakeGrid(mjCModel* model, mjCBody* body, char* error, int err
   for (int ix=0; ix<count[0]; ix++) {
     for (int iy=0; iy<count[1]; iy++) {
       // create body
-      mjCBody* b = body->AddBody(NULL);
+      mjmBody* b = mjm_addBody(body, NULL);
       mju::sprintf_arr(txt, "%sB%d_%d", prefix.c_str(), ix, iy);
-      b->name = txt;
+      mjm_setString(b->name, txt);
 
       // set body position
       b->pos[0] = offset[0] + spacing*(ix - 0.5*count[0]);
@@ -598,18 +600,18 @@ bool mjCComposite::MakeGrid(mjCModel* model, mjCBody* body, char* error, int err
       b->pos[2] = offset[2];
 
       // add geom
-      mjCGeom* g = b->AddGeom(def);
+      mjCGeom* g = (mjCGeom*)mjm_addGeom(b, def);
-      g->def = body->def;
+      g->def = (mjCDef*)mjm_getDefault(body->element);
       g->type = mjGEOM_SPHERE;
       mju::sprintf_arr(txt, "%sG%d_%d", prefix.c_str(), ix, iy);
       g->name = txt;
 
       // add site
-      mjCSite* s = b->AddSite(def);
+      mjmSite* s = mjm_addSite(b, def);
-      s->def = body->def;
+      mjm_setDefault(s->element, mjm_getDefault(body->element));
-      s->spec.type = mjGEOM_SPHERE;
+      s->type = mjGEOM_SPHERE;
       mju::sprintf_arr(txt, "%sS%d_%d", prefix.c_str(), ix, iy);
-      s->name = txt;
+      mjm_setString(s->name, txt);
 
       // skip pinned elements
       bool skip = false;
@@ -626,8 +628,8 @@ bool mjCComposite::MakeGrid(mjCModel* model, mjCBody* body, char* error, int err
       // add slider joint
       mjCJoint* jnt[3];
       for (int i=0; i<3; i++) {
-        jnt[i] = b->AddJoint(&defjoint[mjCOMPKIND_JOINT][0]);
+        jnt[i] = (mjCJoint*)mjm_addJoint(b, &defjoint[mjCOMPKIND_JOINT][0]);
-        jnt[i]->def = body->def;
+        jnt[i]->def = (mjCDef*)mjm_getDefault(body->element);
         mju::sprintf_arr(txt, "%sJ%d_%d_%d", prefix.c_str(), i, ix, iy);
         jnt[i]->name = txt;
         jnt[i]->type = mjJNT_SLIDE;
@@ -682,7 +684,7 @@ bool mjCComposite::MakeGrid(mjCModel* model, mjCBody* body, char* error, int err
 
 
 
-bool mjCComposite::MakeCable(mjCModel* model, mjCBody* body, char* error, int error_sz) {
+bool mjCComposite::MakeCable(mjCModel* model, mjmBody* body, char* error, int error_sz) {
   // check dim
   if (dim!=1) {
     return comperr(error, "Cable must be one-dimensional", error_sz);
@@ -753,7 +755,7 @@ bool mjCComposite::MakeCable(mjCModel* model, mjCBody* body, char* error, int er
 
 
 
-mjCBody* mjCComposite::AddCableBody(mjCModel* model, mjCBody* body, int ix, mjtNum normal[3], mjtNum prev_quat[4]) {
+mjmBody* mjCComposite::AddCableBody(mjCModel* model, mjmBody* body, int ix, mjtNum normal[3], mjtNum prev_quat[4]) {
   char txt_geom[100], txt_site[100], txt_slide[100];
   char this_body[100], next_body[100], this_joint[100];
   mjtNum dquat[4], this_quat[4];
@@ -809,8 +811,8 @@ mjCBody* mjCComposite::AddCableBody(mjCModel* model, mjCBody* body, int ix, mjtN
   mju::sprintf_arr(txt_slide, "%sJs%d", prefix.c_str(), ix);
 
   // add body
-  body = body->AddBody();
+  body = mjm_addBody(body, 0);
-  body->name = this_body;
+  mjm_setString(body->name, this_body);
   if (first) {
     mjuu_setvec(body->pos, offset[0]+uservert[3*ix],
                            offset[1]+uservert[3*ix+1],
@@ -824,8 +826,8 @@ mjCBody* mjCComposite::AddCableBody(mjCModel* model, mjCBody* body, int ix, mjtN
   }
 
   // add geom
-  mjCGeom* geom = body->AddGeom(def);
+  mjCGeom* geom = (mjCGeom*)mjm_addGeom(body, def);
-  geom->def = body->def;
+  geom->def = (mjCDef*)mjm_getDefault(body->element);
   geom->name = txt_geom;
   if (def[0].geom.type==mjGEOM_CYLINDER ||
       def[0].geom.type==mjGEOM_CAPSULE) {
@@ -839,10 +841,11 @@ mjCBody* mjCComposite::AddCableBody(mjCModel* model, mjCBody* body, int ix, mjtN
 
   // add plugin
   if (plugin_instance) {
-    body->is_plugin = true;
+    mjmPlugin* plugin = &body->plugin;
-    body->plugin_name = plugin_name;
+    plugin->active = true;
-    body->plugin_instance = plugin_instance;
+    plugin->instance = (mjElement)plugin_instance;
-    body->plugin_instance_name = plugin_instance_name;
+    mjm_setString(plugin->name, plugin_name.c_str());
+    mjm_setString(plugin->instance_name, plugin_instance_name.c_str());
   }
 
   // update orientation
@@ -850,8 +853,8 @@ mjCBody* mjCComposite::AddCableBody(mjCModel* model, mjCBody* body, int ix, mjtN
 
   // add curvature joint
   if (!first || strcmp(initial.c_str(), "none")) {
-    mjCJoint* jnt = body->AddJoint(&defjoint[mjCOMPKIND_JOINT][0]);
+    mjCJoint* jnt = (mjCJoint*)mjm_addJoint(body, &defjoint[mjCOMPKIND_JOINT][0]);
-    jnt->def = body->def;
+    jnt->def = (mjCDef*)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;
     jnt->armature = jnt->type==mjJNT_FREE ? 0 : jnt->armature;
@@ -868,11 +871,11 @@ mjCBody* mjCComposite::AddCableBody(mjCModel* model, mjCBody* body, int ix, mjtN
 
   // add site at the boundary
   if (last || first) {
-    mjCSite* site = body->AddSite(def);
+    mjmSite* site = mjm_addSite(body, def);
-    site->def = body->def;
+    mjm_setDefault(site->element, mjm_getDefault(body->element));
-    site->name = txt_site;
+    mjm_setString(site->name, txt_site);
-    mjuu_setvec(site->spec.pos, last ? length : 0, 0, 0);
+    mjuu_setvec(site->pos, last ? length : 0, 0, 0);
-    mjuu_setvec(site->spec.quat, 1, 0, 0, 0);
+    mjuu_setvec(site->quat, 1, 0, 0, 0);
   }
 
   return body;
@@ -880,7 +883,7 @@ mjCBody* mjCComposite::AddCableBody(mjCModel* model, mjCBody* body, int ix, mjtN
 
 
 // make rope
-bool mjCComposite::MakeRope(mjCModel* model, mjCBody* body, char* error, int error_sz) {
+bool mjCComposite::MakeRope(mjCModel* model, mjmBody* body, char* error, int error_sz) {
   // check dim
   if (dim!=1) {
     return comperr(error, "Rope must be one-dimensional", error_sz);
@@ -889,13 +892,14 @@ bool mjCComposite::MakeRope(mjCModel* model, mjCBody* body, char* error, int err
   // check root body name prefix
   char txt[200];
   mju::sprintf_arr(txt, "%sB", prefix.c_str());
-  if (std::strncmp(txt, body->name.substr(0, strlen(txt)).c_str(), mju::sizeof_arr(txt))) {
+  std::string body_name = mjm_getString(body->name);
+  if (std::strncmp(txt, body_name.substr(0, strlen(txt)).c_str(), mju::sizeof_arr(txt))) {
     mju::strcat_arr(txt, " must be the beginning of root body name");
     return comperr(error, txt, error_sz);
   }
 
   // read origin coordinate from root body
-  mju::strcpy_arr(txt, body->name.substr(strlen(txt)).c_str());
+  mju::strcpy_arr(txt, body_name.substr(strlen(txt)).c_str());
   int ox = -1;
   if (sscanf(txt, "%d", &ox)!=1) {
     return comperr(error, "Root body name must contain X coordinate", error_sz);
@@ -908,7 +912,7 @@ bool mjCComposite::MakeRope(mjCModel* model, mjCBody* body, char* error, int err
   AddRopeBody(model, body, ox, ox);
 
   // add elements: right
-  mjCBody* pbody = body;
+  mjmBody* pbody = body;
   for (int ix=ox; ix<count[0]-1; ix++) {
     pbody = AddRopeBody(model, pbody, ix, ix+1);
   }
@@ -946,16 +950,16 @@ bool mjCComposite::MakeRope(mjCModel* model, mjCBody* body, char* error, int err
 
 
 // add child body for cloth
-mjCBody* mjCComposite::AddRopeBody(mjCModel* model, mjCBody* body, int ix, int ix1) {
+mjmBody* mjCComposite::AddRopeBody(mjCModel* model, mjmBody* body, int ix, int ix1) {
   char txt[100];
   bool isroot = (ix==ix1);
   double dx = spacing*(ix1-ix);
 
   // add child if not root
   if (!isroot) {
-    body = body->AddBody();
+    body = mjm_addBody(body, 0);
     mju::sprintf_arr(txt, "%sB%d", prefix.c_str(), ix1);
-    body->name = txt;
+    mjm_setString(body->name, txt);
 
     // loop
     if (type==mjCOMPTYPE_LOOP) {
@@ -978,8 +982,8 @@ mjCBody* mjCComposite::AddRopeBody(mjCModel* model, mjCBody* body, int ix, int i
   }
 
   // add geom
-  mjCGeom* geom = body->AddGeom(def);
+  mjCGeom* geom = (mjCGeom*)mjm_addGeom(body, def);
-  geom->def = body->def;
+  geom->def = (mjCDef*)mjm_getDefault(body->element);
   mju::sprintf_arr(txt, "%sG%d", prefix.c_str(), ix1);
   geom->name = txt;
   mjuu_setvec(geom->pos, 0, 0, 0);
@@ -993,8 +997,8 @@ mjCBody* mjCComposite::AddRopeBody(mjCModel* model, mjCBody* body, int ix, int i
   // add main joint
   for (int i=0; i<2; i++) {
     // add joint
-    mjCJoint* jnt = body->AddJoint(&defjoint[mjCOMPKIND_JOINT][0]);
+    mjCJoint* jnt = (mjCJoint*)mjm_addJoint(body, &defjoint[mjCOMPKIND_JOINT][0]);
-    jnt->def = body->def;
+    jnt->def = (mjCDef*)mjm_getDefault(body->element);
     mju::sprintf_arr(txt, "%sJ%d_%d", prefix.c_str(), i, ix1);
     jnt->name = txt;
     jnt->type = mjJNT_HINGE;
@@ -1006,8 +1010,8 @@ mjCBody* mjCComposite::AddRopeBody(mjCModel* model, mjCBody* body, int ix, int i
   // add twist joint
   if (add[mjCOMPKIND_TWIST]) {
     // add joint
-    mjCJoint* jnt = body->AddJoint(&defjoint[mjCOMPKIND_TWIST][0]);
+    mjCJoint* jnt = (mjCJoint*)mjm_addJoint(body, &defjoint[mjCOMPKIND_TWIST][0]);
-    jnt->def = body->def;
+    jnt->def = (mjCDef*)mjm_getDefault(body->element);
     mju::sprintf_arr(txt, "%sJT%d", prefix.c_str(), ix1);
     jnt->name = txt;
     jnt->type = mjJNT_HINGE;
@@ -1024,8 +1028,8 @@ mjCBody* mjCComposite::AddRopeBody(mjCModel* model, mjCBody* body, int ix, int i
   // add stretch joint
   if (add[mjCOMPKIND_STRETCH]) {
     // add joint
-    mjCJoint* jnt = body->AddJoint(&defjoint[mjCOMPKIND_STRETCH][0]);
+    mjCJoint* jnt = (mjCJoint*)mjm_addJoint(body, &defjoint[mjCOMPKIND_STRETCH][0]);
-    jnt->def = body->def;
+    jnt->def = (mjCDef*)mjm_getDefault(body->element);
     mju::sprintf_arr(txt, "%sJS%d", prefix.c_str(), ix1);
     jnt->name = txt;
     jnt->type = mjJNT_SLIDE;
@@ -1081,7 +1085,7 @@ void mjCComposite::BoxProject(double* pos) {
 
 
 // make 3d box, ellipsoid or cylinder
-bool mjCComposite::MakeBox(mjCModel* model, mjCBody* body, char* error, int error_sz) {
+bool mjCComposite::MakeBox(mjCModel* model, mjmBody* body, char* error, int error_sz) {
   char txt[100];
 
   // check dim
@@ -1090,8 +1094,8 @@ bool mjCComposite::MakeBox(mjCModel* model, mjCBody* body, char* error, int erro
   }
 
   // center geom: two times bigger
-  mjCGeom* geom = body->AddGeom(def);
+  mjCGeom* geom = (mjCGeom*)mjm_addGeom(body, def);
-  geom->def = body->def;
+  geom->def = (mjCDef*)mjm_getDefault(body->element);
   geom->type = mjGEOM_SPHERE;
   mju::sprintf_arr(txt, "%sGcenter", prefix.c_str());
   geom->name = txt;
@@ -1114,9 +1118,9 @@ bool mjCComposite::MakeBox(mjCModel* model, mjCBody* body, char* error, int erro
             iy==0 || iy==count[1]-1 ||
             iz==0 || iz==count[2]-1) {
           // create body
-          mjCBody* b = body->AddBody(NULL);
+          mjmBody* b = mjm_addBody(body, NULL);
           mju::sprintf_arr(txt, "%sB%d_%d_%d", prefix.c_str(), ix, iy, iz);
-          b->name = txt;
+          mjm_setString(b->name, txt);
 
           // set body position (+/- 1)
           b->pos[0] = 2.0*ix/(count[0]-1) - 1;
@@ -1131,8 +1135,8 @@ bool mjCComposite::MakeBox(mjCModel* model, mjCBody* body, char* error, int erro
           mjuu_normvec(b->alt.zaxis, 3);
 
           // add geom
-          mjCGeom* g = b->AddGeom(def);
+          mjCGeom* g = (mjCGeom*) mjm_addGeom(b, def);
-          g->def = body->def;
+          g->def = (mjCDef*)mjm_getDefault(body->element);
           mju::sprintf_arr(txt, "%sG%d_%d_%d", prefix.c_str(), ix, iy, iz);
           g->name = txt;
 
@@ -1145,8 +1149,8 @@ bool mjCComposite::MakeBox(mjCModel* model, mjCBody* body, char* error, int erro
           }
 
           // add slider joint
-          mjCJoint* jnt = b->AddJoint(&defjoint[mjCOMPKIND_JOINT][0]);
+          mjCJoint* jnt = (mjCJoint*)mjm_addJoint(b, &defjoint[mjCOMPKIND_JOINT][0]);
-          jnt->def = body->def;
+          jnt->def = (mjCDef*)mjm_getDefault(body->element);
           mju::sprintf_arr(txt, "%sJ%d_%d_%d", prefix.c_str(), ix, iy, iz);
           jnt->name = txt;
           jnt->type = mjJNT_SLIDE;

src/user/user_composite.h

@@ -68,13 +68,13 @@ class mjCComposite {
   bool AddDefaultJoint(char* error = NULL, int error_sz = 0);
   void AdjustSoft(mjtNum* solref, mjtNum* solimp, int level);
 
-  bool Make(mjCModel* model, mjCBody* body, char* error, int error_sz);
+  bool Make(mjCModel* model, mjmBody* body, char* error, int error_sz);
 
-  bool MakeParticle(mjCModel* model, mjCBody* body, char* error, int error_sz);
+  bool MakeParticle(mjCModel* model, mjmBody* body, char* error, int error_sz);
-  bool MakeGrid(mjCModel* model, mjCBody* body, char* error, int error_sz);
+  bool MakeGrid(mjCModel* model, mjmBody* body, char* error, int error_sz);
-  bool MakeRope(mjCModel* model, mjCBody* body, char* error, int error_sz);
+  bool MakeRope(mjCModel* model, mjmBody* body, char* error, int error_sz);
-  bool MakeCable(mjCModel* model, mjCBody* body, char* error, int error_sz);
+  bool MakeCable(mjCModel* model, mjmBody* body, char* error, int error_sz);
-  bool MakeBox(mjCModel* model, mjCBody* body, char* error, int error_sz);
+  bool MakeBox(mjCModel* model, mjmBody* body, char* error, int error_sz);
   void MakeShear(mjCModel* model);
 
   void MakeSkin2(mjCModel* model, mjtNum inflate);
@@ -136,8 +136,8 @@ class mjCComposite {
   int dim;                        // dimensionality
 
  private:
-  mjCBody* AddRopeBody(mjCModel* model, mjCBody* body, int ix, int ix1);
+  mjmBody* AddRopeBody(mjCModel* model, mjmBody* body, int ix, int ix1);
-  mjCBody* AddCableBody(mjCModel* model, mjCBody* body, int ix, mjtNum normal[3], mjtNum prev_quat[4]);
+  mjmBody* AddCableBody(mjCModel* model, mjmBody* body, int ix, mjtNum normal[3], mjtNum prev_quat[4]);
 };
 
 #endif  // MUJOCO_SRC_USER_USER_COMPOSITE_H_

src/user/user_flexcomp.cc

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#include "user/user_api.h"
 #include "user/user_flexcomp.h"
 #include <stdio.h>
 
@@ -76,7 +77,7 @@ mjCFlexcomp::mjCFlexcomp(void) {
 
 
 // make flexcomp object
-bool mjCFlexcomp::Make(mjCModel* model, mjCBody* body, char* error, int error_sz) {
+bool mjCFlexcomp::Make(mjCModel* model, mjmBody* body, char* error, int error_sz) {
   bool radial = (type==mjFCOMPTYPE_BOX ||
                  type==mjFCOMPTYPE_CYLINDER ||
                  type==mjFCOMPTYPE_ELLIPSOID);
@@ -85,7 +86,7 @@ bool mjCFlexcomp::Make(mjCModel* model, mjCBody* body, char* error, int error_sz
                  type==mjFCOMPTYPE_GMSH);
 
   // check parent body name
-  if (body->name.empty()) {
+  if (std::string(mjm_getString(body->name)).empty()) {
     return comperr(error, "Parent body must have name", error_sz);
   }
 
@@ -381,7 +382,7 @@ bool mjCFlexcomp::Make(mjCModel* model, mjCBody* body, char* error, int error_sz
 
   // rigid: set parent name, nothing else to do
   if (rigid) {
-    pf->vertbody.push_back(body->name);
+    pf->vertbody.push_back(std::string(mjm_getString(body->name)).c_str());
     return true;
   }
 
@@ -398,21 +399,22 @@ bool mjCFlexcomp::Make(mjCModel* model, mjCBody* body, char* error, int error_sz
 
     // pinned: parent body
     if (pinned[i]) {
-      pf->vertbody.push_back(body->name);
+      pf->vertbody.push_back(std::string(mjm_getString(body->name)).c_str());
 
       // add plugin
       if (plugin_instance) {
-        body->is_plugin = true;
+        mjmPlugin* plugin = &body->plugin;
-        body->plugin_name = plugin_name;
+        plugin->active = true;
-        body->plugin_instance = plugin_instance;
+        plugin->instance = (mjElement)plugin_instance;
-        body->plugin_instance_name = plugin_instance_name;
+        mjm_setString(plugin->name, plugin_name.c_str());
+        mjm_setString(plugin->instance_name, plugin_instance_name.c_str());
       }
     }
 
     // not pinned: new body
     else {
       // add new body at vertex coordinates
-      mjCBody* pb = body->AddBody();
+      mjmBody* pb = mjm_addBody(body, 0);
 
       // set frame and inertial
       pb->pos[0] = point[3*i];
@@ -423,11 +425,11 @@ bool mjCFlexcomp::Make(mjCModel* model, mjCBody* body, char* error, int error_sz
       pb->inertia[0] = bodyinertia;
       pb->inertia[1] = bodyinertia;
       pb->inertia[2] = bodyinertia;
-      pb->MakeInertialExplicit();
+      pb->explicitinertial = true;
 
       // add radial slider
       if (radial) {
-        mjCJoint* jnt = pb->AddJoint();
+        mjCJoint* jnt = (mjCJoint*)mjm_addJoint(pb, 0);
 
         // set properties
         jnt->type = mjJNT_SLIDE;
@@ -440,7 +442,7 @@ bool mjCFlexcomp::Make(mjCModel* model, mjCBody* body, char* error, int error_sz
       else {
         for (int j=0; j<3; j++) {
           // add joint to body
-          mjCJoint* jnt = pb->AddJoint();
+          mjCJoint* jnt = (mjCJoint*)mjm_addJoint(pb, 0);
 
           // set properties
           jnt->type = mjJNT_SLIDE;
@@ -453,8 +455,8 @@ bool mjCFlexcomp::Make(mjCModel* model, mjCBody* body, char* error, int error_sz
       // construct body name, add to vertbody
       char txt[100];
       mju::sprintf_arr(txt, "%s_%d", name.c_str(), i);
-      pb->name = txt;
+      mjm_setString(pb->name, txt);
-      pf->vertbody.push_back(pb->name);
+      pf->vertbody.push_back(std::string(mjm_getString(pb->name)).c_str());
 
       // clear flex vertex coordinates if allocated
       if (!centered) {
@@ -465,10 +467,11 @@ bool mjCFlexcomp::Make(mjCModel* model, mjCBody* body, char* error, int error_sz
 
       // add plugin
       if (plugin_instance) {
-        pb->is_plugin = true;
+        mjmPlugin* plugin = &pb->plugin;
-        pb->plugin_name = plugin_name;
+        plugin->active = true;
-        pb->plugin_instance = plugin_instance;
+        plugin->instance = (mjElement)plugin_instance;
-        pb->plugin_instance_name = plugin_instance_name;
+        mjm_setString(plugin->name, plugin_name.c_str());
+        mjm_setString(plugin->instance_name, plugin_instance_name.c_str());
       }
     }
   }

src/user/user_flexcomp.h

@@ -39,7 +39,7 @@ typedef enum _mjtFcompType {
 class mjCFlexcomp {
  public:
   mjCFlexcomp(void);
-  bool Make(mjCModel* model, mjCBody* body, char* error, int error_sz);
+  bool Make(mjCModel* model, mjmBody* body, char* error, int error_sz);
 
   bool MakeGrid(char* error, int error_sz);
   bool MakeBox(char* error, int error_sz);

src/user/user_mesh.cc

@@ -26,6 +26,8 @@
 #include <utility>
 #include <vector>
 
+#include "user/user_api.h"
+
 #ifdef MUJOCO_TINYOBJLOADER_IMPL
 #define TINYOBJLOADER_IMPLEMENTATION
 #endif
@@ -176,6 +178,10 @@ mjCMesh::mjCMesh(mjCModel* _model, mjCDef* _def) {
   // set model, def
   model = _model;
   def = (_def ? _def : (_model ? _model->defaults[0] : 0));
+
+  // point to local (needs to be after defaults)
+  plugin.name = (mjString)&plugin_name;
+  plugin.instance_name = (mjString)&plugin_instance_name;
 }
 
 
@@ -302,26 +308,27 @@ void mjCMesh::LoadSDF() {
                    name.c_str(), id);
   }
 
+  mjCPlugin* plugin_instance = (mjCPlugin*)plugin.instance;
   model->ResolvePlugin(this, plugin_name, plugin_instance_name, &plugin_instance);
-  const mjpPlugin* plugin = mjp_getPluginAtSlot(plugin_instance->plugin_slot);
+  const mjpPlugin* pplugin = mjp_getPluginAtSlot(plugin_instance->plugin_slot);
-  if (!(plugin->capabilityflags & mjPLUGIN_SDF)) {
+  if (!(pplugin->capabilityflags & mjPLUGIN_SDF)) {
-    throw mjCError(this, "plugin '%s' does not support signed distance fields", plugin->name);
+    throw mjCError(this, "plugin '%s' does not support signed distance fields", pplugin->name);
   }
 
-  std::vector<mjtNum> attributes(plugin->nattribute, 0);
+  std::vector<mjtNum> attributes(pplugin->nattribute, 0);
-  std::vector<const char*> names(plugin->nattribute, 0);
+  std::vector<const char*> names(pplugin->nattribute, 0);
-  std::vector<const char*> values(plugin->nattribute, 0);
+  std::vector<const char*> values(pplugin->nattribute, 0);
-  for (int i=0; i < plugin->nattribute; i++) {
+  for (int i=0; i < pplugin->nattribute; i++) {
-    names[i] = plugin->attributes[i];
+    names[i] = pplugin->attributes[i];
     values[i] = plugin_instance->config_attribs[names[i]].c_str();
   }
 
-  if (plugin->sdf_attribute) {
+  if (pplugin->sdf_attribute) {
-    plugin->sdf_attribute(attributes.data(), names.data(), values.data());
+    pplugin->sdf_attribute(attributes.data(), names.data(), values.data());
   }
 
   mjtNum aabb[6] = {0};
-  plugin->sdf_aabb(aabb, attributes.data());
+  pplugin->sdf_aabb(aabb, attributes.data());
   mjtNum total = aabb[3] + aabb[4] + aabb[5];
 
   const mjtNum n = 300;
@@ -337,7 +344,7 @@ void mjCMesh::LoadSDF() {
         mjtNum point[] = {aabb[0]-aabb[3] + 2 * aabb[3] * i / (nx-1),
                           aabb[1]-aabb[4] + 2 * aabb[4] * j / (ny-1),
                           aabb[2]-aabb[5] + 2 * aabb[5] * k / (nz-1)};
-        field[(k * ny + j) * nx + i] =  plugin->sdf_staticdistance(point, attributes.data());
+        field[(k * ny + j) * nx + i] =  pplugin->sdf_staticdistance(point, attributes.data());
       }
     }
   }
@@ -410,7 +417,7 @@ void mjCMesh::Compile(const mjVFS* vfs) {
   }
 
   // create using marching cubes
-  else if (is_plugin) {
+  else if (plugin.active) {
     LoadSDF();
   }
 
@@ -2444,9 +2451,10 @@ void mjCFlex::Compile(const mjVFS* vfs) {
   mjXUtil::Vector2String(useredge, edgeidx);
 
   for (int i=0; i<(int)vertbodyid.size(); i++) {
-    if (model->bodies[vertbodyid[i]]->plugin_instance) {
+    if (model->bodies[vertbodyid[i]]->plugin.instance) {
-      model->bodies[vertbodyid[i]]->plugin_instance->config_attribs["face"] = userface;
+      mjCPlugin* plugin_instance = (mjCPlugin*)model->bodies[vertbodyid[i]]->plugin.instance;
-      model->bodies[vertbodyid[i]]->plugin_instance->config_attribs["edge"] = useredge;
+      plugin_instance->config_attribs["face"] = userface;
+      plugin_instance->config_attribs["edge"] = useredge;
     }
   }
 

src/user/user_model.cc

@@ -1491,7 +1491,7 @@ void mjCModel::CopyTree(mjModel* m) {
     m->body_mass[i] = (mjtNum)pb->mass;
     copyvec(m->body_inertia+3*i, pb->inertia, 3);
     m->body_gravcomp[i] = pb->gravcomp;
-    copyvec(m->body_user+nuser_body*i, pb->userdata.data(), nuser_body);
+    copyvec(m->body_user+nuser_body*i, pb->get_userdata().data(), nuser_body);
 
     m->body_contype[i] = pb->contype;
     m->body_conaffinity[i] = pb->conaffinity;
@@ -2883,7 +2883,7 @@ void mjCModel::TryCompile(mjModel*& m, mjData*& d, const mjVFS* vfs) {
   if (nuser_body == -1) {
     nuser_body = 0;
     for (int i=0; i<bodies.size(); i++) {
-      nuser_body = mjMAX(nuser_body, bodies[i]->userdata.size());
+      nuser_body = mjMAX(nuser_body, bodies[i]->spec_userdata_.size());
     }
   }
   if (nuser_jnt == -1) {
@@ -3050,8 +3050,8 @@ void mjCModel::TryCompile(mjModel*& m, mjData*& d, const mjVFS* vfs) {
     // set actuator_plugin to the plugin instance ID
     std::vector<std::vector<int>> plugin_to_actuators(nplugin);
     for (int i = 0; i < nu; ++i) {
-      if (actuators[i]->is_plugin) {
+      if (actuators[i]->plugin.active) {
-        int actuator_plugin = actuators[i]->plugin_instance->id;
+        int actuator_plugin = ((mjCPlugin*)actuators[i]->plugin.instance)->id;
         m->actuator_plugin[i] = actuator_plugin;
         plugin_to_actuators[actuator_plugin].push_back(i);
       } else {
@@ -3060,16 +3060,16 @@ void mjCModel::TryCompile(mjModel*& m, mjData*& d, const mjVFS* vfs) {
     }
 
     for (int i = 0; i < nbody; ++i) {
-      if (bodies[i]->is_plugin) {
+      if (bodies[i]->plugin.active) {
-        m->body_plugin[i] = bodies[i]->plugin_instance->id;
+        m->body_plugin[i] = ((mjCPlugin*)bodies[i]->plugin.instance)->id;
       } else {
         m->body_plugin[i] = -1;
       }
     }
 
     for (int i = 0; i < ngeom; ++i) {
-      if (geoms[i]->is_plugin) {
+      if (geoms[i]->plugin.active) {
-        m->geom_plugin[i] = geoms[i]->plugin_instance->id;
+        m->geom_plugin[i] = ((mjCPlugin*)geoms[i]->plugin.instance)->id;
       } else {
         m->geom_plugin[i] = -1;
       }
@@ -3078,7 +3078,7 @@ void mjCModel::TryCompile(mjModel*& m, mjData*& d, const mjVFS* vfs) {
     std::vector<std::vector<int>> plugin_to_sensors(nplugin);
     for (int i = 0; i < nsensor; ++i) {
       if (sensors[i]->type == mjSENS_PLUGIN) {
-        int sensor_plugin = sensors[i]->plugin_instance->id;
+        int sensor_plugin = ((mjCPlugin*)sensors[i]->plugin.instance)->id;
         m->sensor_plugin[i] = sensor_plugin;
         plugin_to_sensors[sensor_plugin].push_back(i);
       } else {
@@ -3321,7 +3321,7 @@ bool mjCModel::CopyBack(const mjModel* m) {
     copyvec(pb->inertia, m->body_inertia+3*i, 3);
 
     if (nuser_body) {
-      copyvec(pb->userdata.data(), m->body_user + nuser_body*i, nuser_body);
+      copyvec(pb->userdata_.data(), m->body_user + nuser_body*i, nuser_body);
     }
   }
 

src/user/user_objects.cc

@@ -493,10 +493,12 @@ mjCBase::mjCBase() {
   frame = nullptr;
 
   // plugin variables
-  is_plugin = false;
+  plugin.active = false;
-  plugin_instance = nullptr;
+  plugin.instance = nullptr;
   plugin_name = "";
   plugin_instance_name = "";
+  plugin.name = (mjString)&plugin_name;
+  plugin.instance_name = (mjString)&plugin_instance_name;
 }
 
 
@@ -550,26 +552,32 @@ mjCBody::mjCBody(mjCModel* _model) {
   // set model pointer
   model = _model;
 
-  // missing information, must be supplied later
+  spec.pos[0] = spec.ipos[0] = mjNAN;
-  pos[0] = ipos[0] = mjNAN;
 
   // clear variables
-  fullinertia[0] = mjNAN;
+  spec.fullinertia[0] = mjNAN;
-  explicitinertial = false;
+  spec.explicitinertial = false;
-  mocap = false;
+  spec.mocap = false;
-  mjuu_setvec(quat, 1, 0, 0, 0);
+  mjuu_setvec(spec.quat, 1, 0, 0, 0);
-  mjuu_setvec(iquat, 1, 0, 0, 0);
+  mjuu_setvec(spec.iquat, 1, 0, 0, 0);
-  mjuu_zerovec(pos+1, 2);
+  mjuu_zerovec(spec.pos+1, 2);
-  mjuu_zerovec(ipos+1, 2);
+  mjuu_zerovec(spec.ipos+1, 2);
-  mass = 0;
+  spec.mass = 0;
-  mjuu_setvec(inertia, 0, 0, 0);
+  mjuu_setvec(spec.inertia, 0, 0, 0);
   parentid = -1;
   weldid = -1;
   dofnum = 0;
   lastdof = -1;
   subtreedofs = 0;
-  gravcomp = 0;
+  spec.gravcomp = 0;
-  userdata.clear();
+  spec_userdata_.clear();
+  spec.alt.axisangle[0] = spec.alt.xyaxes[0] = spec.alt.zaxis[0] =
+      spec.alt.euler[0] = mjNAN;
+  spec.ialt.axisangle[0] = spec.ialt.xyaxes[0] = spec.ialt.zaxis[0] =
+      spec.ialt.euler[0] = mjNAN;
+
+  spec.plugin.active = false;
+  spec.plugin.instance = nullptr;
 
   contype = 0;
   conaffinity = 0;
@@ -585,6 +593,37 @@ mjCBody::mjCBody(mjCModel* _model) {
   sites.clear();
   cameras.clear();
   lights.clear();
+
+  // point to local
+  spec.element = (mjElement)this;
+  spec.name = (mjString)&name;
+  spec.classname = (mjString)&classname;
+  spec.userdata = (mjDouble)&spec_userdata_;
+  spec.plugin.name = (mjString)&plugin_name;
+  spec.plugin.instance_name = (mjString)&plugin_instance_name;
+
+  // in case this body is not compiled
+  CopyFromSpec();
+}
+
+
+void mjCBody::CopyFromSpec() {
+  *static_cast<mjmBody*>(this) = spec;
+  userdata_ = spec_userdata_;
+  userdata = (mjDouble)&userdata_;
+  spec.info = (mjString)&info;
+  mju_copy4(alt_.axisangle, alt.axisangle);
+  mju_copy(alt_.xyaxes, alt.xyaxes, 6);
+  mju_copy3(alt_.zaxis, alt.zaxis);
+  mju_copy3(alt_.euler, alt.euler);
+  mju_copy4(ialt_.axisangle, ialt.axisangle);
+  mju_copy(ialt_.xyaxes, ialt.xyaxes, 6);
+  mju_copy3(ialt_.zaxis, ialt.zaxis);
+  mju_copy3(ialt_.euler, ialt.euler);
+  plugin.active = spec.plugin.active;
+  plugin.instance = spec.plugin.instance;
+  plugin.name = spec.plugin.name;
+  plugin.instance_name = spec.plugin.instance_name;
 }
 
 
@@ -934,18 +973,20 @@ const char* mjCBody::FullInertia(double quat[4], double inertia[3]) {
 
 // set explicitinertial to true
 void mjCBody::MakeInertialExplicit() {
-  explicitinertial = true;
+  spec.explicitinertial = true;
 }
 
 
 // compiler
 void mjCBody::Compile(void) {
+  CopyFromSpec();
+
   // resize userdata
-  if (userdata.size() > model->nuser_body) {
+  if (userdata_.size() > model->nuser_body) {
     throw mjCError(this, "user has more values than nuser_body in body '%s' (id = %d)",
                    name.c_str(), id);
   }
-  userdata.resize(model->nuser_body);
+  userdata_.resize(model->nuser_body);
 
   // pos defaults to (0,0,0)
   if (!mjuu_defined(pos[0])) {
@@ -963,7 +1004,7 @@ void mjCBody::Compile(void) {
   }
 
   // check and process orientation alternatives for body
-  const char* err = alt.Set(quat, model->degree, model->euler);
+  const char* err = alt_.Set(quat, model->degree, model->euler);
   if (err) {
     throw mjCError(this, "error '%s' in frame alternative", err);
   }
@@ -1101,17 +1142,18 @@ void mjCBody::Compile(void) {
   for (int i=0; i<lights.size(); i++) lights[i]->Compile();
 
   // plugin
-  if (is_plugin) {
+  if (plugin.active) {
     if (plugin_name.empty() && plugin_instance_name.empty()) {
       throw mjCError(
           this, "neither 'plugin' nor 'instance' is specified for body '%s', (id = %d)",
           name.c_str(), id);
     }
 
-    model->ResolvePlugin(this, plugin_name, plugin_instance_name, &plugin_instance);
+    mjCPlugin** plugin_instance = (mjCPlugin**)&plugin.instance;
-    const mjpPlugin* plugin = mjp_getPluginAtSlot(plugin_instance->plugin_slot);
+    model->ResolvePlugin(this, plugin_name, plugin_instance_name, plugin_instance);
-    if (!(plugin->capabilityflags & mjPLUGIN_PASSIVE)) {
+    const mjpPlugin* pplugin = mjp_getPluginAtSlot((*plugin_instance)->plugin_slot);
-      throw mjCError(this, "plugin '%s' does not support passive forces", plugin->name);
+    if (!(pplugin->capabilityflags & mjPLUGIN_PASSIVE)) {
+      throw mjCError(this, "plugin '%s' does not support passive forces", pplugin->name);
     }
   }
 
@@ -1399,11 +1441,9 @@ mjCGeom::mjCGeom(mjCModel* _model, mjCDef* _def) {
   model = _model;
   def = (_def ? _def : (_model ? _model->defaults[0] : 0));
 
-  // plugin variables
+  // point to local (needs to be after defaults)
-  is_plugin = false;
+  plugin.name = (mjString)&plugin_name;
-  plugin_instance = nullptr;
+  plugin.instance_name = (mjString)&plugin_instance_name;
-  plugin_name = "";
-  plugin_instance_name = "";
 }
 
 
@@ -1915,17 +1955,18 @@ void mjCGeom::Compile(void) {
   }
 
   // plugin
-  if (is_plugin) {
+  if (plugin.active) {
     if (plugin_name.empty() && plugin_instance_name.empty()) {
       throw mjCError(
           this, "neither 'plugin' nor 'instance' is specified for geom '%s', (id = %d)",
           name.c_str(), id);
     }
 
-    model->ResolvePlugin(this, plugin_name, plugin_instance_name, &plugin_instance);
+    mjCPlugin** plugin_instance = (mjCPlugin**)&plugin.instance;
-    const mjpPlugin* plugin = mjp_getPluginAtSlot(plugin_instance->plugin_slot);
+    model->ResolvePlugin(this, plugin_name, plugin_instance_name, plugin_instance);
-    if (!(plugin->capabilityflags & mjPLUGIN_SDF)) {
+    const mjpPlugin* pplugin = mjp_getPluginAtSlot((*plugin_instance)->plugin_slot);
-      throw mjCError(this, "plugin '%s' does not support sign distance fields", plugin->name);
+    if (!(pplugin->capabilityflags & mjPLUGIN_SDF)) {
+      throw mjCError(this, "plugin '%s' does not support sign distance fields", pplugin->name);
     }
   }
 
@@ -3921,10 +3962,9 @@ mjCActuator::mjCActuator(mjCModel* _model, mjCDef* _def) {
   model = _model;
   def = (_def ? _def : (_model ? _model->defaults[0] : 0));
 
-  is_plugin = false;
+  // point to local (needs to be after defaults)
-  plugin_instance = nullptr;
+  plugin.name = (mjString)&plugin_name;
-  plugin_name = "";
+  plugin.instance_name = (mjString)&plugin_instance_name;
-  plugin_instance_name = "";
 }
 
 
@@ -4106,17 +4146,18 @@ void mjCActuator::Compile(void) {
   }
 
   // plugin
-  if (is_plugin) {
+  if (plugin.active) {
     if (plugin_name.empty() && plugin_instance_name.empty()) {
       throw mjCError(
           this, "neither 'plugin' nor 'instance' is specified for actuator '%s', (id = %d)",
           name.c_str(), id);
     }
 
-    model->ResolvePlugin(this, plugin_name, plugin_instance_name, &plugin_instance);
+    mjCPlugin** plugin_instance = (mjCPlugin**)&plugin.instance;
-    const mjpPlugin* plugin = mjp_getPluginAtSlot(plugin_instance->plugin_slot);
+    model->ResolvePlugin(this, plugin_name, plugin_instance_name, plugin_instance);
-    if (!(plugin->capabilityflags & mjPLUGIN_ACTUATOR)) {
+    const mjpPlugin* pplugin = mjp_getPluginAtSlot((*plugin_instance)->plugin_slot);
-      throw mjCError(this, "plugin '%s' does not support actuators", plugin->name);
+    if (!(pplugin->capabilityflags & mjPLUGIN_ACTUATOR)) {
+      throw mjCError(this, "plugin '%s' does not support actuators", pplugin->name);
     }
   }
 }
@@ -4147,9 +4188,9 @@ mjCSensor::mjCSensor(mjCModel* _model) {
   obj = nullptr;
   refid = -1;
 
-  plugin_instance = nullptr;
+  // point to local (needs to be after defaults)
-  plugin_name = "";
+  plugin.name = (mjString)&plugin_name;
-  plugin_instance_name = "";
+  plugin.instance_name = (mjString)&plugin_instance_name;
 }
 
 
@@ -4520,12 +4561,13 @@ void mjCSensor::Compile(void) {
 
     // resolve plugin instance, or create one if using the "plugin" attribute shortcut
     {
-      model->ResolvePlugin(this, plugin_name, plugin_instance_name, &plugin_instance);
+      mjCPlugin** plugin_instance = (mjCPlugin**)&plugin.instance;
-      const mjpPlugin* plugin = mjp_getPluginAtSlot(plugin_instance->plugin_slot);
+      model->ResolvePlugin(this, plugin_name, plugin_instance_name, plugin_instance);
-      if (!(plugin->capabilityflags & mjPLUGIN_SENSOR)) {
+      const mjpPlugin* pplugin = mjp_getPluginAtSlot((*plugin_instance)->plugin_slot);
-        throw mjCError(this, "plugin '%s' does not support sensors", plugin->name);
+      if (!(pplugin->capabilityflags & mjPLUGIN_SENSOR)) {
+        throw mjCError(this, "plugin '%s' does not support sensors", pplugin->name);
       }
-      needstage = static_cast<mjtStage>(plugin->needstage);
+      needstage = static_cast<mjtStage>(pplugin->needstage);
     }
 
     break;

src/user/user_objects.h

@@ -173,7 +173,6 @@ class mjCBoundingVolumeHierarchy {
 //------------------------- class mjCBase ----------------------------------------------------------
 // Generic functionality for all derived classes
 
-class mjCPlugin;
 class mjCBase {
   friend class mjCDef;
 
@@ -197,11 +196,9 @@ class mjCBase {
   mjCFrame* frame;                // pointer to frame transformation
 
   // plugin support
-  bool is_plugin;
+  mjmPlugin plugin;
   std::string plugin_name;
   std::string plugin_instance_name;
-  mjCPlugin* plugin_instance;
-
  protected:
   mjCBase();                      // constructor
 };
@@ -211,13 +208,15 @@ class mjCBase {
 //------------------------- class mjCBody -----------------------------------------------
 // Describes a rigid body
 
-class mjCBody : public mjCBase {
+class mjCBody : public mjCBase, private mjmBody {
   friend class mjCJoint;
   friend class mjCGeom;
   friend class mjCSite;
   friend class mjCCamera;
+  friend class mjCComposite;
   friend class mjCLight;
   friend class mjCFlex;
+  friend class mjCFlexcomp;
   friend class mjCEquality;
   friend class mjCPair;
   friend class mjCModel;
@@ -248,33 +247,32 @@ class mjCBody : public mjCBase {
   // return nullptr on success, error string on failure
   const char* FullInertia(double quat[4], double inertia[3]);
 
-  // variables set by user or 'Compile'
+  // variables set by user
-  bool mocap;                     // is this a mocap body
+  mjmBody spec;
-  double pos[3];                  // frame position
+
-  double quat[4];                 // frame orientation
+  // inherited
-  double ipos[3];                 // inertial frame position
+  using mjCBase::name;
-  double iquat[4];                // inertial frame orientation
+  using mjCBase::classname;
-  double mass;                    // mass
+  using mjCBase::info;
-  double inertia[3];              // diagonal inertia (in i-frame)
+  using mjCBase::plugin;
-  double gravcomp;                // gravity compensation
+
-  std::vector<double> userdata;   // user data
+  // used by mjXWriter and mjCModel
-  double fullinertia[6];          // non-axis-aligned inertia matrix
+  const std::vector<double>& get_userdata() { return userdata_; }
-  mjCAlternative alt;             // alternative orientation specification
+
-  mjCAlternative ialt;            // alternative for inertial frame
+  mjCAlternative alt_;
+  mjCAlternative ialt_;
 
   // variables computed by 'Compile' and 'AddXXX'
  private:
   mjCBody(mjCModel*);             // constructor
   ~mjCBody();                     // destructor
-  void Compile(void);                 // compiler
+  void Compile(void);             // compiler
-
   void GeomFrame(void);           // get inertial info from geoms
 
   int parentid;                   // parent index in global array
   int weldid;                     // top index of body we are welded to
   int dofnum;                     // number of motion dofs for body
   int mocapid;                    // mocap id, -1: not mocap
-  bool explicitinertial;          // whether to save the body with an explicit inertial clause
 
   int contype;                    // OR over geom contypes
   int conaffinity;                // OR over geom conaffinities
@@ -296,6 +294,12 @@ class mjCBody : public mjCBase {
   std::vector<mjCSite*>    sites;      // sites attached to this body
   std::vector<mjCCamera*>  cameras;    // cameras attached to this body
   std::vector<mjCLight*>   lights;     // lights attached to this body
+
+  void CopyFromSpec();                 // copy spec into attributes
+
+  // variable-size data
+  std::vector<double> userdata_;
+  std::vector<double> spec_userdata_;
 };
 
 
@@ -482,7 +486,6 @@ class mjCSite : public mjCBase, private mjmSite {
   void CopyFromSpec();                    // copy spec into attributes
 
   mjCAlternative alt_;
-  mjCAlternative spec_alt_;
 
   // variable-size data
   std::string material_;
@@ -1223,11 +1226,6 @@ class mjCSensor : public mjCBase {
   double noise;                   // noise stdev
   std::vector<double> userdata;   // user data
 
-  // plugin support
-  std::string plugin_name;
-  std::string plugin_instance_name;
-  mjCPlugin* plugin_instance;
-
  private:
   mjCSensor(mjCModel*);           // constructor
   void Compile(void);             // compiler

src/xml/xml_native_reader.cc

@@ -873,7 +873,7 @@ void mjXReader::Parse(XMLElement* root) {
 
   for (XMLElement* section = root->FirstChildElement("worldbody"); section;
        section = section->NextSiblingElement("worldbody")) {
-    Body(section, model->GetWorld(), nullptr);
+    Body(section, &model->GetWorld()->spec, nullptr);
   }
 
   for (XMLElement* section = root->FirstChildElement("contact"); section;
@@ -1350,7 +1350,7 @@ void mjXReader::OneMesh(XMLElement* elem, mjCMesh* pmesh) {
 
   XMLElement* eplugin = elem->FirstChildElement("plugin");
   if (eplugin) {
-    OnePlugin(eplugin, pmesh);
+    OnePlugin(eplugin, &pmesh->plugin);
   }
 
   if (MapValue(elem, "smoothnormal", &n, bool_map, 2)) {
@@ -1542,7 +1542,7 @@ void mjXReader::OneGeom(XMLElement* elem, mjCGeom* pgeom) {
   // plugin sub-element
   XMLElement* eplugin = elem->FirstChildElement("plugin");
   if (eplugin) {
-    OnePlugin(eplugin, pgeom);
+    OnePlugin(eplugin, &pgeom->plugin);
   }
 
   // remaining attributes
@@ -2044,7 +2044,7 @@ void mjXReader::OneActuator(XMLElement* elem, mjCActuator* pact) {
   }
 
   else if (type == "plugin") {
-    OnePlugin(elem, pact);
+    OnePlugin(elem, &pact->plugin);
     int n;
     if (MapValue(elem, "dyntype", &n, dyn_map, dyn_sz)) {
       pact->dyntype = (mjtDyn)n;
@@ -2068,7 +2068,7 @@ void mjXReader::OneActuator(XMLElement* elem, mjCActuator* pact) {
 
 
 // make composite
-void mjXReader::OneComposite(XMLElement* elem, mjCBody* pbody, mjCDef* def) {
+void mjXReader::OneComposite(XMLElement* elem, mjmBody* pbody, mjCDef* def) {
   string text;
   int n;
 
@@ -2091,7 +2091,7 @@ void mjXReader::OneComposite(XMLElement* elem, mjCBody* pbody, mjCDef* def) {
     ReadAttrTxt(eplugin, "plugin", comp.plugin_name);
     ReadAttrTxt(eplugin, "instance", comp.plugin_instance_name);
     if (comp.plugin_instance_name.empty()) {
-      comp.plugin_instance = model->AddPlugin();
+      comp.plugin_instance = (mjCPlugin*)mjm_addPlugin(model);
       comp.plugin_instance->name = "composite"+comp.prefix;
       comp.plugin_instance_name = comp.plugin_instance->name;
     } else {
@@ -2286,7 +2286,7 @@ void mjXReader::OneComposite(XMLElement* elem, mjCBody* pbody, mjCDef* def) {
 
   // make composite
   char error[200];
-  bool res = comp.Make(pbody->model, pbody, error, 200);
+  bool res = comp.Make((mjCModel*)mjm_getModel(pbody), pbody, error, 200);
 
   // throw error
   if (!res) {
@@ -2297,7 +2297,7 @@ void mjXReader::OneComposite(XMLElement* elem, mjCBody* pbody, mjCDef* def) {
 
 
 // make flexcomp
-void mjXReader::OneFlexcomp(XMLElement* elem, mjCBody* pbody) {
+void mjXReader::OneFlexcomp(XMLElement* elem, mjmBody* pbody) {
   string text;
   int n;
 
@@ -2407,7 +2407,7 @@ void mjXReader::OneFlexcomp(XMLElement* elem, mjCBody* pbody) {
     ReadAttrTxt(eplugin, "plugin", fcomp.plugin_name);
     ReadAttrTxt(eplugin, "instance", fcomp.plugin_instance_name);
     if (fcomp.plugin_instance_name.empty()) {
-      fcomp.plugin_instance = model->AddPlugin();
+      fcomp.plugin_instance = (mjCPlugin*)mjm_addPlugin(model);
       fcomp.plugin_instance->name = "flexcomp_" + fcomp.name;
       fcomp.plugin_instance_name = fcomp.plugin_instance->name;
     } else {
@@ -2418,7 +2418,7 @@ void mjXReader::OneFlexcomp(XMLElement* elem, mjCBody* pbody) {
 
   // make flexcomp
   char error[200];
-  bool res = fcomp.Make(pbody->model, pbody, error, 200);
+  bool res = fcomp.Make((mjCModel*)mjm_getModel(pbody), pbody, error, 200);
 
   // throw error
   if (!res) {
@@ -2429,13 +2429,17 @@ void mjXReader::OneFlexcomp(XMLElement* elem, mjCBody* pbody) {
 
 
 // add plugin
-void mjXReader::OnePlugin(XMLElement* elem, mjCBase* object) {
+void mjXReader::OnePlugin(XMLElement* elem, mjmPlugin* plugin) {
-  object->is_plugin = true;
+  plugin->active = true;
-  ReadAttrTxt(elem, "plugin", object->plugin_name);
+  std::string name = "";
-  ReadAttrTxt(elem, "instance", object->plugin_instance_name);
+  std::string instance_name = "";
-  if (object->plugin_instance_name.empty()) {
+  ReadAttrTxt(elem, "plugin", name);
-    object->plugin_instance = model->AddPlugin();
+  ReadAttrTxt(elem, "instance", instance_name);
-    ReadPluginConfigs(elem, object->plugin_instance);
+  mjm_setString(plugin->name, name.c_str());
+  mjm_setString(plugin->instance_name, instance_name.c_str());
+  if (instance_name.empty()) {
+    plugin->instance = mjm_addPlugin(model);
+    ReadPluginConfigs(elem, (mjCPlugin*)plugin->instance);
   } else {
     model->hasImplicitPluginElem = true;
   }
@@ -2579,7 +2583,7 @@ void mjXReader::Extension(XMLElement* section) {
             throw mjXError(
                 child, "explicit plugin instance must appear before implicit plugin elements");
           }
-          mjCPlugin* pp = model->AddPlugin();
+          mjCPlugin* pp = (mjCPlugin*)mjm_addPlugin(model);
           GetXMLPos(child, pp);
           ReadAttrTxt(child, "name", pp->name, /* required = */ true);
           if (pp->name.empty()) {
@@ -2983,7 +2987,7 @@ void mjXReader::Asset(XMLElement* section) {
 
 
 // body/world section parser; recursive
-void mjXReader::Body(XMLElement* section, mjCBody* pbody, mjCFrame* frame) {
+void mjXReader::Body(XMLElement* section, mjmBody* pbody, mjCFrame* frame) {
   string text, name;
   XMLElement* elem;
   int n;
@@ -2994,7 +2998,7 @@ void mjXReader::Body(XMLElement* section, mjCBody* pbody, mjCFrame* frame) {
   }
 
   // no attributes allowed in world body
-  if (pbody->id==0 && section->FirstAttribute() && !frame) {
+  if (mjm_getId(pbody->element)==0 && section->FirstAttribute() && !frame) {
     throw mjXError(section, "World body cannot have attributes");
   }
 
@@ -3007,13 +3011,13 @@ void mjXReader::Body(XMLElement* section, mjCBody* pbody, mjCFrame* frame) {
     // get class if specified, otherwise use body
     mjCDef* def = GetClass(elem);
     if (!def) {
-      def = pbody->def;
+      def = (mjCDef*)mjm_getDefault(pbody->element);
     }
 
     // inertial sub-element
     if (name=="inertial") {
       // no inertia allowed in world body
-      if (pbody->id==0) {
+      if (mjm_getId(pbody->element)==0) {
         throw mjXError(elem, "World body cannot have inertia");
       }
       pbody->explicitinertial = true;
@@ -3031,12 +3035,12 @@ void mjXReader::Body(XMLElement* section, mjCBody* pbody, mjCFrame* frame) {
     // joint sub-element
     else if (name=="joint") {
       // no joints allowed in world body
-      if (pbody->id==0) {
+      if (mjm_getId(pbody->element)==0) {
         throw mjXError(elem, "World body cannot have joints");
       }
 
       // create joint and parse
-      mjCJoint* pjoint = pbody->AddJoint(def);
+      mjCJoint* pjoint = (mjCJoint*)mjm_addJoint(pbody, def);
       OneJoint(elem, pjoint);
       pjoint->SetFrame(frame);
     }
@@ -3044,12 +3048,12 @@ void mjXReader::Body(XMLElement* section, mjCBody* pbody, mjCFrame* frame) {
     // freejoint sub-element
     else if (name=="freejoint") {
       // no joints allowed in world body
-      if (pbody->id==0) {
+      if (mjm_getId(pbody->element)==0) {
         throw mjXError(elem, "World body cannot have joints");
       }
 
       // create free joint without defaults
-      mjCJoint* pjoint = pbody->AddFreeJoint();
+      mjCJoint* pjoint = (mjCJoint*)mjm_addFreeJoint(pbody);
       pjoint->SetFrame(frame);
 
       // save defaults after creation, to make sure writing is ok
@@ -3063,7 +3067,7 @@ void mjXReader::Body(XMLElement* section, mjCBody* pbody, mjCFrame* frame) {
     // geom sub-element
     else if (name=="geom") {
       // create geom and parse
-      mjCGeom* pgeom = pbody->AddGeom(def);
+      mjCGeom* pgeom = (mjCGeom*)mjm_addGeom(pbody, def);
       OneGeom(elem, pgeom);
       pgeom->SetFrame(frame);
     }
@@ -3073,13 +3077,13 @@ void mjXReader::Body(XMLElement* section, mjCBody* pbody, mjCFrame* frame) {
       // create site and parse
       mjmSite* site = mjm_addSite(pbody,  def);
       OneSite(elem, *site);
-      mjm_setFrame(site, frame);
+      mjm_setFrame(site->element, frame);
     }
 
     // camera sub-element
     else if (name=="camera") {
       // create camera and parse
-      mjCCamera* pcam = pbody->AddCamera(def);
+      mjCCamera* pcam = (mjCCamera*)mjm_addCamera(pbody, def);
       OneCamera(elem, pcam);
       pcam->SetFrame(frame);
     }
@@ -3087,14 +3091,14 @@ void mjXReader::Body(XMLElement* section, mjCBody* pbody, mjCFrame* frame) {
     // light sub-element
     else if (name=="light") {
       // create light and parse
-      mjCLight* plight = pbody->AddLight(def);
+      mjCLight* plight = (mjCLight*)mjm_addLight(pbody, def);
       OneLight(elem, plight);
       plight->SetFrame(frame);
     }
 
     // plugin sub-element
     else if (name == "plugin") {
-      OnePlugin(elem, pbody);
+      OnePlugin(elem, &(pbody->plugin));
     }
 
     // composite sub-element
@@ -3111,7 +3115,7 @@ void mjXReader::Body(XMLElement* section, mjCBody* pbody, mjCFrame* frame) {
 
     // frame sub-element
     else if (name=="frame") {
-      mjCFrame* pframe = pbody->AddFrame(frame);
+      mjCFrame* pframe = (mjCFrame*)mjm_addFrame(pbody, frame);
       GetXMLPos(elem, pframe);
 
       ReadAttr(elem, "pos", 3, pframe->pos, text);
@@ -3133,12 +3137,16 @@ void mjXReader::Body(XMLElement* section, mjCBody* pbody, mjCFrame* frame) {
       }
 
       // create child body
-      mjCBody* pchild = pbody->AddBody(childdef);
+      mjmBody* pchild = mjm_addBody(pbody, childdef);
-      GetXMLPos(elem, pchild);
+      mjm_setString(pchild->info,
+                    std::string("line = " + std::to_string(elem->GetLineNum())).c_str());
 
       // read attributes
-      ReadAttrTxt(elem, "name", pchild->name);
+      std::string name, childclass;
-      ReadAttrTxt(elem, "childclass", pchild->classname);
+      ReadAttrTxt(elem, "name", name);
+      mjm_setString(pchild->name, name.c_str());
+      ReadAttrTxt(elem, "childclass", childclass);
+      mjm_setString(pchild->classname, childclass.c_str());
       ReadAttr(elem, "pos", 3, pchild->pos, text);
       ReadQuat(elem, "quat", pchild->quat, text);
       if (MapValue(elem, "mocap", &n, bool_map, 2)) {
@@ -3150,10 +3158,12 @@ void mjXReader::Body(XMLElement* section, mjCBody* pbody, mjCFrame* frame) {
       ReadAttr(elem, "gravcomp", 1, &pchild->gravcomp, text);
 
       // read userdata
-      ReadVector(elem, "user", pchild->userdata, text);
+      std::vector<double> userdata;
+      ReadVector(elem, "user", userdata, text);
+      mjm_setDouble(pchild->userdata, userdata.data(), userdata.size());
 
       // add frame
-      pchild->SetFrame(frame);
+      mjm_setFrame(pchild->element, frame);
 
       // make recursive call
       Body(elem, pchild, nullptr);
@@ -3373,6 +3383,8 @@ void mjXReader::Sensor(XMLElement* section) {
     // create sensor, get string type
     mjCSensor* psen = model->AddSensor();
     string type = elem->Value();
+    string plugin_name = "";
+    string instance_name = "";
 
     // read name, noise, userdata
     ReadAttrTxt(elem, "name", psen->name);
@@ -3630,14 +3642,16 @@ void mjXReader::Sensor(XMLElement* section) {
 
     else if (type=="plugin") {
       psen->type = mjSENS_PLUGIN;
-      ReadAttrTxt(elem, "plugin", psen->plugin_name);
+      ReadAttrTxt(elem, "plugin", plugin_name);
-      ReadAttrTxt(elem, "instance", psen->plugin_instance_name);
+      ReadAttrTxt(elem, "instance", instance_name);
-      if (psen->plugin_instance_name.empty()) {
+      mjm_setString(psen->plugin.name, plugin_name.c_str());
-        psen->plugin_instance = model->AddPlugin();
+      mjm_setString(psen->plugin.instance_name, instance_name.c_str());
+      if (instance_name.empty()) {
+        psen->plugin.instance = mjm_addPlugin(model);
       } else {
         model->hasImplicitPluginElem = true;
       }
-      ReadPluginConfigs(elem, psen->plugin_instance);
+      ReadPluginConfigs(elem, (mjCPlugin*)psen->plugin.instance);
       ReadAttrTxt(elem, "objtype", text);
       psen->objtype = (mjtObj)mju_str2Type(text.c_str());
       ReadAttrTxt(elem, "objname", psen->objname);

src/xml/xml_native_reader.h

@@ -42,7 +42,7 @@ class mjXReader : public mjXBase {
   void Visual(tinyxml2::XMLElement* section);                          // visual section
   void Statistic(tinyxml2::XMLElement* section);                       // statistic section
   void Asset(tinyxml2::XMLElement* section);                           // asset section
-  void Body(tinyxml2::XMLElement* section, mjCBody* pbody,
+  void Body(tinyxml2::XMLElement* section, mjmBody* pbody,
             mjCFrame* pframe);                                         // body/world section
   void Contact(tinyxml2::XMLElement* section);                         // contact section
   void Deformable(tinyxml2::XMLElement* section);                      // deformable section
@@ -66,9 +66,9 @@ class mjXReader : public mjXBase {
   void OneEquality(tinyxml2::XMLElement* elem, mjCEquality* pequality);
   void OneTendon(tinyxml2::XMLElement* elem, mjCTendon* ptendon);
   void OneActuator(tinyxml2::XMLElement* elem, mjCActuator* pactuator);
-  void OneComposite(tinyxml2::XMLElement* elem, mjCBody* pbody, mjCDef* def);
+  void OneComposite(tinyxml2::XMLElement* elem, mjmBody* pbody, mjCDef* def);
-  void OneFlexcomp(tinyxml2::XMLElement* elem, mjCBody* pbody);
+  void OneFlexcomp(tinyxml2::XMLElement* elem, mjmBody* pbody);
-  void OnePlugin(tinyxml2::XMLElement* elem, mjCBase* object);
+  void OnePlugin(tinyxml2::XMLElement* elem, mjmPlugin* plugin);
 
   mjXSchema schema;                                                   // schema used for validation
   mjCDef* GetClass(tinyxml2::XMLElement* section);                    // get default class name

src/xml/xml_native_writer.cc

@@ -26,6 +26,7 @@
 #include "engine/engine_plugin.h"
 #include "engine/engine_util_errmem.h"
 #include "engine/engine_util_misc.h"
+#include "user/user_api.h"
 #include "user/user_model.h"
 #include "user/user_objects.h"
 #include "user/user_util.h"
@@ -417,7 +418,7 @@ void mjXWriter::OneGeom(XMLElement* elem, mjCGeom* pgeom, mjCDef* def) {
   }
 
   // write plugin
-  if (pgeom->is_plugin) {
+  if (pgeom->plugin.active) {
     OnePlugin(InsertEnd(elem, "plugin"), pgeom);
   }
 }
@@ -725,7 +726,7 @@ void mjXWriter::OneActuator(XMLElement* elem, mjCActuator* pact, mjCDef* def) {
   WriteAttr(elem, "dynprm", mjNDYN, pact->dynprm, def->actuator.dynprm);
 
   // plugins: write config attributes
-  if (pact->is_plugin) {
+  if (pact->plugin.active) {
     OnePlugin(elem, pact);
   }
 
@@ -756,13 +757,15 @@ void mjXWriter::OneActuator(XMLElement* elem, mjCActuator* pact, mjCDef* def) {
 
 // write plugin
 void mjXWriter::OnePlugin(XMLElement* elem, mjCBase* object) {
-  if (!object->plugin_instance_name.empty()) {
+  const std::string instance_name = std::string(mjm_getString(object->plugin.instance_name));
-    WriteAttrTxt(elem, "instance", object->plugin_instance_name);
+  const std::string plugin_name = std::string(mjm_getString(object->plugin.name));
+  if (!instance_name.empty()) {
+    WriteAttrTxt(elem, "instance", instance_name);
   } else {
-    WriteAttrTxt(elem, "plugin", object->plugin_name);
+    WriteAttrTxt(elem, "plugin", plugin_name);
     const mjpPlugin* plugin = mjp_getPluginAtSlot(
-        object->plugin_instance->plugin_slot);
+        ((mjCPlugin*)object->plugin.instance)->plugin_slot);
-    const char* c = &object->plugin_instance->flattened_attributes[0];
+    const char* c = &((mjCPlugin*)object->plugin.instance)->flattened_attributes[0];
     for (int i = 0; i < plugin->nattribute; ++i) {
       std::string value(c);
       if (!value.empty()) {
@@ -1424,7 +1427,7 @@ void mjXWriter::Asset(XMLElement* root) {
   for (int i=0; i<nmesh; i++) {
     // create element and write
     mjCMesh* pmesh = (mjCMesh*)model->GetObject(mjOBJ_MESH, i);
-    if (pmesh->is_plugin) {
+    if (pmesh->plugin.active) {
       elem = InsertEnd(section, "mesh");
       WriteAttrTxt(elem, "name", pmesh->name);
       OnePlugin(InsertEnd(elem, "plugin"), pmesh);
@@ -1487,7 +1490,7 @@ void mjXWriter::Body(XMLElement* elem, mjCBody* body) {
       WriteAttr(elem, "gravcomp", 1, &body->gravcomp);
     }
     // userdata
-    WriteVector(elem, "user", body->userdata);
+    WriteVector(elem, "user", body->get_userdata());
 
     // write inertial
     if (body->explicitinertial &&
@@ -1526,7 +1529,7 @@ void mjXWriter::Body(XMLElement* elem, mjCBody* body) {
   }
 
   // write plugin
-  if (body->is_plugin) {
+  if (body->plugin.active) {
     OnePlugin(InsertEnd(elem, "plugin"), body);
   }
 
@@ -1709,7 +1712,7 @@ void mjXWriter::Actuator(XMLElement* root) {
   for (int i=0; i<num; i++) {
     mjCActuator* pact = (mjCActuator*)model->GetObject(mjOBJ_ACTUATOR, i);
     XMLElement* elem;
-    if (pact->is_plugin) {
+    if (pact->plugin.active) {
       elem = InsertEnd(section, "plugin");
     } else {
       elem = InsertEnd(section, "general");
@@ -1737,6 +1740,8 @@ void mjXWriter::Sensor(XMLElement* root) {
   for (int i=0; i<num; i++) {
     XMLElement* elem = 0;
     mjCSensor* psen = model->sensors[i];
+    std::string instance_name = "";
+    std::string plugin_name = "";
 
     // write sensor type and type-specific attributes
     switch (psen->type) {
@@ -1923,13 +1928,15 @@ void mjXWriter::Sensor(XMLElement* root) {
         WriteAttrTxt(elem, "objtype", mju_type2Str(psen->objtype));
         WriteAttrTxt(elem, "objname", psen->objname);
       }
-      if (!psen->plugin_instance_name.empty()) {
+      instance_name = std::string(mjm_getString(psen->plugin.instance_name));
-        WriteAttrTxt(elem, "instance", psen->plugin_instance_name);
+      plugin_name = std::string(mjm_getString(psen->plugin.name));
+      if (!instance_name.empty()) {
+        WriteAttrTxt(elem, "instance", instance_name);
       } else {
-        WriteAttrTxt(elem, "plugin", psen->plugin_name);
+        WriteAttrTxt(elem, "plugin", plugin_name);
         const mjpPlugin* plugin = mjp_getPluginAtSlot(
-            psen->plugin_instance->plugin_slot);
+            ((mjCPlugin*)psen->plugin.instance)->plugin_slot);
-        const char* c = &psen->plugin_instance->flattened_attributes[0];
+        const char* c = &((mjCPlugin*)psen->plugin.instance)->flattened_attributes[0];
         for (int i = 0; i < plugin->nattribute; ++i) {
           std::string value(c);
           if (!value.empty()) {

src/xml/xml_urdf.cc

@@ -18,6 +18,7 @@
 #include <vector>
 
 #include <mujoco/mjmodel.h>
+#include "user/user_api.h"
 #include "user/user_model.h"
 #include "user/user_objects.h"
 #include "user/user_util.h"
@@ -201,14 +202,15 @@ void mjXURDF::Parse(
   // override the pose for the base link and add a free joint
   for (int i = 0; i < (int)urName.size(); i++) {
     if (urParent[i] < 0) {
-      mjCBody* pbody = (mjCBody*)model->GetWorld()->FindObject(mjOBJ_BODY, urName[i]);
+      mjmBody* world = mjm_findBody(model, "world");
+      mjmBody* pbody = mjm_findChild(world, urName[i].c_str());
       mjuu_copyvec(pbody->pos, pos, 3);
       mjuu_copyvec(pbody->quat, quat, 4);
 
       // add a free joint to allow motion of the body
       // if the mass is 0, assume the object is static
       if (!static_body && pbody->mass > 0) {
-        auto pjoint = pbody->AddJoint();
+        mjCJoint* pjoint = (mjCJoint*)mjm_addJoint(pbody, 0);
         pjoint->name = urName[i] + "_free_joint";
         pjoint->type = mjJNT_FREE;
       }
@@ -220,13 +222,14 @@ void mjXURDF::Parse(
 void mjXURDF::Body(XMLElement* body_elem) {
   std::string name, text;
   XMLElement *elem, *temp, *temp1;
-  mjCBody* pbody;
+  mjmBody *pbody, *world;
   mjCGeom* pgeom;
 
-  // get body name and pointer to mjCBody
+  // get body name and pointer to mjmBody
   ReadAttrTxt(body_elem, "name", name, true);
   name = GetPrefixedName(name);
-  pbody = (mjCBody*) model->GetWorld()->FindObject(mjOBJ_BODY, name);
+  world = mjm_findBody(model, "world");
+  pbody = mjm_findChild(world, name.c_str());
   if (!pbody) {
     throw mjXError(body_elem, "URDF body not found");  // SHOULD NOT OCCUR
   }
@@ -264,7 +267,7 @@ void mjXURDF::Body(XMLElement* body_elem) {
     // process inertia
     //  lquat = rotation from specified to default (joint/body) inertial frame
     double lquat[4], tmpquat[4];
-    const char* altres = pbody->FullInertia(lquat, pbody->inertia);
+    const char* altres = mjm_setFullInertia(pbody, lquat, pbody->inertia);
 
     // inertia are sometimes 0 in URDF files: ignore error in altres, fix later
     (void) altres;
@@ -366,7 +369,7 @@ void mjXURDF::Parse(XMLElement* root) {
 void mjXURDF::Joint(XMLElement* joint_elem) {
   std::string jntname, name, text;
   XMLElement *elem;
-  mjCBody *pbody, *parent;
+  mjmBody *pbody, *parent, *world;
   mjCJoint *pjoint=0, *pjoint1=0, *pjoint2=0;
   int jointtype;
 
@@ -382,7 +385,8 @@ void mjXURDF::Joint(XMLElement* joint_elem) {
   elem = FindSubElem(joint_elem, "parent", true);
   ReadAttrTxt(elem, "link", name, true);
   name = GetPrefixedName(name);
-  parent = (mjCBody*) model->GetWorld()->FindObject(mjOBJ_BODY, name);
+  world = mjm_findBody(model, "world");
+  parent = mjm_findChild(world, name.c_str());
   if (!parent) {                      // SHOULD NOT OCCUR
     throw mjXError(elem, "invalid parent name in URDF joint definition");
   }
@@ -391,7 +395,8 @@ void mjXURDF::Joint(XMLElement* joint_elem) {
   elem = FindSubElem(joint_elem, "child", true);
   ReadAttrTxt(elem, "link", name, true);
   name = GetPrefixedName(name);
-  pbody = (mjCBody*) model->GetWorld()->FindObject(mjOBJ_BODY, name);
+  world = mjm_findBody(model, "world");
+  pbody = mjm_findChild(world, name.c_str());
   if (!pbody) {                       // SHOULD NOT OCCUR
     throw mjXError(elem, "invalid child name in URDF joint definition");
   }
@@ -408,7 +413,7 @@ void mjXURDF::Joint(XMLElement* joint_elem) {
   switch (jointtype) {
   case 0:     // revolute
   case 1:     // continuous
-    pjoint = pbody->AddJoint();
+    pjoint = (mjCJoint*)mjm_addJoint(pbody, 0);
     pjoint->name = jntname;
     pjoint->type = mjJNT_HINGE;
     mjuu_setvec(pjoint->pos, 0, 0, 0);
@@ -416,7 +421,7 @@ void mjXURDF::Joint(XMLElement* joint_elem) {
     break;
 
   case 2:     // prismatic
-    pjoint = pbody->AddJoint();
+    pjoint = (mjCJoint*)mjm_addJoint(pbody, 0);
     pjoint->name = jntname;
     pjoint->type = mjJNT_SLIDE;
     mjuu_setvec(pjoint->pos, 0, 0, 0);
@@ -427,7 +432,7 @@ void mjXURDF::Joint(XMLElement* joint_elem) {
     return;
 
   case 4:     // floating
-    pjoint = pbody->AddJoint();
+    pjoint = (mjCJoint*)mjm_addJoint(pbody, 0);
     pjoint->name = jntname;
     pjoint->type = mjJNT_FREE;
     break;
@@ -438,7 +443,7 @@ void mjXURDF::Joint(XMLElement* joint_elem) {
     mjuu_quat2mat(mat, quat);
 
     // construct slider along x
-    pjoint = pbody->AddJoint();
+    pjoint = (mjCJoint*)mjm_addJoint(pbody, 0);
     pjoint->name = jntname + "_TX";
     pjoint->type = mjJNT_SLIDE;
     tmpaxis[0] = mat[0];
@@ -448,7 +453,7 @@ void mjXURDF::Joint(XMLElement* joint_elem) {
     mjuu_copyvec(pjoint->axis, tmpaxis, 3);
 
     // construct slider along y
-    pjoint1 = pbody->AddJoint();
+    pjoint1 = (mjCJoint*)mjm_addJoint(pbody, 0);
     pjoint1->name = jntname + "_TY";
     pjoint1->type = mjJNT_SLIDE;
     tmpaxis[0] = mat[1];
@@ -458,7 +463,7 @@ void mjXURDF::Joint(XMLElement* joint_elem) {
     mjuu_copyvec(pjoint1->axis, tmpaxis, 3);
 
     // construct hinge around z = locaxis
-    pjoint2 = pbody->AddJoint();
+    pjoint2 = (mjCJoint*)mjm_addJoint(pbody, 0);
     pjoint2->name = jntname + "_RZ";
     pjoint2->type = mjJNT_HINGE;
     mjuu_setvec(pjoint2->pos, 0, 0, 0);
@@ -495,7 +500,7 @@ void mjXURDF::Joint(XMLElement* joint_elem) {
 
 
 // parse origin and geometry elements of visual or collision
-mjCGeom* mjXURDF::Geom(XMLElement* geom_elem, mjCBody* pbody, bool collision) {
+mjCGeom* mjXURDF::Geom(XMLElement* geom_elem, mjmBody* pbody, bool collision) {
   XMLElement *elem, *temp;
   std::string text, meshfile;
 
@@ -503,7 +508,7 @@ mjCGeom* mjXURDF::Geom(XMLElement* geom_elem, mjCBody* pbody, bool collision) {
   elem = FindSubElem(geom_elem, "geometry", true);
 
   // add BOX geom, modify type later
-  mjCGeom* pgeom = pbody->AddGeom();
+  mjCGeom* pgeom = (mjCGeom*)mjm_addGeom(pbody, 0);
   pgeom->name = "";
   pgeom->type = mjGEOM_BOX;
   if (collision) {
@@ -664,21 +669,22 @@ void mjXURDF::AddBody(std::string name) {
 // add body with given number to the mjCModel tree, process children
 void mjXURDF::AddToTree(int n) {
   // get pointer to parent in mjCModel tree
-  mjCBody *parent = 0, *child = 0;
+  mjmBody *parent = 0, *child = 0, *world = 0;
   if (urParent[n]>=0) {
-    parent = (mjCBody*) model->GetWorld()->FindObject(mjOBJ_BODY, urName[urParent[n]]);
+    world = mjm_findBody(model, "world");
+    parent = mjm_findChild(world, urName[urParent[n]].c_str());
 
     if (!parent)
       throw mjXError(0, "URDF body parent should already be in tree: %s",
                      urName[urParent[n]].c_str());       // SHOULD NOT OCCUR
   } else {
-    parent = model->GetWorld();
+    parent = &model->GetWorld()->spec;
   }
 
   // add this body
   if (urName[n] != "world") {
-    child = parent->AddBody();
+    child = mjm_addBody(parent, 0);
-    child->name = urName[n];
+    mjm_setString(child->name, urName[n].c_str());
   }
 
   // add children recursively

src/xml/xml_urdf.h

@@ -19,6 +19,7 @@
 #include <unordered_set>
 #include <vector>
 
+#include "user/user_api.h"
 #include "user/user_model.h"
 #include "xml/xml_base.h"
 #include "tinyxml2.h"
@@ -52,7 +53,7 @@ class mjXURDF : public mjXBase {
   void Body(tinyxml2::XMLElement* body_elem);                      // parse body
   void Joint(tinyxml2::XMLElement* joint_elem);                    // parse joint
   mjCGeom* Geom(tinyxml2::XMLElement* geom_elem,
-                mjCBody* pbody, bool collision);      // parse origin and geometry of geom
+                mjmBody* pbody, bool collision);      // parse origin and geometry of geom
   void Origin(tinyxml2::XMLElement* origin_elem, double* pos, double* quat); // parse origin element
 
   void MakeMaterials(tinyxml2::XMLElement* elem);     // find all materials recursively

test/engine/engine_plugin_test.cc

@@ -17,6 +17,7 @@
 
 #include "src/engine/engine_plugin.h"
 
+#include <array>
 #include <cstdint>
 #include <sstream>
 #include <string>
@@ -421,6 +422,22 @@ constexpr char xml[] = R"(
 </mujoco>
 )";
 
+TEST_F(MujocoTest, EmptyPluginDisallowed) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <worldbody>
+      <plugin/>
+    </worldbody>
+  </mujoco>
+  )";
+  std::array<char, 1024> error;
+  mjModel* m = LoadModelFromString(xml, error.data(), error.size());
+  EXPECT_THAT(m, testing::IsNull()) << error.data();
+  EXPECT_THAT(error.data(), HasSubstr(
+              "neither 'plugin' nor 'instance' is specified for body 'world'"));
+  mj_deleteModel(m);
+}
+
 TEST_F(PluginTest, FirstPartyPlugins) {
   EXPECT_THAT(mjp_pluginCount(), kNumTruePlugins);
 }

test/user/user_api_test.cc

@@ -18,7 +18,6 @@
 #include <gmock/gmock.h>
 #include <gtest/gtest.h>
 #include "src/user/user_api.h"
-#include <mujoco/mjmodel.h>
 #include "test/fixture.h"
 
 namespace mujoco {
@@ -31,8 +30,8 @@ using ::testing::HasSubstr;
 
 TEST_F(MujocoTest, ReadWriteData) {
   void* model = mjm_createModel();
-  void* world = mjm_findObject(model, mjOBJ_BODY, "world");
+  mjmBody* world = mjm_findBody(model, "world");
-  void* body = mjm_addBody(world, 0);
+  mjmBody* body = mjm_addBody(world, 0);
   mjmSite* site = mjm_addSite(body, 0);
 
   {