Add mjmActuator and mjmSensor.

PiperOrigin-RevId: 605600704
Change-Id: I0a2f5eb9d894c068ca9ee08940c752066021b6ec

src/user/user_api.cc

@@ -123,6 +123,25 @@ void* mjm_addFrame(mjmBody* bodyspec, void* parentframe) {
 
 
 
+// add actuator to model
+mjmActuator* mjm_addActuator(void* model, void* defspec) {
+  mjCModel* modelC = static_cast<mjCModel*>(model);
+  mjCDef* def = static_cast<mjCDef*>(defspec);
+  mjCActuator* actuator = modelC->AddActuator(def);
+  return &actuator->spec;
+}
+
+
+
+// add sensor to model
+mjmSensor* mjm_addSensor(void* model) {
+  mjCModel* modelC = static_cast<mjCModel*>(model);
+  mjCSensor* sensor = modelC->AddSensor();
+  return &sensor->spec;
+}
+
+
+
 // Add plugin to model.
 mjElement mjm_addPlugin(void* model) {
   mjCModel* modelC = static_cast<mjCModel*>(model);

src/user/user_api.h

@@ -211,6 +211,59 @@ typedef struct _mjmLight {
 } mjmLight;
 
 
+typedef struct _mjmActuator {
+  mjElement element;              // compiler only, do not modify
+  mjString name;                  // name
+  mjString classname;             // class name
+  mjString info;                  // message appended to errors
+  int group;                      // group for visualization
+  int ctrllimited;                // are control limits defined: 0 false, 1 true, 2 auto
+  int forcelimited;               // are force limits defined: 0 false, 1 true, 2 auto
+  int actlimited;                 // are activation limits defined: 0 false, 1 true, 2 auto
+  int actdim;                     // dimension of associated activations
+  int plugin_actdim;              // actuator state size for plugins
+  mjtDyn dyntype;                 // dynamics type
+  mjtTrn trntype;                 // transmission type
+  mjtGain gaintype;               // gain type
+  mjtBias biastype;               // bias type
+  double dynprm[mjNDYN];          // dynamics parameters
+  double gainprm[mjNGAIN];        // gain parameters
+  double biasprm[mjNGAIN];        // bias parameters
+  mjtByte actearly;               // apply activations to qfrc instantly
+  double ctrlrange[2];            // control range
+  double forcerange[2];           // force range
+  double actrange[2];             // activation range
+  double lengthrange[2];          // length range
+  double gear[6];                 // length and transmitted force scaling
+  double cranklength;             // crank length, for slider-crank only
+  mjDouble userdata;              // user data
+  mjString target;                // transmission target name
+  mjString slidersite;            // site defining cylinder, for slider-crank only
+  mjString refsite;               // reference site, for site transmission only
+  mjmPlugin plugin;               // actuator plugin
+} mjmActuator;
+
+
+typedef struct _mjmSensor {
+  mjElement element;              // compiler only, do not modify
+  mjString name;                  // name
+  mjString classname;             // class name
+  mjString info;                  // message appended to errors
+  mjtSensor type;                 // type of sensor
+  mjtDataType datatype;           // data type for sensor measurement
+  mjtStage needstage;             // compute stage needed to simulate sensor
+  mjtObj objtype;                 // type of sensorized object
+  mjString objname;               // name of sensorized object
+  mjtObj reftype;                 // type of referenced object
+  mjString refname;               // name of referenced object
+  int dim;                        // number of scalar outputs
+  double cutoff;                  // cutoff for real and positive datatypes
+  double noise;                   // noise stdev
+  mjDouble userdata;              // user data
+  mjmPlugin plugin;               // sensor plugin
+} mjmSensor;
+
+
 //---------------------------------- Public API ----------------------------------------------------
 
 // Create model.
@@ -246,6 +299,12 @@ MJAPI mjmLight* mjm_addLight(mjmBody* body, void* defspec);
 // Add frame to body.
 MJAPI void* mjm_addFrame(mjmBody* body, void* parentframe);
 
+// Add actuator to model.
+MJAPI mjmActuator* mjm_addActuator(void* model, void* defspec);
+
+// Add sensor to model.
+MJAPI mjmSensor* mjm_addSensor(void* model);
+
 // Add plugin to model.
 MJAPI mjElement mjm_addPlugin(void* model);
 
@@ -306,6 +365,12 @@ MJAPI void mjm_defaultCamera(mjmCamera& camera);
 // Default light attributes.
 MJAPI void mjm_defaultLight(mjmLight& light);
 
+// Default actuator attributes.
+MJAPI void mjm_defaultActuator(mjmActuator& actuator);
+
+// Default sensor attributes.
+MJAPI void mjm_defaultSensor(mjmSensor& sensor);
+
 #ifdef __cplusplus
 }
 #endif

src/user/user_init.cc

@@ -81,6 +81,8 @@ void mjm_defaultGeom(mjmGeom& geom) {
   geom.fluid_coefs[2] = 1.5;       // angular drag coefficient
   geom.fluid_coefs[3] = 1.0;       // kutta lift coefficient
   geom.fluid_coefs[4] = 1.0;       // magnus lift coefficient
+  geom.plugin.active = false;
+  geom.plugin.instance = nullptr;
 }
 
 
@@ -131,3 +133,44 @@ void mjm_defaultLight(mjmLight& light) {
 }
 
 
+
+// default actuator attributes
+void mjm_defaultActuator(mjmActuator& actuator) {
+  memset(&actuator, 0, sizeof(mjmActuator));
+  actuator.group = 0;
+  actuator.ctrllimited = 2;
+  actuator.forcelimited = 2;
+  actuator.actlimited = 2;
+  actuator.actdim = -1;
+  actuator.plugin_actdim = 0;
+  actuator.trntype = mjTRN_UNDEFINED;
+  actuator.dyntype = mjDYN_NONE;
+  actuator.gaintype = mjGAIN_FIXED;
+  actuator.biastype = mjBIAS_NONE;
+  actuator.actearly = 0;
+  actuator.gear[0] = 1;
+  actuator.dynprm[0] = 1;
+  actuator.gainprm[0] = 1;
+  actuator.cranklength = 0;
+  actuator.plugin.active = false;
+  actuator.plugin.instance = nullptr;
+}
+
+
+
+// default sensor attributes
+void mjm_defaultSensor(mjmSensor& sensor) {
+  memset(&sensor, 0, sizeof(mjmSensor));
+  sensor.type = mjSENS_TOUCH;
+  sensor.datatype = mjDATATYPE_REAL;
+  sensor.needstage = mjSTAGE_ACC;
+  sensor.objtype = mjOBJ_UNKNOWN;
+  sensor.reftype = mjOBJ_UNKNOWN;
+  sensor.cutoff = 0;
+  sensor.noise = 0;
+  sensor.dim = 0;
+}
+
+
+
+

src/user/user_model.cc

@@ -2298,7 +2298,7 @@ void mjCModel::CopyObjects(mjModel* m) {
     copyvec(m->actuator_forcerange + 2*i, pac->forcerange, 2);
     copyvec(m->actuator_actrange + 2*i, pac->actrange, 2);
     copyvec(m->actuator_lengthrange + 2*i, pac->lengthrange, 2);
-    copyvec(m->actuator_user+nuser_actuator*i, pac->userdata.data(), nuser_actuator);
+    copyvec(m->actuator_user+nuser_actuator*i, pac->get_userdata().data(), nuser_actuator);
   }
 
   // sensors
@@ -2318,7 +2318,7 @@ void mjCModel::CopyObjects(mjModel* m) {
     m->sensor_dim[i] = psen->dim;
     m->sensor_cutoff[i] = (mjtNum)psen->cutoff;
     m->sensor_noise[i] = (mjtNum)psen->noise;
-    copyvec(m->sensor_user+nuser_sensor*i, psen->userdata.data(), nuser_sensor);
+    copyvec(m->sensor_user+nuser_sensor*i, psen->get_userdata().data(), nuser_sensor);
 
     // calculate address and advance
     m->sensor_adr[i] = adr;
@@ -2920,13 +2920,13 @@ void mjCModel::TryCompile(mjModel*& m, mjData*& d, const mjVFS* vfs) {
   if (nuser_actuator == -1) {
     nuser_actuator = 0;
     for (int i=0; i<actuators.size(); i++) {
-      nuser_actuator = mjMAX(nuser_actuator, actuators[i]->userdata.size());
+      nuser_actuator = mjMAX(nuser_actuator, actuators[i]->spec_userdata_.size());
     }
   }
   if (nuser_sensor == -1) {
     nuser_sensor = 0;
     for (int i=0; i<sensors.size(); i++) {
-      nuser_sensor = mjMAX(nuser_sensor, sensors[i]->userdata.size());
+      nuser_sensor = mjMAX(nuser_sensor, sensors[i]->spec_userdata_.size());
     }
   }
 
@@ -3500,7 +3500,7 @@ bool mjCModel::CopyBack(const mjModel* m) {
     pa->cranklength = (double)m->actuator_cranklength[i];
 
     if (nuser_actuator) {
-      copyvec(pa->userdata.data(), m->actuator_user + nuser_actuator*i, nuser_actuator);
+      copyvec(pa->userdata_.data(), m->actuator_user + nuser_actuator*i, nuser_actuator);
     }
   }
 
@@ -3510,7 +3510,7 @@ bool mjCModel::CopyBack(const mjModel* m) {
     sensors[i]->noise = (double)m->sensor_noise[i];
 
     if (nuser_sensor) {
-      copyvec(sensors[i]->userdata.data(), m->sensor_user + nuser_sensor*i, nuser_sensor);
+      copyvec(sensors[i]->userdata_.data(), m->sensor_user + nuser_sensor*i, nuser_sensor);
     }
   }
 

src/user/user_objects.cc

@@ -475,7 +475,7 @@ void mjCDef::Compile(const mjCModel* model) {
   site.userdata_.resize(model->nuser_site);
   camera.userdata_.resize(model->nuser_cam);
   tendon.userdata.resize(model->nuser_tendon);
-  actuator.userdata.resize(model->nuser_actuator);
+  actuator.userdata_.resize(model->nuser_actuator);
 }
 
 
@@ -3963,35 +3963,13 @@ void mjCWrap::Compile(void) {
 
 // initialize defaults
 mjCActuator::mjCActuator(mjCModel* _model, mjCDef* _def) {
-  // actuator defaults
-  group = 0;
-  ctrllimited = 2;
-  forcelimited = 2;
-  actlimited = 2;
-  actdim = -1;
-  plugin_actdim = 0;
-  trntype = mjTRN_UNDEFINED;
-  dyntype = mjDYN_NONE;
-  gaintype = mjGAIN_FIXED;
-  biastype = mjBIAS_NONE;
-  mjuu_zerovec(dynprm, mjNDYN);
-  mjuu_zerovec(gainprm, mjNGAIN);
-  mjuu_zerovec(biasprm, mjNBIAS);
-  mjuu_zerovec(ctrlrange, 2);
-  mjuu_zerovec(forcerange, 2);
-  mjuu_zerovec(actrange, 2);
-  mjuu_zerovec(lengthrange, 2);
-  mjuu_zerovec(gear, 6);
-  gear[0] = 1;
-  dynprm[0] = 1;
-  gainprm[0] = 1;
-  cranklength = 0;
-  target.clear();
-  slidersite.clear();
-  refsite.clear();
-  userdata.clear();
+  mjm_defaultActuator(spec);
 
   // clear private variables
+  spec_target_.clear();
+  spec_slidersite_.clear();
+  spec_refsite_.clear();
+  spec_userdata_.clear();
   trnid[0] = trnid[1] = -1;
 
   // reset to default if given
@@ -4003,23 +3981,59 @@ mjCActuator::mjCActuator(mjCModel* _model, mjCDef* _def) {
   model = _model;
   def = (_def ? _def : (_model ? _model->defaults[0] : 0));
 
+  // in case this actuator is not compiled
+  CopyFromSpec();
+
   // point to local (needs to be after defaults)
-  plugin.name = (mjString)&plugin_name;
-  plugin.instance_name = (mjString)&plugin_instance_name;
+  MakePointerLocal();
+}
+
+
+
+void mjCActuator::MakePointerLocal() {
+  spec.element = (mjElement)this;
+  spec.name = (mjString)&name;
+  spec.classname = (mjString)&classname;
+  spec.userdata = (mjDouble)&spec_userdata_;
+  spec.target = (mjString)&spec_target_;
+  spec.refsite = (mjString)&spec_refsite_;
+  spec.slidersite = (mjString)&spec_slidersite_;
+  spec.plugin.name = (mjString)&plugin_name;
+  spec.plugin.instance_name = (mjString)&plugin_instance_name;
+  spec.info = (mjString)&info;
+}
+
+
+
+void mjCActuator::CopyFromSpec() {
+  *static_cast<mjmActuator*>(this) = spec;
+  userdata_ = spec_userdata_;
+  target_ = spec_target_;
+  refsite_ = spec_refsite_;
+  slidersite_ = spec_slidersite_;
+  userdata = (mjDouble)&userdata_;
+  target = (mjString)&target_;
+  refsite = (mjString)&refsite_;
+  slidersite = (mjString)&slidersite_;
+  plugin.active = spec.plugin.active;
+  plugin.instance = spec.plugin.instance;
+  plugin.name = spec.plugin.name;
+  plugin.instance_name = spec.plugin.instance_name;
 }
 
 
 
 // compiler
 void mjCActuator::Compile(void) {
+  CopyFromSpec();
   mjCJoint* pjnt;
 
   // resize userdata
-  if (userdata.size() > model->nuser_actuator) {
+  if (userdata_.size() > model->nuser_actuator) {
     throw mjCError(this, "user has more values than nuser_actuator in actuator '%s' (id = %d)",
                    name.c_str(), id);
   }
-  userdata.resize(model->nuser_actuator);
+  userdata_.resize(model->nuser_actuator);
 
   // if limited is auto, set to 1 if range is specified, otherwise unlimited
   if (forcelimited==2) {
@@ -4104,7 +4118,7 @@ void mjCActuator::Compile(void) {
   }
 
   // check for missing target name
-  if (target.empty()) {
+  if (target_.empty()) {
     throw mjCError(this,
                    "missing transmission target for actuator '%s' (id = %d)", name.c_str(), id);
   }
@@ -4115,10 +4129,10 @@ void mjCActuator::Compile(void) {
   case mjTRN_JOINT:
   case mjTRN_JOINTINPARENT:
     // get joint
-    ptarget = model->FindObject(mjOBJ_JOINT, target);
+    ptarget = model->FindObject(mjOBJ_JOINT, target_);
     if (!ptarget) {
       throw mjCError(this,
-                     "unknown transmission target '%s' for actuator id = %d", target.c_str(), id);
+                     "unknown transmission target '%s' for actuator id = %d", target_.c_str(), id);
     }
     pjnt = (mjCJoint*) ptarget;
 
@@ -4132,12 +4146,12 @@ void mjCActuator::Compile(void) {
 
   case mjTRN_SLIDERCRANK:
     // get slidersite, copy in trnid[1]
-    if (slidersite.empty()) {
+    if (slidersite_.empty()) {
       throw mjCError(this, "missing base site for slider-crank '%s' (id = %d)", name.c_str(), id);
     }
-    ptarget = model->FindObject(mjOBJ_SITE, slidersite);
+    ptarget = model->FindObject(mjOBJ_SITE, slidersite_);
     if (!ptarget) {
-      throw mjCError(this, "base site '%s' not found for actuator %d", slidersite.c_str(), id);
+      throw mjCError(this, "base site '%s' not found for actuator %d", slidersite_.c_str(), id);
     }
     trnid[1] = ptarget->id;
 
@@ -4148,31 +4162,31 @@ void mjCActuator::Compile(void) {
     }
 
     // proceed with regular target
-    ptarget = model->FindObject(mjOBJ_SITE, target);
+    ptarget = model->FindObject(mjOBJ_SITE, target_);
     break;
 
   case mjTRN_TENDON:
     // get tendon
-    ptarget = model->FindObject(mjOBJ_TENDON, target);
+    ptarget = model->FindObject(mjOBJ_TENDON, target_);
     break;
 
   case mjTRN_SITE:
     // get refsite, copy into trnid[1]
-    if (!refsite.empty()) {
-      ptarget = model->FindObject(mjOBJ_SITE, refsite);
+    if (!refsite_.empty()) {
+      ptarget = model->FindObject(mjOBJ_SITE, refsite_);
       if (!ptarget) {
-        throw mjCError(this, "reference site '%s' not found for actuator %d", refsite.c_str(), id);
+        throw mjCError(this, "reference site '%s' not found for actuator %d", refsite_.c_str(), id);
       }
       trnid[1] = ptarget->id;
     }
 
     // proceed with regular site target
-    ptarget = model->FindObject(mjOBJ_SITE, target);
+    ptarget = model->FindObject(mjOBJ_SITE, target_);
     break;
 
   case mjTRN_BODY:
     // get body
-    ptarget = model->FindObject(mjOBJ_BODY, target);
+    ptarget = model->FindObject(mjOBJ_BODY, target_);
     break;
 
   default:
@@ -4181,7 +4195,7 @@ void mjCActuator::Compile(void) {
 
   // assign and check
   if (!ptarget) {
-    throw mjCError(this, "transmission target '%s' not found in actuator %d", target.c_str(), id);
+    throw mjCError(this, "transmission target '%s' not found in actuator %d", target_.c_str(), id);
   } else {
     trnid[0] = ptarget->id;
   }
@@ -4209,41 +4223,67 @@ void mjCActuator::Compile(void) {
 
 // initialize defaults
 mjCSensor::mjCSensor(mjCModel* _model) {
+  mjm_defaultSensor(spec);
+
   // set model
   model = _model;
 
-  // set sensor defaults (somewhat arbitrary)
-  type = mjSENS_TOUCH;
-  datatype = mjDATATYPE_REAL;
-  needstage = mjSTAGE_ACC;
-  objtype = mjOBJ_UNKNOWN;
-  objname.clear();
-  reftype = mjOBJ_UNKNOWN;
-  refname.clear();
-  cutoff = 0;
-  noise = 0;
-  userdata.clear();
-  dim = 0;
-
   // clear private variables
+  spec_objname_.clear();
+  spec_refname_.clear();
+  spec_userdata_.clear();
   obj = nullptr;
   refid = -1;
 
-  // point to local (needs to be after defaults)
-  plugin.name = (mjString)&plugin_name;
-  plugin.instance_name = (mjString)&plugin_instance_name;
+  // in case this sensor is not compiled
+  CopyFromSpec();
+
+  // point to local
+  MakePointerLocal();
+}
+
+
+
+void mjCSensor::MakePointerLocal() {
+  spec.element = (mjElement)this;
+  spec.name = (mjString)&name;
+  spec.classname = (mjString)&classname;
+  spec.userdata = (mjDouble)&spec_userdata_;
+  spec.objname = (mjString)&spec_objname_;
+  spec.refname = (mjString)&spec_refname_;
+  spec.plugin.name = (mjString)&plugin_name;
+  spec.plugin.instance_name = (mjString)&plugin_instance_name;
+  spec.info = (mjString)&info;
+}
+
+
+
+void mjCSensor::CopyFromSpec() {
+  *static_cast<mjmSensor*>(this) = spec;
+  userdata_ = spec_userdata_;
+  objname_ = spec_objname_;
+  refname_ = spec_refname_;
+  userdata = (mjDouble)&userdata_;
+  objname = (mjString)&objname_;
+  refname = (mjString)&refname_;
+  plugin.active = spec.plugin.active;
+  plugin.instance = spec.plugin.instance;
+  plugin.name = spec.plugin.name;
+  plugin.instance_name = spec.plugin.instance_name;
 }
 
 
 
 // compiler
 void mjCSensor::Compile(void) {
+  CopyFromSpec();
+
   // resize userdata
-  if (userdata.size() > model->nuser_sensor) {
+  if (userdata_.size() > model->nuser_sensor) {
     throw mjCError(this, "user has more values than nuser_sensor in sensor '%s' (id = %d)",
                    name.c_str(), id);
   }
-  userdata.resize(model->nuser_sensor);
+  userdata_.resize(model->nuser_sensor);
 
   // require non-negative noise
   if (noise<0) {
@@ -4258,14 +4298,14 @@ void mjCSensor::Compile(void) {
   // get objid from objtype and objname
   if (objtype!=mjOBJ_UNKNOWN) {
     // check for missing object name
-    if (objname.empty()) {
+    if (objname_.empty()) {
       throw mjCError(this,
                      "missing name of sensorized object in sensor '%s' (id = %d)",
                      name.c_str(), id);
     }
 
     // find name
-    obj = model->FindObject(objtype, objname);
+    obj = model->FindObject(objtype, objname_);
     if (!obj) {
       throw mjCError(this,
                      "unrecognized name of sensorized object in sensor '%s' (id = %d)",
@@ -4285,14 +4325,14 @@ void mjCSensor::Compile(void) {
   // get refid from reftype and refname
   if (reftype!=mjOBJ_UNKNOWN) {
     // check for missing object name
-    if (refname.empty()) {
+    if (refname_.empty()) {
       throw mjCError(this,
                      "missing name of reference frame object in sensor '%s' (id = %d)",
                      name.c_str(), id);
     }
 
     // find name
-    mjCBase* pref = model->FindObject(reftype, refname);
+    mjCBase* pref = model->FindObject(reftype, refname_);
     if (!pref) {
       throw mjCError(this,
                      "unrecognized name of reference frame object in sensor '%s' (id = %d)",
@@ -4351,7 +4391,7 @@ void mjCSensor::Compile(void) {
 
     // check for camera resolution for camera projection sensor
     if (type==mjSENS_CAMPROJECTION) {
-      mjCCamera* camref = (mjCCamera*) model->FindObject(mjOBJ_CAMERA, refname);
+      mjCCamera* camref = (mjCCamera*) model->FindObject(mjOBJ_CAMERA, refname_);
       if (!camref->resolution[0] || !camref->resolution[1]) {
         throw mjCError(this,
                        "camera projection sensor requires camera resolution '%s' (id = %d)",

src/user/user_objects.h

@@ -1141,43 +1141,41 @@ class mjCPlugin : public mjCBase {
 //------------------------- class mjCActuator ------------------------------------------------------
 // Describes an actuator
 
-class mjCActuator : public mjCBase {
+class mjCActuator : public mjCBase, private mjmActuator {
   friend class mjCDef;
   friend class mjCModel;
   friend class mjXWriter;
 
  public:
-  // variables set by user or API
-  int group;                      // group for visualization
-  int ctrllimited;                // are control limits defined: 0 false, 1 true, 2 auto
-  int forcelimited;               // are force limits defined: 0 false, 1 true, 2 auto
-  int actlimited;                 // are activation limits defined: 0 false, 1 true, 2 auto
-  int actdim;                     // dimension of associated activations
-  int plugin_actdim;              // actuator state size for plugins
-  mjtDyn dyntype;                 // dynamics type
-  mjtTrn trntype;                 // transmission type
-  mjtGain gaintype;               // gain type
-  mjtBias biastype;               // bias type
-  double dynprm[mjNDYN];          // dynamics parameters
-  double gainprm[mjNGAIN];        // gain parameters
-  double biasprm[mjNGAIN];        // bias parameters
-  bool actearly = false;          // apply activations to qfrc instantly
-  double ctrlrange[2];            // control range
-  double forcerange[2];           // force range
-  double actrange[2];             // activation range
-  double lengthrange[2];          // length range
-  double gear[6];                 // length and transmitted force scaling
-  double cranklength;             // crank length, for slider-crank only
-  std::vector<double> userdata;   // user data
-  std::string target;             // transmission target name
-  std::string slidersite;         // site defining cylinder, for slider-crank only
-  std::string refsite;            // reference site, for site transmission only
+  mjmActuator spec;
+  using mjCBase::name;
+  using mjCBase::classname;
+  using mjCBase::info;
+  using mjCBase::plugin;
+
+  // used by mjXWriter and mjCModel
+  const std::vector<double>& get_userdata() { return userdata_; }
+  const std::string& get_target() { return spec_target_; }
+  const std::string& get_slidersite() { return spec_slidersite_; }
+  const std::string& get_refsite() { return spec_refsite_; }
 
  private:
   mjCActuator(mjCModel* = 0, mjCDef* = 0);  // constructor
   void Compile(void);                       // compiler
+  void CopyFromSpec();
+  void MakePointerLocal();
 
   int trnid[2];                   // id of transmission target
+
+  // variable-size data
+  std::string target_;
+  std::string slidersite_;
+  std::string refsite_;
+  std::vector<double> userdata_;
+  std::string spec_target_;
+  std::string spec_slidersite_;
+  std::string spec_refsite_;
+  std::vector<double> spec_userdata_;
 };
 
 
@@ -1185,31 +1183,39 @@ class mjCActuator : public mjCBase {
 //------------------------- class mjCSensor --------------------------------------------------------
 // Describes a sensor
 
-class mjCSensor : public mjCBase {
+class mjCSensor : public mjCBase, private mjmSensor {
   friend class mjCDef;
   friend class mjCModel;
   friend class mjXWriter;
 
  public:
-  // variables set by user or API
-  mjtSensor type;                 // type of sensor
-  mjtDataType datatype;           // data type for sensor measurement
-  mjtStage needstage;             // compute stage needed to simulate sensor
-  mjtObj objtype;                 // type of sensorized object
-  std::string objname;            // name of sensorized object
-  mjtObj reftype;
-  std::string refname;
-  int dim;                        // number of scalar outputs
-  double cutoff;                  // cutoff for real and positive datatypes
-  double noise;                   // noise stdev
-  std::vector<double> userdata;   // user data
+  mjmSensor spec;
+  using mjCBase::name;
+  using mjCBase::classname;
+  using mjCBase::info;
+  using mjCBase::plugin;
+
+  // used by mjXWriter and mjCModel
+  const std::vector<double>& get_userdata() { return userdata_; }
+  const std::string& get_objname() { return spec_objname_; }
+  const std::string& get_refname() { return spec_refname_; }
 
  private:
   mjCSensor(mjCModel*);           // constructor
   void Compile(void);             // compiler
+  void CopyFromSpec();
+  void MakePointerLocal();
 
   mjCBase* obj;                   // sensorized object
   int refid;                      // id of reference frame
+
+  // variable-size data
+  std::string objname_;
+  std::string refname_;
+  std::vector<double> userdata_;
+  std::string spec_objname_;
+  std::string spec_refname_;
+  std::vector<double> spec_userdata_;
 };