Replace mjCModel with mjmModel in mjXBase.

PiperOrigin-RevId: 611762880
Change-Id: I22254689b6cbf10fe7b61f9742b6ffd6c2074477

src/user/user_api.cc

@@ -28,12 +28,44 @@
 
 // create model
 mjmModel* mjm_createModel() {
-  mjCModel* modelC = new mjCModel();
+  mjCModel* modelC = new mjCModel;
   return &modelC->spec;
 }
 
 
 
+// copy back model
+void mjm_copyBack(mjmModel* model, const mjModel* m) {
+  mjCModel* modelC = reinterpret_cast<mjCModel*>(model->element);
+  modelC->CopyBack(m);
+}
+
+
+
+// compile model
+mjModel* mjm_compileModel(mjmModel* model, const mjVFS* vfs) {
+  mjCModel* modelC = reinterpret_cast<mjCModel*>(model->element);
+  return modelC->Compile(vfs);
+}
+
+
+
+// get error message from model
+const char* mjm_getError(mjmModel* model) {
+  mjCModel* modelC = reinterpret_cast<mjCModel*>(model->element);
+  return modelC->GetError().message;
+}
+
+
+
+// check if model has warnings
+int mjm_isWarning(mjmModel* model) {
+  mjCModel* modelC = reinterpret_cast<mjCModel*>(model->element);
+  return modelC->GetError().warning;
+}
+
+
+
 // delete model
 void mjm_deleteModel(mjmModel* modelspec) {
   mjCModel* model = reinterpret_cast<mjCModel*>(modelspec->element);
@@ -320,8 +352,9 @@ mjmPlugin* mjm_addPlugin(mjmModel* model) {
 
 
 // add default to model
-mjmDefault* mjm_addDefault(mjmModel* model, const char* classname, int parentid) {
+mjmDefault* mjm_addDefault(mjmModel* model, const char* classname, int parentid, int* id) {
   mjCModel* modelC = reinterpret_cast<mjCModel*>(model->element);
+  *id = (int)modelC->defaults.size();
   mjCDef* def = modelC->AddDef(classname, parentid);
   if (def) {
     return &def->spec;
@@ -346,10 +379,10 @@ mjmDefault* mjm_getDefault(mjElement element) {
 
 
 
-// find default in model by class name
-mjmDefault* mjm_findDefault(mjmModel* modelspec, const char* classname) {
-  mjCModel* model = reinterpret_cast<mjCModel*>(modelspec->element);
-  mjCDef* cdef = model->FindDef(classname);
+// Find default with given name in model.
+mjmDefault* mjm_findDefault(mjmModel* model, const char* classname) {
+  mjCModel* modelC = reinterpret_cast<mjCModel*>(model->element);
+  mjCDef* cdef = modelC->FindDef(classname);
   if (!cdef) {
     return nullptr;
   }
@@ -358,6 +391,18 @@ mjmDefault* mjm_findDefault(mjmModel* modelspec, const char* classname) {
 
 
 
+// get default[0] from model
+mjmDefault* mjm_getModelDefault(mjmModel* model) {
+  mjCModel* modelC = reinterpret_cast<mjCModel*>(model->element);
+  mjCDef* def = modelC->defaults[0];
+  if (!def) {
+    return nullptr;
+  }
+  return &def->spec;
+}
+
+
+
 // find body in model by name
 mjmBody* mjm_findBody(mjmModel* modelspec, const char* name) {
   mjCModel* model = reinterpret_cast<mjCModel*>(modelspec->element);
@@ -539,6 +584,16 @@ void mjm_setPluginAttributes(mjmPlugin* plugin, void* attributes) {
 
 
 
+// Set active plugins.
+void mjm_setActivePlugins(mjmModel* model, void* activeplugins) {
+  mjCModel* modelC = reinterpret_cast<mjCModel*>(model->element);
+  std::vector<std::pair<const mjpPlugin*, int>>* active_plugins =
+      reinterpret_cast<std::vector<std::pair<const mjpPlugin*, int>>*>(activeplugins);
+  modelC->active_plugins = std::move(*active_plugins);
+}
+
+
+
 // compute full inertia
 const char* mjm_setFullInertia(mjmBody* bodyspec, double quat[4], double inertia[3]) {
   mjCBody* body = reinterpret_cast<mjCBody*>(bodyspec->element);

src/user/user_api.h

@@ -729,6 +729,18 @@ typedef struct _mjmDefault {       // default specification
 // Create model.
 MJAPI mjmModel* mjm_createModel();
 
+// Copy back model.
+MJAPI void mjm_copyBack(mjmModel* model, const mjModel* m);
+
+// Compile model.
+MJAPI mjModel* mjm_compileModel(mjmModel* model, const mjVFS* vfs);
+
+// Get error message from model.
+MJAPI const char* mjm_getError(mjmModel* model);
+
+// Return 1 if model has warnings.
+MJAPI int mjm_isWarning(mjmModel* model);
+
 // Delete model.
 MJAPI void mjm_deleteModel(mjmModel* modelspec);
 
@@ -820,7 +832,7 @@ MJAPI mjmKey* mjm_addKey(mjmModel* model);
 MJAPI mjmPlugin* mjm_addPlugin(mjmModel* model);
 
 // Add default to model.
-MJAPI mjmDefault* mjm_addDefault(mjmModel* model, const char* classname, int parentid);
+MJAPI mjmDefault* mjm_addDefault(mjmModel* model, const char* classname, int parentid, int* id);
 
 // Get model from body.
 MJAPI mjmModel* mjm_getModel(mjmBody* body);
@@ -831,6 +843,9 @@ MJAPI mjmDefault* mjm_getDefault(mjElement element);
 // Find default in model by class name.
 MJAPI mjmDefault* mjm_findDefault(mjmModel* model, const char* classname);
 
+// Get global default from model.
+MJAPI mjmDefault* mjm_getModelDefault(mjmModel* model);
+
 // Find body in model by name.
 MJAPI mjmBody* mjm_findBody(mjmModel* model, const char* name);
 
@@ -879,6 +894,9 @@ MJAPI const double* mjm_getDouble(mjDoubleVec source, int* size);
 // Set plugin attributes.
 MJAPI void mjm_setPluginAttributes(mjmPlugin* plugin, void* attributes);
 
+// Set active plugins.
+MJAPI void mjm_setActivePlugins(mjmModel* model, void* activeplugins);
+
 // Set default.
 MJAPI void mjm_setDefault(mjElement element, mjmDefault* def);
 

src/user/user_composite.cc

@@ -223,7 +223,9 @@ void mjCComposite::SetDefault(void) {
 
 
 // make composite object
-bool mjCComposite::Make(mjCModel* model, mjmBody* body, char* error, int error_sz) {
+bool mjCComposite::Make(mjmModel* modelspec, mjmBody* body, char* error, int error_sz) {
+  mjCModel* model = (mjCModel*)modelspec->element;
+
   // check geom type
   if ((def[0].spec.geom->type!=mjGEOM_SPHERE &&
        def[0].spec.geom->type!=mjGEOM_CAPSULE &&

src/user/user_composite.h

@@ -69,7 +69,7 @@ class mjCComposite {
   bool AddDefaultJoint(char* error = NULL, int error_sz = 0);
   void AdjustSoft(mjtNum* solref, mjtNum* solimp, int level);
 
-  bool Make(mjCModel* model, mjmBody* body, char* error, int error_sz);
+  bool Make(mjmModel* modelspec, mjmBody* body, char* error, int error_sz);
 
   bool MakeParticle(mjCModel* model, mjmBody* body, char* error, int error_sz);
   bool MakeGrid(mjCModel* model, mjmBody* body, char* error, int error_sz);

src/user/user_flexcomp.cc

@@ -82,7 +82,8 @@ mjCFlexcomp::mjCFlexcomp(void) {
 
 
 // make flexcomp object
-bool mjCFlexcomp::Make(mjCModel* model, mjmBody* body, char* error, int error_sz) {
+bool mjCFlexcomp::Make(mjmModel* modelspec, mjmBody* body, char* error, int error_sz) {
+  mjCModel* model = (mjCModel*)modelspec->element;
   mjmFlex* dflex = def.spec.flex;
   int dim = dflex->dim;
   bool radial = (type==mjFCOMPTYPE_BOX ||

src/user/user_flexcomp.h

@@ -42,7 +42,7 @@ typedef enum _mjtFcompType {
 class mjCFlexcomp {
  public:
   mjCFlexcomp(void);
-  bool Make(mjCModel* model, mjmBody* body, char* error, int error_sz);
+  bool Make(mjmModel* modelspec, mjmBody* body, char* error, int error_sz);
 
   bool MakeGrid(char* error, int error_sz);
   bool MakeBox(char* error, int error_sz);

src/user/user_model.cc

@@ -175,6 +175,13 @@ mjCModel::mjCModel() {
 
 
 
+mjCModel::mjCModel(const mjCModel& other) {
+  *this = other;
+  PointToLocal();
+}
+
+
+
 void mjCModel::PointToLocal() {
   spec.element = (mjElement)this;
   spec.comment = (mjString)&spec_comment_;
@@ -504,8 +511,8 @@ int mjCModel::GetFixed(void) {
 
 
 
-// copy of error object
-mjCError mjCModel::GetError(void) {
+// get reference of error object
+const mjCError& mjCModel::GetError(void) {
   return errInfo;
 }
 

src/user/user_model.h

@@ -61,6 +61,7 @@ class mjCModel : private mjmModel {
 
  public:
   mjCModel();
+  mjCModel(const mjCModel& other);
   ~mjCModel();
   void CopyFromSpec();  // copy spec to private attributes
   void PointToLocal();
@@ -105,7 +106,7 @@ class mjCModel : private mjmModel {
   // API for access to other variables
   bool IsCompiled();                                       // is model already compiled
   int GetFixed();                                          // number of fixed massless bodies
-  mjCError GetError();                                     // copy of error object
+  const mjCError& GetError(void);                          // get reference of error object
   mjCBody* GetWorld();                                     // pointer to world body
   mjCDef* FindDef(std::string name);                       // find default class name
   mjCDef* AddDef(std::string name, int parentid);          // add default class to array

src/xml/xml.cc

@@ -35,7 +35,7 @@
 #include "engine/engine_crossplatform.h"
 #include "engine/engine_resource.h"
 #include "engine/engine_vfs.h"
-#include "user/user_model.h"
+#include "user/user_api.h"
 #include "user/user_util.h"
 #include "xml/xml_native_reader.h"
 #include "xml/xml_native_writer.h"
@@ -99,7 +99,7 @@ class LocaleOverride {
 }  // namespace
 
 // Main writer function - calls mjXWrite
-std::string mjWriteXML(mjCModel* model, char* error, int error_sz) {
+std::string mjWriteXML(mjmModel* model, char* error, int error_sz) {
   LocaleOverride locale_override;
 
   // check for empty model
@@ -272,7 +272,7 @@ static void mjIncludeXML(mjXReader& reader, XMLElement* elem,
 
 
 // Main parser function
-mjCModel* mjParseXML(const char* filename, const mjVFS* vfs,
+mjmModel* mjParseXML(const char* filename, const mjVFS* vfs,
                      char* error, int error_sz) {
   LocaleOverride locale_override;
 
@@ -285,7 +285,7 @@ mjCModel* mjParseXML(const char* filename, const mjVFS* vfs,
   }
 
   // clear
-  mjCModel* model = 0;
+  mjmModel* model = nullptr;
   if (error) {
     error[0] = '\0';
   }
@@ -344,14 +344,14 @@ mjCModel* mjParseXML(const char* filename, const mjVFS* vfs,
   }
 
   // create model, set filedir
-  model = new mjCModel;
+  model = mjm_createModel();
   const char* dir;
   int ndir = 0;
   mju_getResourceDir(resource, &dir, &ndir);
   if (dir != nullptr) {
-    mjm_setString(model->spec.modelfiledir, std::string(dir, ndir).c_str());
+    mjm_setString(model->modelfiledir, std::string(dir, ndir).c_str());
   } else {
-    mjm_setString(model->spec.modelfiledir, "");
+    mjm_setString(model->modelfiledir, "");
   }
 
   // close resource
@@ -363,8 +363,8 @@ mjCModel* mjParseXML(const char* filename, const mjVFS* vfs,
       // find include elements, replace them with subtree from xml file
       std::unordered_set<std::string> included = {filename};
       mjXReader parser;
-      parser.SetModelFileDir(mjm_getString(model->spec.modelfiledir));
-      mjIncludeXML(parser, root, mjm_getString(model->spec.modelfiledir), vfs, included);
+      parser.SetModelFileDir(mjm_getString(model->modelfiledir));
+      mjIncludeXML(parser, root, mjm_getString(model->modelfiledir), vfs, included);
 
       // parse MuJoCo model
       parser.SetModel(model);
@@ -377,9 +377,9 @@ mjCModel* mjParseXML(const char* filename, const mjVFS* vfs,
 
       // set reasonable default for parsing a URDF
       // this is separate from the Parser to allow multiple URDFs to be loaded.
-      model->spec.strippath = true;
-      model->spec.fusestatic = true;
-      model->spec.discardvisual = true;
+      model->strippath = true;
+      model->fusestatic = true;
+      model->discardvisual = true;
 
       parser.SetModel(model);
       parser.Parse(root);
@@ -393,7 +393,7 @@ mjCModel* mjParseXML(const char* filename, const mjVFS* vfs,
   // catch known errors
   catch (mjXError err) {
     mjCopyError(error, err.message, error_sz);
-    delete model;
+    mjm_deleteModel(model);
     return nullptr;
   }
 

src/xml/xml.h

@@ -18,15 +18,15 @@
 #include <string>
 
 #include <mujoco/mjmodel.h>
-#include "user/user_model.h"
+#include "user/user_api.h"
 
 // Top level API
 
 // Main writer function
-std::string mjWriteXML(mjCModel* model, char* error, int error_sz);
+std::string mjWriteXML(mjmModel* model, char* error, int error_sz);
 
 // Main parser function
-mjCModel* mjParseXML(const char* filename, const mjVFS* vfs, char* error, int error_sz);
+mjmModel* mjParseXML(const char* filename, const mjVFS* vfs, char* error, int error_sz);
 
 
 #endif  // MUJOCO_SRC_XML_XML_H_

src/xml/xml_api.cc

@@ -17,6 +17,7 @@
 #include <cstdio>
 #include <cstring>
 #include <fstream>
+#include <functional>
 #include <memory>
 #include <mutex>
 #include <optional>
@@ -25,7 +26,7 @@
 #include <type_traits>
 
 #include <mujoco/mjmodel.h>
-#include "user/user_model.h"
+#include "user/user_api.h"
 #include "xml/xml.h"
 #include "xml/xml_native_reader.h"
 #include "xml/xml_util.h"
@@ -36,7 +37,7 @@
 class GlobalModel {
  public:
   // deletes current model and takes ownership of model
-  void Set(mjCModel* model = nullptr);
+  void Set(mjmModel* model = nullptr);
 
   // writes XML to string
   std::optional<std::string> ToXML(const mjModel* m, char* error,
@@ -45,7 +46,7 @@ class GlobalModel {
  private:
   // using raw pointers as GlobalModel needs to be trivially destructible
   std::mutex* mutex_ = new std::mutex();
-  mjCModel* model_ = nullptr;
+  mjmModel* model_ = nullptr;
 };
 
 std::optional<std::string> GlobalModel::ToXML(const mjModel* m, char* error,
@@ -55,7 +56,7 @@ std::optional<std::string> GlobalModel::ToXML(const mjModel* m, char* error,
     mjCopyError(error, "No XML model loaded", error_sz);
     return std::nullopt;
   }
-  model_->CopyBack(m);
+  mjm_copyBack(model_, m);
   std::string result = mjWriteXML(model_, error, error_sz);
   if (result.empty()) {
     return std::nullopt;
@@ -63,10 +64,10 @@ std::optional<std::string> GlobalModel::ToXML(const mjModel* m, char* error,
   return result;
 }
 
-void GlobalModel::Set(mjCModel* model) {
+void GlobalModel::Set(mjmModel* model) {
   std::lock_guard<std::mutex> lock(*mutex_);
   if (model_ != nullptr) {
-    delete model_;
+    mjm_deleteModel(model_);
   }
   model_ = model;
 }
@@ -90,21 +91,23 @@ mjModel* mj_loadXML(const char* filename, const mjVFS* vfs,
                     char* error, int error_sz) {
 
   // parse new model
-  std::unique_ptr<mjCModel> model(mjParseXML(filename, vfs, error, error_sz));
+  std::unique_ptr<mjmModel, std::function<void(mjmModel*)>> model(
+      mjParseXML(filename, vfs, error, error_sz),
+      [](mjmModel* m) { mjm_deleteModel(m); });
   if (!model) {
     return nullptr;
   }
 
   // compile new model
-  mjModel* m = model->Compile(vfs);
+  mjModel* m = mjm_compileModel(model.get(), vfs);
   if (!m) {
-    mjCopyError(error, model->GetError().message, error_sz);
+    mjCopyError(error, mjm_getError(model.get()), error_sz);
     return nullptr;
   }
 
   // handle compile warning
-  if (model->GetError().warning) {
-    mjCopyError(error, model->GetError().message, error_sz);
+  if (mjm_isWarning(model.get())) {
+    mjCopyError(error, mjm_getError(model.get()), error_sz);
   } else if (error) {
     error[0] = '\0';
   }

src/xml/xml_base.cc

@@ -44,7 +44,7 @@ mjXBase::mjXBase() {
 
 
 // set model field
-void mjXBase::SetModel(mjCModel* _model) {
+void mjXBase::SetModel(mjmModel* _model) {
   model = _model;
 }
 

src/xml/xml_base.h

@@ -19,9 +19,7 @@
 #include <string>
 
 #include "tinyxml2.h"
-#include <mujoco/mjmodel.h>
-#include "user/user_model.h"
-#include "user/user_objects.h"
+#include "user/user_api.h"
 #include "xml/xml_util.h"
 
 
@@ -89,13 +87,13 @@ class mjXBase : public mjXUtil {
   };
 
   // set the model allocated externally
-  void SetModel(mjCModel*);
+  virtual void SetModel(mjmModel*);
 
   // read alternative orientation specification
   static int ReadAlternative(tinyxml2::XMLElement* elem, mjmOrientation& alt);
 
  protected:
-  mjCModel* model;                    // internally-allocated mjCModel object
+  mjmModel* model;                    // internally-allocated model
 };
 
 #endif  // MUJOCO_SRC_XML_XML_BASE_H_

src/xml/xml_native_reader.cc

@@ -808,31 +808,31 @@ void mjXReader::Parse(XMLElement* root) {
   // get model name
   string modelname;
   if (ReadAttrTxt(root, "model", modelname)) {
-    mjm_setString(model->spec.modelname, modelname.c_str());
+    mjm_setString(model->modelname, modelname.c_str());
   }
 
   // get comment
   if (root->FirstChild() && root->FirstChild()->ToComment()) {
-    mjm_setString(model->spec.comment, root->FirstChild()->Value());
+    mjm_setString(model->comment, root->FirstChild()->Value());
   } else {
-    mjm_setString(model->spec.comment, "");
+    mjm_setString(model->comment, "");
   }
 
   //------------------- parse MuJoCo sections embedded in all XML formats
 
   for (XMLElement* section = FirstChildElement(root, "compiler"); section;
        section = NextSiblingElement(section, "compiler")) {
-    Compiler(section, &model->spec);
+    Compiler(section, model);
   }
 
   for (XMLElement* section = FirstChildElement(root, "option"); section;
        section = NextSiblingElement(section, "option")) {
-    Option(section, &model->spec.option);
+    Option(section, &model->option);
   }
 
   for (XMLElement* section = FirstChildElement(root, "size"); section;
        section = NextSiblingElement(section, "size")) {
-    Size(section, &model->spec);
+    Size(section, model);
   }
 
   //------------------ parse MJCF-specific sections
@@ -871,7 +871,7 @@ void mjXReader::Parse(XMLElement* root) {
 
   for (XMLElement* section = FirstChildElement(root, "worldbody"); section;
        section = NextSiblingElement(section, "worldbody")) {
-    Body(section, &model->GetWorld()->spec, nullptr);
+    Body(section, mjm_findBody(model, "world"), nullptr);
   }
 
   for (XMLElement* section = FirstChildElement(root, "contact"); section;
@@ -1261,15 +1261,14 @@ void mjXReader::Statistic(XMLElement* section) {
   string text;
 
   // read statistics
-  mjmModel* pmodel = &model->spec;
-  ReadAttr(section, "meaninertia", 1, &pmodel->stat.meaninertia, text);
-  ReadAttr(section, "meanmass", 1, &pmodel->stat.meanmass, text);
-  ReadAttr(section, "meansize", 1, &pmodel->stat.meansize, text);
-  ReadAttr(section, "extent", 1, &pmodel->stat.extent, text);
-  if (mjuu_defined(pmodel->stat.extent) && pmodel->stat.extent<=0) {
+  ReadAttr(section, "meaninertia", 1, &model->stat.meaninertia, text);
+  ReadAttr(section, "meanmass", 1, &model->stat.meanmass, text);
+  ReadAttr(section, "meansize", 1, &model->stat.meansize, text);
+  ReadAttr(section, "extent", 1, &model->stat.extent, text);
+  if (mjuu_defined(model->stat.extent) && model->stat.extent<=0) {
     throw mjXError(section, "extent must be strictly positive");
   }
-  ReadAttr(section, "center", 3, pmodel->stat.center, text);
+  ReadAttr(section, "center", 3, model->stat.center, text);
 }
 
 
@@ -2617,7 +2616,7 @@ void mjXReader::OnePlugin(XMLElement* elem, mjmPlugin* plugin) {
   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->spec)->instance;
+    plugin->instance = mjm_addPlugin(model)->instance;
     ReadPluginConfigs(elem, plugin);
   } else {
     model->hasImplicitPluginElem = true;
@@ -2646,14 +2645,13 @@ void mjXReader::Default(XMLElement* section, int parentid) {
     }
   }
   if (parentid>=0) {
-    thisid = (int)model->defaults.size();
-    def = mjm_addDefault(&model->spec, text.c_str(), parentid);
+    def = mjm_addDefault(model, text.c_str(), parentid, &thisid);
     if (!def) {
       throw mjXError(section, "repeated default class name");
     }
   } else {
     thisid = 0;
-    def = &model->defaults[0]->spec;
+    def = mjm_getModelDefault(model);
     mjm_setString(def->name, text.c_str());
   }
 
@@ -2731,6 +2729,8 @@ void mjXReader::Default(XMLElement* section, int parentid) {
 // extension section parser
 void mjXReader::Extension(XMLElement* section) {
   XMLElement* elem = FirstChildElement(section);
+  std::vector<std::pair<const mjpPlugin*, int>> active_plugins;
+
   while (elem) {
     // get sub-element name
     std::string_view name = elem->Value();
@@ -2745,14 +2745,14 @@ void mjXReader::Extension(XMLElement* section) {
       }
 
       bool already_declared = false;
-      for (const auto& [existing_plugin, existing_slot] : model->active_plugins) {
+      for (const auto& [existing_plugin, existing_slot] : active_plugins) {
         if (plugin == existing_plugin) {
           already_declared = true;
           break;
         }
       }
       if (!already_declared) {
-        model->active_plugins.emplace_back(std::make_pair(plugin, plugin_slot));
+        active_plugins.emplace_back(std::make_pair(plugin, plugin_slot));
       }
 
       XMLElement* child = FirstChildElement(elem);
@@ -2763,7 +2763,7 @@ void mjXReader::Extension(XMLElement* section) {
                 child, "explicit plugin instance must appear before implicit plugin elements");
           }
           string name;
-          mjmPlugin* p = mjm_addPlugin(&model->spec);
+          mjmPlugin* p = mjm_addPlugin(model);
           mjm_setString(p->info, ("line = " + std::to_string(elem->GetLineNum())).c_str());
           ReadAttrTxt(child, "name", name, /* required = */ true);
           mjm_setString(p->name, name.c_str());
@@ -2780,6 +2780,8 @@ void mjXReader::Extension(XMLElement* section) {
     // advance to next element
     elem = NextSiblingElement(elem);
   }
+
+  mjm_setActivePlugins(model, &active_plugins);
 }
 
 
@@ -2800,7 +2802,7 @@ void mjXReader::Custom(XMLElement* section) {
     // numeric
     if (name=="numeric") {
       // create custom
-      mjmNumeric* pnum = mjm_addNumeric(&model->spec);
+      mjmNumeric* pnum = mjm_addNumeric(model);
 
       // write error info
       mjm_setString(pnum->info, ("line = " + std::to_string(elem->GetLineNum())).c_str());
@@ -2831,7 +2833,7 @@ void mjXReader::Custom(XMLElement* section) {
     // text
     else if (name=="text") {
       // create custom
-      mjmText* pte = mjm_addText(&model->spec);
+      mjmText* pte = mjm_addText(model);
 
       // write error info
       mjm_setString(pte->info, ("line = " + std::to_string(elem->GetLineNum())).c_str());
@@ -2851,7 +2853,7 @@ void mjXReader::Custom(XMLElement* section) {
     // tuple
     else if (name=="tuple") {
       // create custom
-      mjmTuple* ptu = mjm_addTuple(&model->spec);
+      mjmTuple* ptu = mjm_addTuple(model);
 
       // write error info
       mjm_setString(ptu->info, ("line = " + std::to_string(elem->GetLineNum())).c_str());
@@ -2910,7 +2912,7 @@ void mjXReader::Custom(XMLElement* section) {
 void mjXReader::Visual(XMLElement* section) {
   string text, name;
   XMLElement* elem;
-  mjVisual* vis = &model->spec.visual;
+  mjVisual* vis = &model->visual;
 
   // iterate over child elements
   elem = FirstChildElement(section);
@@ -3052,13 +3054,13 @@ void mjXReader::Asset(XMLElement* section) {
     // get class if specified, otherwise use default0
     mjmDefault* def = GetClass(elem);
     if (!def) {
-      def = &model->defaults[0]->spec;
+      def = mjm_getModelDefault(model);
     }
 
     // texture sub-element
     if (name=="texture") {
       // create texture
-      mjmTexture* ptex = mjm_addTexture(&model->spec);
+      mjmTexture* ptex = mjm_addTexture(model);
 
       // write error info
       mjm_setString(ptex->info, ("line = " + std::to_string(elem->GetLineNum())).c_str());
@@ -3126,28 +3128,28 @@ void mjXReader::Asset(XMLElement* section) {
     // material sub-element
     else if (name=="material") {
       // create material and parse
-      mjmMaterial* pmat = mjm_addMaterial(&model->spec, def);
+      mjmMaterial* pmat = mjm_addMaterial(model, def);
       OneMaterial(elem, pmat);
     }
 
     // mesh sub-element
     else if (name=="mesh") {
       // create mesh and parse
-      mjmMesh* pmesh = mjm_addMesh(&model->spec, def);
+      mjmMesh* pmesh = mjm_addMesh(model, def);
       OneMesh(elem, pmesh);
     }
 
     // skin sub-element... deprecate ???
     else if (name=="skin") {
       // create skin and parse
-      mjmSkin* pskin = mjm_addSkin(&model->spec);
+      mjmSkin* pskin = mjm_addSkin(model);
       OneSkin(elem, pskin);
     }
 
     // hfield sub-element
     else if (name=="hfield") {
       // create hfield
-      mjmHField* phf = mjm_addHField(&model->spec);
+      mjmHField* phf = mjm_addHField(model);
 
       // write error info
       mjm_setString(phf->info, ("line = " + std::to_string(elem->GetLineNum())).c_str());
@@ -3356,7 +3358,8 @@ void mjXReader::Body(XMLElement* section, mjmBody* pbody, mjmFrame* frame) {
       // read childdef
       mjmDefault* childdef = 0;
       if (ReadAttrTxt(elem, "childclass", text)) {
-        childdef = &model->FindDef(text)->spec;
+        childdef = mjm_findDefault(model, text.c_str());
+        mjm_findDefault(model, text.c_str());
         if (!childdef) {
           throw mjXError(elem, "unknown default childclass");
         }
@@ -3423,19 +3426,19 @@ void mjXReader::Contact(XMLElement* section) {
     // get class if specified, otherwise use default0
     mjmDefault* def = GetClass(elem);
     if (!def) {
-      def = &model->defaults[0]->spec;
+      def = mjm_getModelDefault(model);
     }
 
     // geom pair to include
     if (name=="pair") {
       // create pair and parse
-      mjmPair* ppair = mjm_addPair(&model->spec, def);
+      mjmPair* ppair = mjm_addPair(model, def);
       OnePair(elem, ppair);
     }
 
     // body pair to exclude
     else if (name=="exclude") {
-      mjmExclude* pexclude = mjm_addExclude(&model->spec);
+      mjmExclude* pexclude = mjm_addExclude(model);
       string exname, exbody1, exbody2;
 
       // write error info
@@ -3468,11 +3471,11 @@ void mjXReader::Equality(XMLElement* section) {
     // get class if specified, otherwise use default0
     mjmDefault* def = GetClass(elem);
     if (!def) {
-      def = &model->defaults[0]->spec;
+      def = mjm_getModelDefault(model);
     }
 
     // create equality constraint and parse
-    mjmEquality* pequality = mjm_addEquality(&model->spec, def);
+    mjmEquality* pequality = mjm_addEquality(model, def);
     OneEquality(elem, pequality);
 
     // advance to next element
@@ -3496,20 +3499,20 @@ void mjXReader::Deformable(XMLElement* section) {
     // get class if specified, otherwise use default0
     mjmDefault* def = GetClass(elem);
     if (!def) {
-      def = &model->defaults[0]->spec;
+      def = mjm_getModelDefault(model);
     }
 
     // flex sub-element
     if (name=="flex") {
       // create flex and parse
-      mjmFlex* pflex = mjm_addFlex(&model->spec);
+      mjmFlex* pflex = mjm_addFlex(model);
       OneFlex(elem, pflex);
     }
 
     // skin sub-element
     else if (name=="skin") {
       // create skin and parse
-      mjmSkin* pskin = mjm_addSkin(&model->spec);
+      mjmSkin* pskin = mjm_addSkin(model);
       OneSkin(elem, pskin);
     }
 
@@ -3532,11 +3535,11 @@ void mjXReader::Tendon(XMLElement* section) {
     // get class if specified, otherwise use default0
     mjmDefault* def = GetClass(elem);
     if (!def) {
-      def = &model->defaults[0]->spec;
+      def = mjm_getModelDefault(model);
     }
 
     // create equality constraint and parse
-    mjmTendon* pten = mjm_addTendon(&model->spec, def);
+    mjmTendon* pten = mjm_addTendon(model, def);
     OneTendon(elem, pten);
 
     // process wrap sub-elements
@@ -3598,11 +3601,11 @@ void mjXReader::Actuator(XMLElement* section) {
     // get class if specified, otherwise use default0
     mjmDefault* def = GetClass(elem);
     if (!def) {
-      def = &model->defaults[0]->spec;
+      def = mjm_getModelDefault(model);
     }
 
     // create actuator and parse
-    mjmActuator* pact = mjm_addActuator(&model->spec, def);
+    mjmActuator* pact = mjm_addActuator(model, def);
     OneActuator(elem, pact);
 
     // advance to next element
@@ -3618,7 +3621,7 @@ void mjXReader::Sensor(XMLElement* section) {
   XMLElement* elem = FirstChildElement(section);
   while (elem) {
     // create sensor, get string type
-    mjmSensor* psen = mjm_addSensor(&model->spec);
+    mjmSensor* psen = mjm_addSensor(model);
     string type = elem->Value();
     string text, name, objname, refname;
     std::vector<double> userdata;
@@ -3935,7 +3938,7 @@ void mjXReader::Keyframe(XMLElement* section) {
     string text, name = "";
 
     // add keyframe
-    mjmKey* pk = mjm_addKey(&model->spec);
+    mjmKey* pk = mjm_addKey(model);
 
     // read name, time
     ReadAttrTxt(elem, "name", name);
@@ -3991,7 +3994,7 @@ mjmDefault* mjXReader::GetClass(XMLElement* section) {
   mjmDefault* def = nullptr;
 
   if (ReadAttrTxt(section, "class", text)) {
-    def = mjm_findDefault(&model->spec, text.c_str());
+    def = mjm_findDefault(model, text.c_str());
     if (!def) {
       throw mjXError(
           section,

src/xml/xml_native_writer.cc

@@ -764,6 +764,14 @@ mjXWriter::mjXWriter(void) {
 }
 
 
+// cast model
+void mjXWriter::SetModel(mjmModel* modelspec) {
+  if (modelspec) {
+    model = (mjCModel*)modelspec->element;
+  }
+}
+
+
 // save existing model in MJCF canonical format, must be compiled
 string mjXWriter::Write(char *error, size_t error_sz) {
   // check model

src/xml/xml_native_writer.h

@@ -18,6 +18,8 @@
 #include <cstdlib>
 #include <string>
 
+#include "user/user_api.h"
+#include "user/user_objects.h"
 #include "xml/xml_base.h"
 #include "tinyxml2.h"
 
@@ -25,6 +27,7 @@ class mjXWriter : public mjXBase {
  public:
   mjXWriter();                                        // constructor
   virtual ~mjXWriter() = default;                     // destructor
+  void SetModel(mjmModel* modelspec);
 
   // write XML document to string
   std::string Write(char *error, std::size_t error_sz);
@@ -33,6 +36,9 @@ class mjXWriter : public mjXBase {
   // insert end child with given name, return child
   tinyxml2::XMLElement* InsertEnd(tinyxml2::XMLElement* parent, const char* name);
 
+  // compiled model
+  mjCModel* model = 0;
+
   // XML section writers
   void Compiler(tinyxml2::XMLElement* root);                    // compiler section
   void Option(tinyxml2::XMLElement* root);                      // option section

src/xml/xml_urdf.cc

@@ -20,6 +20,7 @@
 
 #include <mujoco/mjmodel.h>
 #include "user/user_api.h"
+#include "user/user_objects.h"
 #include "user/user_util.h"
 #include "xml/xml_native_reader.h"
 #include "xml/xml_urdf.h"
@@ -93,25 +94,25 @@ void mjXURDF::Parse(
   if (mjc) {
     XMLElement *section;
     if ((section = FindSubElem(mjc, "compiler"))) {
-      mjXReader::Compiler(section, &model->spec);
+      mjXReader::Compiler(section, model);
     }
 
     if ((section = FindSubElem(mjc, "option"))) {
-      mjXReader::Option(section, &model->spec.option);
+      mjXReader::Option(section, &model->option);
     }
 
     if ((section = FindSubElem(mjc, "size"))) {
-      mjXReader::Size(section, &model->spec);
+      mjXReader::Size(section, model);
     }
   }
 
   // enforce required compiler defaults for URDF
-  model->spec.degree = false;
+  model->degree = false;
 
   // get model name
   std::string modelname;
   if (ReadAttrTxt(root, "name", modelname)) {
-    mjm_setString(model->spec.modelname, modelname.c_str());
+    mjm_setString(model->modelname, modelname.c_str());
   }
 
   // find and register all materials
@@ -205,7 +206,7 @@ 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) {
-      mjmBody* world = mjm_findBody(&model->spec, "world");
+      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);
@@ -231,7 +232,7 @@ void mjXURDF::Body(XMLElement* body_elem) {
   // get body name and pointer to mjmBody
   ReadAttrTxt(body_elem, "name", name, true);
   name = GetPrefixedName(name);
-  world = mjm_findBody(&model->spec, "world");
+  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
@@ -313,7 +314,7 @@ void mjXURDF::Body(XMLElement* body_elem) {
         }
       }
       // create geom if not discarded
-      if (!model->spec.discardvisual) {
+      if (!model->discardvisual) {
         pgeom = Geom(elem, pbody, false);
 
         // save color
@@ -389,7 +390,7 @@ void mjXURDF::Joint(XMLElement* joint_elem) {
   elem = FindSubElem(joint_elem, "parent", true);
   ReadAttrTxt(elem, "link", name, true);
   name = GetPrefixedName(name);
-  world = mjm_findBody(&model->spec, "world");
+  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");
@@ -399,7 +400,7 @@ void mjXURDF::Joint(XMLElement* joint_elem) {
   elem = FindSubElem(joint_elem, "child", true);
   ReadAttrTxt(elem, "link", name, true);
   name = GetPrefixedName(name);
-  world = mjm_findBody(&model->spec, "world");
+  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");
@@ -566,7 +567,7 @@ mjmGeom* mjXURDF::Geom(XMLElement* geom_elem, mjmBody* pbody, bool collision) {
                                       .value_or(default_meshscale);
 
     // strip file name if necessary
-    if (model->spec.strippath) {
+    if (model->strippath) {
       meshfile = mjuu_strippath(meshfile);
     }
 
@@ -575,19 +576,19 @@ mjmGeom* mjXURDF::Geom(XMLElement* geom_elem, mjmBody* pbody, bool collision) {
     meshname = mjuu_stripext(meshname);
 
     // look for existing mesh
-    mjmMesh* mesh = mjm_findMesh(&model->spec, meshname.c_str());
+    mjmMesh* mesh = mjm_findMesh(model, meshname.c_str());
     mjmMesh* pmesh = 0;
 
     // does not exist: create
     if (!mesh) {
-      pmesh = mjm_addMesh(&model->spec, 0);
+      pmesh = mjm_addMesh(model, 0);
     }
 
     // exists with different scale: append name with '1', create
     else if (mesh->scale[0]!=meshscale[0] ||
              mesh->scale[1]!=meshscale[1] ||
              mesh->scale[2]!=meshscale[2]) {
-      pmesh = mjm_addMesh(&model->spec, 0);
+      pmesh = mjm_addMesh(model, 0);
       meshname = meshname + "1";
     }
 
@@ -684,14 +685,14 @@ void mjXURDF::AddToTree(int n) {
   // get pointer to parent in mjCModel tree
   mjmBody *parent = 0, *child = 0, *world = 0;
   if (urParent[n]>=0) {
-    world = mjm_findBody(&model->spec, "world");
+    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()->spec;
+    parent = mjm_findBody(model, "world");
   }
 
   // add this body

src/xml/xml_urdf.h

@@ -20,7 +20,6 @@
 #include <vector>
 
 #include "user/user_api.h"
-#include "user/user_model.h"
 #include "xml/xml_base.h"
 #include "tinyxml2.h"