From b224367a9b5de430e868c0215a56034ce998b87e Mon Sep 17 00:00:00 2001
From: Alessio Quaglino <quaglino@google.com>
Date: Mon, 12 Feb 2024 08:46:57 -0800
Subject: [PATCH] Add mjmFlex to C API.

PiperOrigin-RevId: 606264589
Change-Id: Ifd94f5241a3eaaf23653b136cece81c054f43202
---
 src/user/user_api.cc         |  50 +++++++++++-
 src/user/user_api.h          |  84 +++++++++++++++++---
 src/user/user_flexcomp.cc    |  61 ++++++++-------
 src/user/user_init.cc        |  26 ++++++-
 src/user/user_mesh.cc        | 143 +++++++++++++++++++----------------
 src/user/user_model.cc       |  10 +--
 src/user/user_objects.cc     |  32 ++++----
 src/user/user_objects.h      |  62 +++++++--------
 src/xml/xml_native_reader.cc |  85 +++++++++++++--------
 src/xml/xml_native_reader.h  |   2 +-
 src/xml/xml_native_writer.cc |  16 ++--
 11 files changed, 368 insertions(+), 203 deletions(-)

diff --git a/src/user/user_api.cc b/src/user/user_api.cc
index 84c11eeb..f29a7429 100644
--- a/src/user/user_api.cc
+++ b/src/user/user_api.cc
@@ -19,6 +19,7 @@
 #include <mujoco/mujoco.h>
 #include "user/user_model.h"
 #include "user/user_objects.h"
+#include "xml/xml_util.h"
 
 
 
@@ -114,6 +115,15 @@ mjmLight* mjm_addLight(mjmBody* bodyspec, void* defspec) {
 
 
 
+// add flex to model
+mjmFlex* mjm_addFlex(void* model) {
+  mjCModel* modelC = static_cast<mjCModel*>(model);
+  mjCFlex* flex = modelC->AddFlex();
+  return &flex->spec;
+}
+
+
+
 // add frame to body
 mjmFrame* mjm_addFrame(mjmBody* bodyspec, mjmFrame* parentframe) {
   mjCFrame* parentframeC = 0;
@@ -292,8 +302,44 @@ void mjm_setString(mjString dest, const char* text) {
 
 
 
+// split text and copy into string array
+void mjm_setStringVec(mjStringVec dest, const char* text) {
+  std::vector<std::string>* v = reinterpret_cast<std::vector<std::string>*>(dest);
+  mjXUtil::String2Vector(text, *v);
+}
+
+
+// add text entry to destination string vector
+void mjm_addToStringVec(mjStringVec dest, const char* text) {
+  std::vector<std::string>* v = reinterpret_cast<std::vector<std::string>*>(dest);
+  v->push_back(std::string(text));
+}
+
+
+// set int array
+void mjm_setInt(mjIntVec dest, const int* array, int size) {
+  std::vector<int>* v = reinterpret_cast<std::vector<int>*>(dest);
+  v->assign(size, 0.0);
+  for (int i = 0; i < size; ++i) {
+    (*v)[i] = array[i];
+  }
+}
+
+
+
+// set float array
+void mjm_setFloat(mjFloatVec dest, const float* array, int size) {
+  std::vector<float>* v = reinterpret_cast<std::vector<float>*>(dest);
+  v->assign(size, 0.0);
+  for (int i = 0; i < size; ++i) {
+    (*v)[i] = array[i];
+  }
+}
+
+
+
 // set double array
-void mjm_setDouble(mjDouble dest, const double* array, int size) {
+void mjm_setDouble(mjDoubleVec dest, const double* array, int size) {
   std::vector<double>* v = reinterpret_cast<std::vector<double>*>(dest);
   v->assign(size, 0.0);
   for (int i = 0; i < size; ++i) {
@@ -312,7 +358,7 @@ const char* mjm_getString(const mjString source) {
 
 
 // get double array
-const double* mjm_getDouble(const mjDouble source, int* size) {
+const double* mjm_getDouble(const mjDoubleVec source, int* size) {
   std::vector<double>* v = reinterpret_cast<std::vector<double>*>(source);
   if (size) {
     *size = v->size();
diff --git a/src/user/user_api.h b/src/user/user_api.h
index 7148be46..d904f0d4 100644
--- a/src/user/user_api.h
+++ b/src/user/user_api.h
@@ -30,7 +30,10 @@ extern "C" {
 
 typedef struct _mjElement* mjElement;
 typedef struct _mjString* mjString;
-typedef struct _mjDouble* mjDouble;
+typedef struct _mjStringVec* mjStringVec;
+typedef struct _mjIntVec* mjIntVec;
+typedef struct _mjDoubleVec* mjDoubleVec;
+typedef struct _mjFloatVec* mjFloatVec;
 
 
 //---------------------------------- enum types (mjt) ----------------------------------------------
@@ -80,7 +83,7 @@ typedef struct _mjmBody {          // body specification
   // other
   mjtByte mocap;                   // is this a mocap body
   double gravcomp;                 // gravity compensation
-  mjDouble userdata;               // user data
+  mjDoubleVec userdata;            // user data
   mjtByte explicitinertial;        // whether to save the body with explicit inertial clause
   mjmPlugin plugin;                // passive force plugin
   mjString info;                   // message appended to compiler errors
@@ -131,7 +134,7 @@ typedef struct _mjmJoint {         // joint specification
   // other
   int group;                       // group
   double urdfeffort;               // effort (urdf)
-  mjDouble userdata;               // user data
+  mjDoubleVec userdata;            // user data
   mjString info;                   // message appended to compiler errors
 } mjmJoint;
 
@@ -179,7 +182,7 @@ typedef struct _mjmGeom {          // geom specification
   mjString hfieldname;             // heightfield attached to geom
   mjString meshname;               // mesh attached to geom
   double fitscale;                 // scale mesh uniformly
-  mjDouble userdata;               // user data
+  mjDoubleVec userdata;            // user data
   mjmPlugin plugin;                // sdf plugin
   mjString info;                   // message appended to compiler errors
 } mjmGeom;
@@ -204,7 +207,7 @@ typedef struct _mjmSite {          // site specification
   float rgba[4];                   // rgba when material is omitted
 
   // other
-  mjDouble userdata;               // user data
+  mjDoubleVec userdata;            // user data
   mjString info;                   // message appended to compiler errors
 } mjmSite;
 
@@ -233,7 +236,7 @@ typedef struct _mjmCamera {        // camera specification
   float principal_pixel[2];        // principal point (pixel)
 
   // other
-  mjDouble userdata;               // user data
+  mjDoubleVec userdata;            // user data
   mjString info;                   // message appended to compiler errors
 } mjmCamera;
 
@@ -265,6 +268,47 @@ typedef struct _mjmLight {         // light specification
 } mjmLight;
 
 
+typedef struct _mjmFlex {
+  mjElement element;               // internal, do not modify
+  mjString name;                   // name
+  mjString classname;              // class name
+
+  // contact properties
+  int contype;                     // contact type
+  int conaffinity;                 // contact affinity
+  int condim;                      // contact dimensionality
+  int priority;                    // contact priority
+  double friction[3];              // one-sided friction coefficients: slide, roll, spin
+  double solmix;                   // solver mixing for contact pairs
+  mjtNum solref[mjNREF];           // solver reference
+  mjtNum solimp[mjNIMP];           // solver impedance
+  double margin;                   // margin for contact detection
+  double gap;                      // include in solver if dist<margin-gap
+
+  // other properties
+  int dim;                         // element dimensionality
+  double radius;                   // radius around primitive element
+  bool internal;                   // enable internal collisions
+  bool flatskin;                   // render flex skin with flat shading
+  int selfcollide;                 // mode for flex self colllision
+  int activelayers;                // number of active element layers in 3D
+  int group;                       // group for visualizatioh
+  double edgestiffness;            // edge stiffness
+  double edgedamping;              // edge damping
+  float rgba[4];                   // rgba when material is omitted
+  mjString material;               // name of material used for rendering
+
+  // mesh properties
+  mjStringVec vertbody;            // vertex body names
+  mjDoubleVec vert;                // vertex positions
+  mjIntVec elem;                   // element vertex ids
+  mjFloatVec texcoord;             // vertex texture coordinates
+
+  // other
+  mjString info;                   // message appended to compiler errors
+} mjmFlex;
+
+
 typedef struct _mjmMaterial {      // material specification
   mjElement element;               // internal, do not modify
   mjString name;                   // name
@@ -323,7 +367,7 @@ typedef struct _mjmTendon {        // tendon specification
   int group;                       // group
 
   // other
-  mjDouble userdata;               // user data
+  mjDoubleVec userdata;            // user data
   mjString info;                   // message appended to errors
 } mjmTendon;
 
@@ -373,7 +417,7 @@ typedef struct _mjmActuator {      // actuator specification
 
   // other
   int group;                       // group
-  mjDouble userdata;               // user data
+  mjDoubleVec userdata;            // user data
   mjmPlugin plugin;                // actuator plugin
   mjString info;                   // message appended to compiler errors
 } mjmActuator;
@@ -401,7 +445,7 @@ typedef struct _mjmSensor {        // sensor specfication
   double noise;                    // noise stdev
 
   // other
-  mjDouble userdata;               // user data
+  mjDoubleVec userdata;            // user data
   mjmPlugin plugin;                // sensor plugin
   mjString info;                   // message appended to compiler errors
 } mjmSensor;
@@ -442,6 +486,9 @@ MJAPI mjmLight* mjm_addLight(mjmBody* body, void* defspec);
 // Add frame to body.
 MJAPI mjmFrame* mjm_addFrame(mjmBody* body, mjmFrame* parentframe);
 
+// Add flex to model.
+MJAPI mjmFlex* mjm_addFlex(void* model);
+
 // Add material to model.
 MJAPI mjmMaterial* mjm_addMaterial(void* model, void* defspec);
 
@@ -490,14 +537,26 @@ MJAPI int mjm_getId(mjElement element);
 // Copy text to destination string.
 MJAPI void mjm_setString(mjString dest, const char* text);
 
+// Split text to entries and copy to destination string vector.
+MJAPI void mjm_setStringVec(mjStringVec dest, const char* text);
+
+// Add text entry to destination string vector.
+MJAPI void mjm_addToStringVec(mjStringVec dest, const char* text);
+
+// Copy int array to destination vector.
+MJAPI void mjm_setInt(mjIntVec dest, const int* array, int size);
+
+// Copy float array to destination vector.
+MJAPI void mjm_setFloat(mjFloatVec dest, const float* array, int size);
+
 // Copy double array to destination vector.
-MJAPI void mjm_setDouble(mjDouble dest, const double* array, int size);
+MJAPI void mjm_setDouble(mjDoubleVec dest, const double* array, int size);
 
 // Get string contents.
 MJAPI const char* mjm_getString(mjString source);
 
 // Get double array contents and optionally its size.
-MJAPI const double* mjm_getDouble(mjDouble source, int* size);
+MJAPI const double* mjm_getDouble(mjDoubleVec source, int* size);
 
 // Set default.
 MJAPI void mjm_setDefault(mjElement element, void* defspec);
@@ -532,6 +591,9 @@ MJAPI void mjm_defaultCamera(mjmCamera& camera);
 // Default light attributes.
 MJAPI void mjm_defaultLight(mjmLight& light);
 
+// Default flex attributes.
+MJAPI void mjm_defaultFlex(mjmFlex& flex);
+
 // Default material attributes.
 MJAPI void mjm_defaultMaterial(mjmMaterial& material);
 
diff --git a/src/user/user_flexcomp.cc b/src/user/user_flexcomp.cc
index 072a1698..a9987df9 100644
--- a/src/user/user_flexcomp.cc
+++ b/src/user/user_flexcomp.cc
@@ -78,6 +78,8 @@ mjCFlexcomp::mjCFlexcomp(void) {
 
 // make flexcomp object
 bool mjCFlexcomp::Make(mjCModel* model, mjmBody* body, char* error, int error_sz) {
+  def.flex.CopyFromSpec();
+
   bool radial = (type==mjFCOMPTYPE_BOX ||
                  type==mjFCOMPTYPE_CYLINDER ||
                  type==mjFCOMPTYPE_ELLIPSOID);
@@ -98,7 +100,7 @@ bool mjCFlexcomp::Make(mjCModel* model, mjmBody* body, char* error, int error_sz
   }
 
   // check spacing
-  double minspace = 2*def.flex.radius + def.flex.margin;
+  double minspace = 2*def.flex.spec.radius + def.flex.spec.margin;
   if (!direct) {
     if (spacing[0]<minspace ||
         spacing[1]<minspace ||
@@ -156,7 +158,7 @@ bool mjCFlexcomp::Make(mjCModel* model, mjmBody* body, char* error, int error_sz
   }
 
   // get dim and check
-  int dim = def.flex.dim;
+  int dim = def.flex.spec.dim;
   if (dim<1 || dim>3) {
     return comperr(error, "Invalid dim, must be between 1 and 3", error_sz);
   }
@@ -164,7 +166,7 @@ bool mjCFlexcomp::Make(mjCModel* model, mjmBody* body, char* error, int error_sz
   // force flatskin shading for box, cylinder and 3D grid
   if (type==mjFCOMPTYPE_BOX || type==mjFCOMPTYPE_CYLINDER ||
       (type==mjFCOMPTYPE_GRID && dim==3)) {
-    def.flex.flatskin = true;
+    def.flex.spec.flatskin = true;
   }
 
   // check pin sizes
@@ -368,21 +370,22 @@ bool mjCFlexcomp::Make(mjCModel* model, mjmBody* body, char* error, int error_sz
   }
 
   // create flex, copy parameters
-  mjCFlex* pf = model->AddFlex();
-  int id = pf->id;
-  *pf = def.flex;
-  pf->model = model;
-  pf->id = id;
-  pf->name = name;
-  pf->elem = element;
-  if (!centered) {
-    pf->vert = point;
-  }
-  pf->texcoord = texcoord;
+  mjCFlex* flex = model->AddFlex();
+  mjmFlex* pf = &flex->spec;
+  int id = flex->id;
+
+  *flex = def.flex;
+  flex->PointToLocal();
+
+  flex->model = model;
+  flex->id = id;
+  mjm_setString(pf->name, name.c_str());
+  mjm_setInt(pf->elem, element.data(), element.size());
+  mjm_setFloat(pf->texcoord, texcoord.data(), texcoord.size());
 
   // rigid: set parent name, nothing else to do
   if (rigid) {
-    pf->vertbody.push_back(std::string(mjm_getString(body->name)).c_str());
+    mjm_addToStringVec(pf->vertbody, mjm_getString(body->name));
     return true;
   }
 
@@ -399,7 +402,7 @@ bool mjCFlexcomp::Make(mjCModel* model, mjmBody* body, char* error, int error_sz
 
     // pinned: parent body
     if (pinned[i]) {
-      pf->vertbody.push_back(std::string(mjm_getString(body->name)).c_str());
+      mjm_addToStringVec(pf->vertbody, mjm_getString(body->name));
 
       // add plugin
       if (plugin_instance) {
@@ -456,13 +459,13 @@ bool mjCFlexcomp::Make(mjCModel* model, mjmBody* body, char* error, int error_sz
       char txt[100];
       mju::sprintf_arr(txt, "%s_%d", name.c_str(), i);
       mjm_setString(pb->name, txt);
-      pf->vertbody.push_back(std::string(mjm_getString(pb->name)).c_str());
+      mjm_addToStringVec(pf->vertbody, mjm_getString(pb->name));
 
       // clear flex vertex coordinates if allocated
       if (!centered) {
-        pf->vert[3*i] = 0;
-        pf->vert[3*i+1] = 0;
-        pf->vert[3*i+2] = 0;
+        point[3*i] = 0;
+        point[3*i+1] = 0;
+        point[3*i+2] = 0;
       }
 
       // add plugin
@@ -476,6 +479,10 @@ bool mjCFlexcomp::Make(mjCModel* model, mjmBody* body, char* error, int error_sz
     }
   }
 
+  if (!centered) {
+    mjm_setDouble(pf->vert, point.data(), point.size());
+  }
+
   // create edge equality constraint
   if (equality) {
     mjmEquality* pe = mjm_addEquality(model, &def);
@@ -502,7 +509,7 @@ int mjCFlexcomp::GridID(int ix, int iy, int iz) {
 
 // make grid
 bool mjCFlexcomp::MakeGrid(char* error, int error_sz) {
-  int dim = def.flex.dim;
+  int dim = def.flex.spec.dim;
   bool hastex = texcoord.empty();
 
   // 1D
@@ -683,7 +690,7 @@ bool mjCFlexcomp::MakeBox(char* error, int error_sz) {
   double pos[3];
 
   // set 3D
-  def.flex.dim = 3;
+  def.flex.spec.dim = 3;
 
   // add center point
   point.push_back(0);
@@ -815,7 +822,7 @@ bool mjCFlexcomp::MakeMesh(mjCModel* model, char* error, int error_sz) {
   }
 
   // check dim
-  if (def.flex.dim!=2) {
+  if (def.flex.spec.dim!=2) {
     return comperr(error, "Flex dim must be 2 in for mesh", error_sz);
   }
 
@@ -1027,7 +1034,7 @@ void mjCFlexcomp::LoadGMSH(mjCModel* model, mjResource* resource) {
     if (entityDim<1 || entityDim>3) {
       throw mjCError(NULL, "Entity must be 1D, 2D or 3D");
     }
-    def.flex.dim = entityDim;
+    def.flex.spec.dim = entityDim;
 
     // read and discard node tags; require range from minNodeTag to maxNodeTag
     for (size_t i=0; i<numNodes; i++) {
@@ -1079,7 +1086,7 @@ void mjCFlexcomp::LoadGMSH(mjCModel* model, mjResource* resource) {
     if (entityDim<1 || entityDim>3) {
       throw mjCError(NULL, "Entity must be 1D, 2D or 3D");
     }
-    def.flex.dim = entityDim;
+    def.flex.spec.dim = entityDim;
 
     // check section byte size
     if (nodeend-nodebegin < 52+numNodes*4*8) {
@@ -1127,7 +1134,7 @@ void mjCFlexcomp::LoadGMSH(mjCModel* model, mjResource* resource) {
     }
 
     // dimensionality must be same as nodes
-    if (entityDim!=def.flex.dim) {
+    if (entityDim!=def.flex.spec.dim) {
       throw mjCError(NULL, "Inconsistent dimensionality in Elements");
     }
 
@@ -1176,7 +1183,7 @@ void mjCFlexcomp::LoadGMSH(mjCModel* model, mjResource* resource) {
     }
 
     // dimensionality must be same as nodes
-    if (entityDim!=def.flex.dim) {
+    if (entityDim!=def.flex.spec.dim) {
       throw mjCError(NULL, "Inconsistent dimensionality in Elements");
     }
 
diff --git a/src/user/user_init.cc b/src/user/user_init.cc
index ed12d2f9..3251e023 100644
--- a/src/user/user_init.cc
+++ b/src/user/user_init.cc
@@ -167,7 +167,31 @@ void mjm_defaultLight(mjmLight& light) {
 
 
 
-// Default material attributes.
+// default flex attributes
+void mjm_defaultFlex(mjmFlex& flex) {
+  memset(&flex, 0, sizeof(mjmFlex));
+
+  // set contact defaults
+  flex.contype = 1;
+  flex.conaffinity = 1;
+  flex.condim = 3;
+  mjuu_setvec(flex.friction, 1, 0.005, 0.0001);
+  flex.solmix = 1.0;
+  mj_defaultSolRefImp(flex.solref, flex.solimp);
+
+  // set other defaults
+  flex.dim = 2;
+  flex.radius = 0.005;
+  flex.internal = true;
+  flex.selfcollide = mjFLEXSELF_AUTO;
+  flex.activelayers = 1;
+  flex.rgba[0] = flex.rgba[1] = flex.rgba[2] = 0.5f;
+  flex.rgba[3] = 1.0f;
+}
+
+
+
+// default material attributes
 void mjm_defaultMaterial(mjmMaterial& material) {
   memset(&material, 0, sizeof(mjmMaterial));
   material.texuniform = false;
diff --git a/src/user/user_mesh.cc b/src/user/user_mesh.cc
index 35e56cbd..8fb4849b 100644
--- a/src/user/user_mesh.cc
+++ b/src/user/user_mesh.cc
@@ -2222,34 +2222,11 @@ constexpr int eledge[3][6][2] = {{{ 0,  1}, {-1, -1}, {-1, -1},
 
 // constructor
 mjCFlex::mjCFlex(mjCModel* _model) {
+  mjm_defaultFlex(spec);
+
   // set model
   model = _model;
 
-  // set contact defaults
-  contype = 1;
-  conaffinity = 1;
-  condim = 3;
-  priority = 0;
-  mjuu_setvec(friction, 1, 0.005, 0.0001);
-  solmix = 1.0;
-  mj_defaultSolRefImp(solref, solimp);
-  margin = 0;
-  gap = 0;
-
-  // set other defaults
-  dim = 2;
-  radius = 0.005;
-  internal = true;
-  flatskin = false;
-  selfcollide = mjFLEXSELF_AUTO;
-  activelayers = 1;
-  group = 0;
-  edgestiffness = 0;
-  edgedamping = 0;
-  material_.clear();
-  rgba[0] = rgba[1] = rgba[2] = 0.5f;
-  rgba[3] = 1.0f;
-
   // clear internal variables
   nvert = 0;
   nedge = 0;
@@ -2257,50 +2234,84 @@ mjCFlex::mjCFlex(mjCModel* _model) {
   matid = -1;
   rigid = false;
   centered = false;
+
+  PointToLocal();
+  CopyFromSpec();
+}
+
+
+void mjCFlex::PointToLocal() {
+  spec.element = (mjElement)this;
+  spec.name = (mjString)&name;
+  spec.classname = (mjString)&classname;
+  spec.material = (mjString)&spec_material_;
+  spec.vertbody = (mjStringVec)&spec_vertbody_;
+  spec.vert = (mjDoubleVec)&spec_vert_;
+  spec.texcoord = (mjFloatVec)&spec_texcoord_;
+  spec.elem = (mjIntVec)&spec_elem_;
+  spec.info = (mjString)&info;
+}
+
+
+void mjCFlex::CopyFromSpec() {
+  *static_cast<mjmFlex*>(this) = spec;
+  spec.info = (mjString)&info;
+  material_ = spec_material_;
+  vertbody_ = spec_vertbody_;
+  vert_ = spec_vert_;
+  texcoord_ = spec_texcoord_;
+  elem_ = spec_elem_;
+  material = (mjString)&material_;
+  vertbody = (mjStringVec)&vertbody_;
+  vert = (mjDoubleVec)&vert_;
+  texcoord = (mjFloatVec)&texcoord_;
+  elem = (mjIntVec)&elem_;
 }
 
 
 bool mjCFlex::HasTexcoord() const {
-  return !texcoord.empty();
+  return !texcoord_.empty();
 }
 
 
 void mjCFlex::DelTexcoord() {
-  texcoord.clear();
+  texcoord_.clear();
 }
 
 
 // compiler
 void mjCFlex::Compile(const mjVFS* vfs) {
+  CopyFromSpec();
+
   // set nelem; check sizes
   if (dim<1 || dim>3) {
       throw mjCError(this, "dim must be 1, 2 or 3");
   }
-  if (elem.empty()) {
+  if (elem_.empty()) {
       throw mjCError(this, "elem is empty");
   }
-  if (elem.size() % (dim+1)) {
+  if (elem_.size() % (dim+1)) {
       throw mjCError(this, "elem size must be multiple of (dim+1)");
   }
-  if (vertbody.empty()) {
+  if (vertbody_.empty()) {
       throw mjCError(this, "vertbody is empty");
   }
-  if (vert.size() % 3) {
+  if (vert_.size() % 3) {
       throw mjCError(this, "vert size must be a multiple of 3");
   }
   if (edgestiffness>0 && dim>1) {
     throw mjCError(this, "edge stiffness only available for dim=1, please use elasticity plugins");
   }
-  nelem = (int)elem.size()/(dim+1);
+  nelem = (int)elem_.size()/(dim+1);
 
   // set nvert, rigid, centered; check size
-  if (vert.empty()) {
+  if (vert_.empty()) {
     centered = true;
-    nvert = (int)vertbody.size();
+    nvert = (int)vertbody_.size();
   }
   else {
-    nvert = (int)vert.size()/3;
-    if (vertbody.size()==1) {
+    nvert = (int)vert_.size()/3;
+    if (vertbody_.size()==1) {
       rigid = true;
     }
   }
@@ -2309,14 +2320,14 @@ void mjCFlex::Compile(const mjVFS* vfs) {
   }
 
   // check elem vertex ids
-  for (int i=0; i<(int)elem.size(); i++) {
-    if (elem[i]<0 || elem[i]>=nvert) {
+  for (int i=0; i<(int)elem_.size(); i++) {
+    if (elem_[i]<0 || elem_[i]>=nvert) {
       throw mjCError(this, "elem vertex id out of range");
     }
   }
 
   // check texcoord
-  if (!texcoord.empty() && texcoord.size()!=2*nvert) {
+  if (!texcoord_.empty() && texcoord_.size()!=2*nvert) {
     throw mjCError(this, "two texture coordinates per vertex expected");
   }
 
@@ -2329,20 +2340,20 @@ void mjCFlex::Compile(const mjVFS* vfs) {
   }
 
   // resolve body ids
-  for (int i=0; i<(int)vertbody.size(); i++) {
-    mjCBase* pbody = model->FindObject(mjOBJ_BODY, vertbody[i]);
+  for (int i=0; i<(int)vertbody_.size(); i++) {
+    mjCBase* pbody = model->FindObject(mjOBJ_BODY, vertbody_[i]);
     if (pbody) {
       vertbodyid.push_back(pbody->id);
     } else {
-        throw mjCError(this, "unkown body '%s' in flex", vertbody[i].c_str());
+        throw mjCError(this, "unkown body '%s' in flex", vertbody_[i].c_str());
     }
   }
 
   // process elements
-  for (int e=0; e<(int)elem.size()/(dim+1); e++) {
+  for (int e=0; e<(int)elem_.size()/(dim+1); e++) {
     // make sorted copy of element
     vector<int> el;
-    el.assign(elem.begin()+e*(dim+1), elem.begin()+(e+1)*(dim+1));
+    el.assign(elem_.begin()+e*(dim+1), elem_.begin()+(e+1)*(dim+1));
     std::sort(el.begin(), el.end());
 
     // check for repeated vertices
@@ -2367,8 +2378,8 @@ void mjCFlex::Compile(const mjVFS* vfs) {
   // determine centered if not already set
   if (!centered) {
     centered = true;
-    for (int i=0; i<(int)vert.size(); i++) {
-      if (vert[i]!=0) {
+    for (int i=0; i<(int)vert_.size(); i++) {
+      if (vert_[i]!=0) {
         centered = false;
         break;
       }
@@ -2385,7 +2396,7 @@ void mjCFlex::Compile(const mjVFS* vfs) {
     // add vertex offset within body if not centered
     if (!centered) {
       mjtNum offset[3];
-      mju_rotVecQuat(offset, vert.data()+3*i, model->bodies[b]->xquat0);
+      mju_rotVecQuat(offset, vert_.data()+3*i, model->bodies[b]->xquat0);
       mju_addTo3(vertxpos.data()+3*i, offset);
     }
   }
@@ -2394,7 +2405,7 @@ void mjCFlex::Compile(const mjVFS* vfs) {
   // faces are (0,1,2); (0,2,3); (0,3,1); (1,3,2)
   if (dim==3) {
     for (int e=0; e<nelem; e++) {
-      const int* edata = elem.data() + e*(dim+1);
+      const int* edata = elem_.data() + e*(dim+1);
       mjtNum* v0 = vertxpos.data() + 3*edata[0];
       mjtNum* v1 = vertxpos.data() + 3*edata[1];
       mjtNum* v2 = vertxpos.data() + 3*edata[2];
@@ -2408,22 +2419,22 @@ void mjCFlex::Compile(const mjVFS* vfs) {
       mju_cross(nrm, v01, v02);
       if (mju_dot3(nrm, v03)>0) {
         // flip orientation
-        int tmp = elem[e*(dim+1)+1];
-        elem[e*(dim+1)+1] = elem[e*(dim+1)+2];
-        elem[e*(dim+1)+2] = tmp;
+        int tmp = elem_[e*(dim+1)+1];
+        elem_[e*(dim+1)+1] = elem_[e*(dim+1)+2];
+        elem_[e*(dim+1)+2] = tmp;
       }
     }
   }
 
   // create edges
-  std::vector<int> edgeidx(elem.size()*kNumEdges[dim-1]);
+  std::vector<int> edgeidx(elem_.size()*kNumEdges[dim-1]);
 
   // map from edge vertices to their index in `edges` vector
   std::unordered_map<std::pair<int, int>, int, PairHash> edge_indices;
 
   // insert local edges into global vector
-  for (int f = 0; f < (int)elem.size()/(dim+1); f++) {
-    int* v = elem.data() + f*(dim+1);
+  for (int f = 0; f < (int)elem_.size()/(dim+1); f++) {
+    int* v = elem_.data() + f*(dim+1);
     for (int e = 0; e < kNumEdges[dim-1]; e++) {
       auto pair = std::pair(
         std::min(v[eledge[dim-1][e][0]], v[eledge[dim-1][e][1]]),
@@ -2447,7 +2458,7 @@ void mjCFlex::Compile(const mjVFS* vfs) {
 
   // add plugins
   std::string userface, useredge;
-  mjXUtil::Vector2String(userface, elem);
+  mjXUtil::Vector2String(userface, elem_);
   mjXUtil::Vector2String(useredge, edgeidx);
 
   for (int i=0; i<(int)vertbodyid.size(); i++) {
@@ -2472,12 +2483,12 @@ void mjCFlex::CreateBVH(void) {
   int nbvh = 0;
 
   // allocate element bounding boxes
-  elemaabb.resize(6*nelem);
+  elemaabb_.resize(6*nelem);
   tree.AllocateBoundingVolumes(nelem);
 
   // construct element bounding boxes, add to hierarchy
   for (int e=0; e<nelem; e++) {
-    const int* edata = elem.data() + e*(dim+1);
+    const int* edata = elem_.data() + e*(dim+1);
 
     // skip inactive in 3D
     if (dim==3 && elemlayer[e]>=activelayers) {
@@ -2496,12 +2507,12 @@ void mjCFlex::CreateBVH(void) {
     }
 
     // compute aabb (center, size)
-    elemaabb[6*e+0] = 0.5*(xmax[0]+xmin[0]);
-    elemaabb[6*e+1] = 0.5*(xmax[1]+xmin[1]);
-    elemaabb[6*e+2] = 0.5*(xmax[2]+xmin[2]);
-    elemaabb[6*e+3] = 0.5*(xmax[0]-xmin[0]) + radius;
-    elemaabb[6*e+4] = 0.5*(xmax[1]-xmin[1]) + radius;
-    elemaabb[6*e+5] = 0.5*(xmax[2]-xmin[2]) + radius;
+    elemaabb_[6*e+0] = 0.5*(xmax[0]+xmin[0]);
+    elemaabb_[6*e+1] = 0.5*(xmax[1]+xmin[1]);
+    elemaabb_[6*e+2] = 0.5*(xmax[2]+xmin[2]);
+    elemaabb_[6*e+3] = 0.5*(xmax[0]-xmin[0]) + radius;
+    elemaabb_[6*e+4] = 0.5*(xmax[1]-xmin[1]) + radius;
+    elemaabb_[6*e+5] = 0.5*(xmax[2]-xmin[2]) + radius;
 
     // add bounding volume for this element
     mjCBoundingVolume* bv = tree.GetBoundingVolume(nbvh++);
@@ -2509,7 +2520,7 @@ void mjCFlex::CreateBVH(void) {
     bv->conaffinity = conaffinity;
     bv->quat = NULL;
     bv->SetId(e);
-    bv->aabb = elemaabb.data() + 6*e;
+    bv->aabb = elemaabb_.data() + 6*e;
     bv->pos = bv->aabb;
   }
 
@@ -2533,7 +2544,7 @@ void mjCFlex::CreateShellPair(void) {
 
     // element vertices in original (unsorted) order
     vector<int> el;
-    el.assign(elem.begin()+n, elem.begin()+n+dim+1);
+    el.assign(elem_.begin()+n, elem_.begin()+n+dim+1);
 
     // line: 2 vertex fragments
     if (dim==1) {
@@ -2702,7 +2713,7 @@ void mjCFlex::CreateShellPair(void) {
 
         // process both elements
         for (int ei=0; ei<2; ei++) {
-          const int* edata = elem.data() + connectspec[n][ei]*(dim+1);
+          const int* edata = elem_.data() + connectspec[n][ei]*(dim+1);
 
           // find element vertex that is not in the common fragment
           for (int i=0; i<=dim; i++) {
diff --git a/src/user/user_model.cc b/src/user/user_model.cc
index f85fc3b2..b9cbecaf 100644
--- a/src/user/user_model.cc
+++ b/src/user/user_model.cc
@@ -1995,15 +1995,15 @@ void mjCModel::CopyObjects(mjModel* m) {
       m->flex_evpairnum[i] = (int)pfl->evpair.size()/2;
       memcpy(m->flex_evpair + 2*evpair_adr, pfl->evpair.data(), pfl->evpair.size()*sizeof(int));
     }
-    if (pfl->texcoord.empty()) {
+    if (pfl->texcoord_.empty()) {
       m->flex_texcoordadr[i] = -1;
     } else {
       m->flex_texcoordadr[i] = texcoord_adr;
       memcpy(m->flex_texcoord + 2*texcoord_adr,
-            pfl->texcoord.data(), pfl->texcoord.size()*sizeof(float));
+            pfl->texcoord_.data(), pfl->texcoord_.size()*sizeof(float));
     }
     m->flex_elemnum[i] = pfl->nelem;
-    memcpy(m->flex_elem + elemdata_adr, pfl->elem.data(), pfl->elem.size()*sizeof(int));
+    memcpy(m->flex_elem + elemdata_adr, pfl->elem_.data(), pfl->elem_.size()*sizeof(int));
     memcpy(m->flex_elemlayer + elem_adr, pfl->elemlayer.data(), pfl->nelem*sizeof(int));
     if (m->flex_shellnum[i]) {
       memcpy(m->flex_shell + shelldata_adr, pfl->shell.data(), pfl->shell.size()*sizeof(int));
@@ -2042,7 +2042,7 @@ void mjCModel::CopyObjects(mjModel* m) {
       mju_zero(m->flex_vert + 3*vert_adr, 3*pfl->nvert);
     }
     else {
-      memcpy(m->flex_vert + 3*vert_adr, pfl->vert.data(), 3*pfl->nvert*sizeof(mjtNum));
+      memcpy(m->flex_vert + 3*vert_adr, pfl->vert_.data(), 3*pfl->nvert*sizeof(mjtNum));
     }
 
     // copy or set vertbodyid
@@ -2073,7 +2073,7 @@ void mjCModel::CopyObjects(mjModel* m) {
     elemdata_adr += (pfl->dim+1) * pfl->nelem;
     shelldata_adr += (int)pfl->shell.size();
     evpair_adr += (int)pfl->evpair.size()/2;
-    texcoord_adr += (int)pfl->texcoord.size()/2;
+    texcoord_adr += (int)pfl->texcoord_.size()/2;
     bvh_adr += pfl->tree.nbvh;
   }
 
diff --git a/src/user/user_objects.cc b/src/user/user_objects.cc
index 942e7699..9f0e9d55 100644
--- a/src/user/user_objects.cc
+++ b/src/user/user_objects.cc
@@ -596,7 +596,7 @@ void mjCBody::PointToLocal() {
   spec.element = (mjElement)this;
   spec.name = (mjString)&name;
   spec.classname = (mjString)&classname;
-  spec.userdata = (mjDouble)&spec_userdata_;
+  spec.userdata = (mjDoubleVec)&spec_userdata_;
   spec.plugin.name = (mjString)&plugin_name;
   spec.plugin.instance_name = (mjString)&plugin_instance_name;
 }
@@ -605,7 +605,7 @@ void mjCBody::PointToLocal() {
 void mjCBody::CopyFromSpec() {
   *static_cast<mjmBody*>(this) = spec;
   userdata_ = spec_userdata_;
-  userdata = (mjDouble)&userdata_;
+  userdata = (mjDoubleVec)&userdata_;
   spec.info = (mjString)&info;
   mju_copy4(alt_.axisangle, alt.axisangle);
   mju_copy(alt_.xyaxes, alt.xyaxes, 6);
@@ -1262,7 +1262,7 @@ void mjCJoint::PointToLocal() {
   spec.element = (mjElement)this;
   spec.name = (mjString)&name;
   spec.classname = (mjString)&classname;
-  spec.userdata = (mjDouble)&spec_userdata_;
+  spec.userdata = (mjDoubleVec)&spec_userdata_;
   spec.info = (mjString)&info;
 }
 
@@ -1271,7 +1271,7 @@ void mjCJoint::PointToLocal() {
 void mjCJoint::CopyFromSpec() {
   *static_cast<mjmJoint*>(this) = spec;
   userdata_ = spec_userdata_;
-  userdata = (mjDouble)&spec_userdata_;
+  userdata = (mjDoubleVec)&spec_userdata_;
 }
 
 
@@ -1460,7 +1460,7 @@ void mjCGeom::PointToLocal(void) {
   spec.name = (mjString)&name;
   spec.info = (mjString)&info;
   spec.classname = (mjString)&classname;
-  spec.userdata = (mjDouble)&spec_userdata_;
+  spec.userdata = (mjDoubleVec)&spec_userdata_;
   spec.material = (mjString)&spec_material_;
   spec.meshname = (mjString)&spec_meshname_;
   spec.hfieldname = (mjString)&spec_hfieldname_;
@@ -1476,7 +1476,7 @@ void mjCGeom::CopyFromSpec() {
   hfieldname_ = spec_hfieldname_;
   meshname_ = spec_meshname_;
   material_ = spec_material_;
-  userdata = (mjDouble)&userdata_;
+  userdata = (mjDoubleVec)&userdata_;
   hfieldname = (mjString)&hfieldname_;
   meshname = (mjString)&meshname_;
   material = (mjString)&material_;
@@ -2061,7 +2061,7 @@ void mjCSite::PointToLocal() {
   spec.info = (mjString)&info;
   spec.classname = (mjString)&classname;
   spec.material = (mjString)&spec_material_;
-  spec.userdata = (mjDouble)&spec_userdata_;
+  spec.userdata = (mjDoubleVec)&spec_userdata_;
 }
 
 
@@ -2070,7 +2070,7 @@ void mjCSite::CopyFromSpec() {
   *static_cast<mjmSite*>(this) = spec;
   userdata_ = spec_userdata_;
   material_ = spec_material_;
-  userdata = (mjDouble)&userdata_;
+  userdata = (mjDoubleVec)&userdata_;
   material = (mjString)&material_;
   mju_copy4(alt_.axisangle, alt.axisangle);
   mju_copy(alt_.xyaxes, alt.xyaxes, 6);
@@ -2201,7 +2201,7 @@ void mjCCamera::PointToLocal() {
   spec.element = (mjElement)this;
   spec.name = (mjString)&name;
   spec.classname = (mjString)&classname;
-  spec.userdata = (mjDouble)&spec_userdata_;
+  spec.userdata = (mjDoubleVec)&spec_userdata_;
   spec.targetbody = (mjString)&spec_targetbody_;
   spec.info = (mjString)&info;
 }
@@ -2212,7 +2212,7 @@ void mjCCamera::CopyFromSpec() {
   *static_cast<mjmCamera*>(this) = spec;
   userdata_ = spec_userdata_;
   targetbody_ = spec_targetbody_;
-  userdata = (mjDouble)&userdata_;
+  userdata = (mjDoubleVec)&userdata_;
   targetbody = (mjString)&targetbody_;
   mju_copy4(alt_.axisangle, alt.axisangle);
   mju_copy(alt_.xyaxes, alt.xyaxes, 6);
@@ -3717,7 +3717,7 @@ void mjCTendon::PointToLocal() {
   spec.name = (mjString)&name;
   spec.classname = (mjString)&classname;
   spec.material = (mjString)&spec_material_;
-  spec.userdata = (mjDouble)&spec_userdata_;
+  spec.userdata = (mjDoubleVec)&spec_userdata_;
   spec.info = (mjString)&info;
 }
 
@@ -3728,7 +3728,7 @@ void mjCTendon::CopyFromSpec() {
   material_ = spec_material_;
   userdata_ = spec_userdata_;
   material = (mjString)&material_;
-  userdata = (mjDouble)&userdata_;
+  userdata = (mjDoubleVec)&userdata_;
 }
 
 
@@ -4097,7 +4097,7 @@ void mjCActuator::MakePointerLocal() {
   spec.element = (mjElement)this;
   spec.name = (mjString)&name;
   spec.classname = (mjString)&classname;
-  spec.userdata = (mjDouble)&spec_userdata_;
+  spec.userdata = (mjDoubleVec)&spec_userdata_;
   spec.target = (mjString)&spec_target_;
   spec.refsite = (mjString)&spec_refsite_;
   spec.slidersite = (mjString)&spec_slidersite_;
@@ -4114,7 +4114,7 @@ void mjCActuator::CopyFromSpec() {
   target_ = spec_target_;
   refsite_ = spec_refsite_;
   slidersite_ = spec_slidersite_;
-  userdata = (mjDouble)&userdata_;
+  userdata = (mjDoubleVec)&userdata_;
   target = (mjString)&target_;
   refsite = (mjString)&refsite_;
   slidersite = (mjString)&slidersite_;
@@ -4359,7 +4359,7 @@ void mjCSensor::MakePointerLocal() {
   spec.element = (mjElement)this;
   spec.name = (mjString)&name;
   spec.classname = (mjString)&classname;
-  spec.userdata = (mjDouble)&spec_userdata_;
+  spec.userdata = (mjDoubleVec)&spec_userdata_;
   spec.objname = (mjString)&spec_objname_;
   spec.refname = (mjString)&spec_refname_;
   spec.plugin.name = (mjString)&plugin_name;
@@ -4374,7 +4374,7 @@ void mjCSensor::CopyFromSpec() {
   userdata_ = spec_userdata_;
   objname_ = spec_objname_;
   refname_ = spec_refname_;
-  userdata = (mjDouble)&userdata_;
+  userdata = (mjDoubleVec)&userdata_;
   objname = (mjString)&objname_;
   refname = (mjString)&refname_;
   plugin.active = spec.plugin.active;
diff --git a/src/user/user_objects.h b/src/user/user_objects.h
index d6ca7b7a..5265842e 100644
--- a/src/user/user_objects.h
+++ b/src/user/user_objects.h
@@ -566,7 +566,7 @@ class mjCLight : public mjCBase, private mjmLight {
 //------------------------- class mjCFlex ----------------------------------------------------------
 // Describes a flex
 
-class mjCFlex: public mjCBase {
+class mjCFlex: public mjCBase, private mjmFlex {
   friend class mjCDef;
   friend class mjCModel;
   friend class mjCFlexcomp;
@@ -574,41 +574,24 @@ class mjCFlex: public mjCBase {
   friend class mjXWriter;
 
  public:
-  void set_material(std::string _material) { material_ = _material; }
-  std::string& get_material() { return material_; }
+  mjmFlex spec;
+  using mjCBase::name;
+  using mjCBase::classname;
+  using mjCBase::info;
 
-  // contact properties
-  int contype;                    // contact type
-  int conaffinity;                // contact affinity
-  int condim;                     // contact dimensionality
-  int priority;                   // contact priority
-  double friction[3];             // one-sided friction coefficients: slide, roll, spin
-  double solmix;                  // solver mixing for contact pairs
-  mjtNum solref[mjNREF];          // solver reference
-  mjtNum solimp[mjNIMP];          // solver impedance
-  double margin;                  // margin for contact detection
-  double gap;                     // include in solver if dist<margin-gap
+  void CopyFromSpec(void);
+  void PointToLocal(void);
 
-  // other properties
-  int dim;                        // element dimensionality
-  double radius;                  // radius around primitive element
-  bool internal;                  // enable internal collisions
-  bool flatskin;                  // render flex skin with flat shading
-  int selfcollide;                // mode for flex self colllision
-  int activelayers;               // number of active element layers in 3D
-  int group;                      // group for visualizatioh
-  double edgestiffness;           // edge stiffness
-  double edgedamping;             // edge damping
-  float rgba[4];                  // rgba when material is omitted
+  // used by mjXWriter and mjCModel
+  const std::string& get_material() { return material_; }
+  const std::vector<std::string>& get_vertbody() { return vertbody_; }
+  const std::vector<double>& get_vert() { return vert_; }
+  const std::vector<double>& get_elemaabb() { return elemaabb_; }
+  const std::vector<int>& get_elem() { return elem_; }
+  const std::vector<float>& get_texcoord() { return texcoord_; }
 
-  std::vector<std::string> vertbody;  // vertex body names
-  std::vector<mjtNum> vert;           // vertex positions
-  std::vector<mjtNum> elemaabb;       // element bounding volume
-  std::vector<int> elem;              // element vertex ids
-  std::vector<float> texcoord;        // vertex texture coordinates
-
-  bool HasTexcoord() const;           // texcoord not null
-  void DelTexcoord();                 // delete texcoord
+  bool HasTexcoord() const;               // texcoord not null
+  void DelTexcoord();                     // delete texcoord
 
  private:
   mjCFlex(mjCModel* = 0);
@@ -629,7 +612,20 @@ class mjCFlex: public mjCBase {
   std::vector<int> evpair;                // element-vertex pairs
   std::vector<mjtNum> vertxpos;           // global vertex positions
   mjCBoundingVolumeHierarchy tree;        // bounding volume hierarchy
+  std::vector<mjtNum> elemaabb_;          // element bounding volume
+
+  // variable-size data
+  std::vector<std::string> vertbody_;     // vertex body names
+  std::vector<mjtNum> vert_;              // vertex positions
+  std::vector<int> elem_;                 // element vertex ids
+  std::vector<float> texcoord_;           // vertex texture coordinates
   std::string material_;                  // name of material used for rendering
+
+  std::string spec_material_;
+  std::vector<std::string> spec_vertbody_;
+  std::vector<mjtNum> spec_vert_;
+  std::vector<int> spec_elem_;
+  std::vector<float> spec_texcoord_;
 };
 
 
diff --git a/src/xml/xml_native_reader.cc b/src/xml/xml_native_reader.cc
index 7b5f7476..ab8e5231 100644
--- a/src/xml/xml_native_reader.cc
+++ b/src/xml/xml_native_reader.cc
@@ -1275,14 +1275,22 @@ void mjXReader::Statistic(XMLElement* section) {
 //---------------------------------- one-element parsers -------------------------------------------
 
 // flex element parser
-void mjXReader::OneFlex(XMLElement* elem, mjCFlex* pflex) {
-  string text;
+void mjXReader::OneFlex(XMLElement* elem, mjmFlex* pflex) {
+  string text, name, classname, material;
   int n;
 
   // read attributes
-  ReadAttrTxt(elem, "name", pflex->name);
+  if (ReadAttrTxt(elem, "name", name)) {
+    mjm_setString(pflex->name, name.c_str());
+  }
+  if (ReadAttrTxt(elem, "classname", classname)) {
+    mjm_setString(pflex->classname, classname.c_str());
+  }
+  if (ReadAttrTxt(elem, "material", material)) {
+    mjm_setString(pflex->material, material.c_str());
+  }
+
   ReadAttr(elem, "radius", 1, &pflex->radius, text);
-  ReadAttrTxt(elem, "material", pflex->get_material());
   ReadAttr(elem, "rgba", 4, pflex->rgba, text);
   if (MapValue(elem, "flatskin", &n, bool_map, 2)) {
     pflex->flatskin = (n==1);
@@ -1292,16 +1300,22 @@ void mjXReader::OneFlex(XMLElement* elem, mjCFlex* pflex) {
 
   // read data vectors
   if (ReadAttrTxt(elem, "body", text, true)) {
-    String2Vector(text, pflex->vertbody);
+    mjm_setStringVec(pflex->vertbody, text.c_str());
   }
   if (ReadAttrTxt(elem, "vertex", text)) {
-    String2Vector(text, pflex->vert);
+    std::vector<double> vert;
+    String2Vector(text, vert);
+    mjm_setDouble(pflex->vert, vert.data(), vert.size());
   }
   if (ReadAttrTxt(elem, "element", text, true)) {
-    String2Vector(text, pflex->elem);
+    std::vector<int> elem;
+    String2Vector(text, elem);
+    mjm_setInt(pflex->elem, elem.data(), elem.size());
   }
   if (ReadAttrTxt(elem, "texcoord", text)) {
-    String2Vector(text, pflex->texcoord);
+    std::vector<float> texcoord;
+    String2Vector(text, texcoord);
+    mjm_setFloat(pflex->texcoord, texcoord.data(), texcoord.size());
   }
 
   // contact subelement
@@ -1331,7 +1345,9 @@ void mjXReader::OneFlex(XMLElement* elem, mjCFlex* pflex) {
     ReadAttr(edge, "damping", 1, &pflex->edgedamping, text);
   }
 
-  GetXMLPos(elem, pflex);
+  // write error info
+  mjm_setString(pflex->info,
+      std::string("line = " + std::to_string(elem->GetLineNum()) + ", column = -1").c_str());
 }
 
 
@@ -2400,7 +2416,7 @@ void mjXReader::OneComposite(XMLElement* elem, mjmBody* pbody, mjCDef* def) {
 
 // make flexcomp
 void mjXReader::OneFlexcomp(XMLElement* elem, mjmBody* pbody) {
-  string text;
+  string text, material;
   int n;
 
   // create out-of-DOM element
@@ -2417,14 +2433,16 @@ void mjXReader::OneFlexcomp(XMLElement* elem, mjmBody* pbody) {
   ReadAttr(elem, "mass", 1, &fcomp.mass, text);
   ReadAttr(elem, "inertiabox", 1, &fcomp.inertiabox, text);
   ReadAttrTxt(elem, "file", fcomp.file);
-  ReadAttrTxt(elem, "material", fcomp.def.flex.get_material());
-  ReadAttr(elem, "rgba", 4, fcomp.def.flex.rgba, text);
-  if (MapValue(elem, "flatskin", &n, bool_map, 2)) {
-    fcomp.def.flex.flatskin = (n==1);
+  if (ReadAttrTxt(elem, "material", material)) {
+    mjm_setString(fcomp.def.flex.spec.material, material.c_str());
   }
-  ReadAttrInt(elem, "dim", &fcomp.def.flex.dim);
-  ReadAttr(elem, "radius", 1, &fcomp.def.flex.radius, text);
-  ReadAttrInt(elem, "group", &fcomp.def.flex.group);
+  ReadAttr(elem, "rgba", 4, fcomp.def.flex.spec.rgba, text);
+  if (MapValue(elem, "flatskin", &n, bool_map, 2)) {
+    fcomp.def.flex.spec.flatskin = (n==1);
+  }
+  ReadAttrInt(elem, "dim", &fcomp.def.flex.spec.dim);
+  ReadAttr(elem, "radius", 1, &fcomp.def.flex.spec.radius, text);
+  ReadAttrInt(elem, "group", &fcomp.def.flex.spec.group);
 
   // pose
   ReadAttr(elem, "pos", 3, fcomp.pos, text);
@@ -2453,28 +2471,28 @@ void mjXReader::OneFlexcomp(XMLElement* elem, mjmBody* pbody) {
     }
     ReadAttr(edge, "solref", mjNREF, fcomp.def.equality.spec.solref, text, false, false);
     ReadAttr(edge, "solimp", mjNIMP, fcomp.def.equality.spec.solimp, text, false, false);
-    ReadAttr(edge, "stiffness", 1, &fcomp.def.flex.edgestiffness, text);
-    ReadAttr(edge, "damping", 1, &fcomp.def.flex.edgedamping, text);
+    ReadAttr(edge, "stiffness", 1, &fcomp.def.flex.spec.edgestiffness, text);
+    ReadAttr(edge, "damping", 1, &fcomp.def.flex.spec.edgedamping, text);
   }
 
   // contact
   XMLElement* cont = FirstChildElement(elem, "contact");
   if (cont) {
-    ReadAttrInt(cont, "contype", &fcomp.def.flex.contype);
-    ReadAttrInt(cont, "conaffinity", &fcomp.def.flex.conaffinity);
-    ReadAttrInt(cont, "condim", &fcomp.def.flex.condim);
-    ReadAttrInt(cont, "priority", &fcomp.def.flex.priority);
-    ReadAttr(cont, "friction", 3, fcomp.def.flex.friction, text, false, false);
-    ReadAttr(cont, "solmix", 1, &fcomp.def.flex.solmix, text);
-    ReadAttr(cont, "solref", mjNREF, fcomp.def.flex.solref, text, false, false);
-    ReadAttr(cont, "solimp", mjNIMP, fcomp.def.flex.solimp, text, false, false);
-    ReadAttr(cont, "margin", 1, &fcomp.def.flex.margin, text);
-    ReadAttr(cont, "gap", 1, &fcomp.def.flex.gap, text);
+    ReadAttrInt(cont, "contype", &fcomp.def.flex.spec.contype);
+    ReadAttrInt(cont, "conaffinity", &fcomp.def.flex.spec.conaffinity);
+    ReadAttrInt(cont, "condim", &fcomp.def.flex.spec.condim);
+    ReadAttrInt(cont, "priority", &fcomp.def.flex.spec.priority);
+    ReadAttr(cont, "friction", 3, fcomp.def.flex.spec.friction, text, false, false);
+    ReadAttr(cont, "solmix", 1, &fcomp.def.flex.spec.solmix, text);
+    ReadAttr(cont, "solref", mjNREF, fcomp.def.flex.spec.solref, text, false, false);
+    ReadAttr(cont, "solimp", mjNIMP, fcomp.def.flex.spec.solimp, text, false, false);
+    ReadAttr(cont, "margin", 1, &fcomp.def.flex.spec.margin, text);
+    ReadAttr(cont, "gap", 1, &fcomp.def.flex.spec.gap, text);
     if (MapValue(cont, "internal", &n, bool_map, 2)) {
-      fcomp.def.flex.internal = (n==1);
+      fcomp.def.flex.spec.internal = (n==1);
     }
-    MapValue(cont, "selfcollide", &fcomp.def.flex.selfcollide, flexself_map, 5);
-    ReadAttrInt(cont, "activelayers", &fcomp.def.flex.activelayers);
+    MapValue(cont, "selfcollide", &fcomp.def.flex.spec.selfcollide, flexself_map, 5);
+    ReadAttrInt(cont, "activelayers", &fcomp.def.flex.spec.activelayers);
   }
 
   // pin
@@ -2639,6 +2657,7 @@ void mjXReader::Default(XMLElement* section, int parentid) {
     mjm_finalize(def->material.spec.element);
     mjm_finalize(def->equality.spec.element);
     mjm_finalize(def->tendon.spec.element);
+    mjm_finalize(def->flex.spec.element);
 
     // advance
     elem = NextSiblingElement(elem);
@@ -3385,7 +3404,7 @@ void mjXReader::Deformable(XMLElement* section) {
     // flex sub-element
     if (name=="flex") {
       // create flex and parse
-      mjCFlex* pflex = model->AddFlex();
+      mjmFlex* pflex = mjm_addFlex(model);
       OneFlex(elem, pflex);
     }
 
diff --git a/src/xml/xml_native_reader.h b/src/xml/xml_native_reader.h
index c20564e7..c3b85317 100644
--- a/src/xml/xml_native_reader.h
+++ b/src/xml/xml_native_reader.h
@@ -53,7 +53,7 @@ class mjXReader : public mjXBase {
   void Keyframe(tinyxml2::XMLElement* section);                        // keyframe section
 
   // single element parsers, used in defaults and main body
-  void OneFlex(tinyxml2::XMLElement* elem, mjCFlex* pflex);
+  void OneFlex(tinyxml2::XMLElement* elem, mjmFlex* pflex);
   void OneMesh(tinyxml2::XMLElement* elem, mjCMesh* pmesh);
   void OneSkin(tinyxml2::XMLElement* elem, mjCSkin* pskin);
   void OneMaterial(tinyxml2::XMLElement* elem, mjmMaterial* pmaterial);
diff --git a/src/xml/xml_native_writer.cc b/src/xml/xml_native_writer.cc
index e5b6099b..03f56732 100644
--- a/src/xml/xml_native_writer.cc
+++ b/src/xml/xml_native_writer.cc
@@ -100,20 +100,20 @@ void mjXWriter::OneFlex(XMLElement* elem, mjCFlex* pflex) {
   WriteAttrInt(elem, "group", pflex->group, defflex.group);
 
   // data vectors
-  if (!pflex->vertbody.empty()) {
-    Vector2String(text, pflex->vertbody);
+  if (!pflex->get_vertbody().empty()) {
+    Vector2String(text, pflex->get_vertbody());
     WriteAttrTxt(elem, "body", text);
   }
-  if (!pflex->vert.empty()) {
-    Vector2String(text, pflex->vert);
+  if (!pflex->get_vert().empty()) {
+    Vector2String(text, pflex->get_vert());
     WriteAttrTxt(elem, "vertex", text);
   }
-  if (!pflex->elem.empty()) {
-    Vector2String(text, pflex->elem);
+  if (!pflex->get_elem().empty()) {
+    Vector2String(text, pflex->get_elem());
     WriteAttrTxt(elem, "element", text);
   }
-  if (!pflex->texcoord.empty()) {
-    Vector2String(text, pflex->texcoord);
+  if (!pflex->get_texcoord().empty()) {
+    Vector2String(text, pflex->get_texcoord());
     WriteAttrTxt(elem, "texcoord", text);
   }