Replace mjCModel with mjmModel in mjXBase.

PiperOrigin-RevId: 611762880
Change-Id: I22254689b6cbf10fe7b61f9742b6ffd6c2074477

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