Add support for printing to stdout in mj_saveLastXML.

PiperOrigin-RevId: 546849864
Change-Id: I799ea3fcd723cf036f82ef5c1ef8152c314072dc

src/xml/xml_base.h

@@ -15,6 +15,7 @@
 #ifndef MUJOCO_SRC_XML_XML_BASE_H_
 #define MUJOCO_SRC_XML_XML_BASE_H_
 
+#include <cstdlib>
 #include <string>
 
 #include "tinyxml2.h"
@@ -82,7 +83,9 @@ class mjXBase : public mjXUtil {
   virtual void Parse(tinyxml2::XMLElement* root) {};
 
   // write: implemented in derived writer class
-  virtual void Write(FILE* fp) {};
+  virtual std::string Write(char *error, std::size_t error_sz) {
+    return "";
+  };
 
   // set the model allocated externally
   void SetModel(mjCModel*);