Return null pointer from mjs_firstChild and mjs_firstElement when the requested list is empty.

Fixes #1824.

PiperOrigin-RevId: 653758711
Change-Id: I6ede762cafd88dcbb0dfc9c2999b7aee24bd2e2a

src/user/user_model.cc

@@ -671,6 +671,13 @@ mjCBase* mjCModel::GetObject(mjtObj type, int id) {
 
 template <class T>
 static mjsElement* GetNext(std::vector<T*>& list, mjsElement* child) {
+  if (!child) {
+    if (list.empty()) {
+      return nullptr;
+    }
+    return list[0]->spec.element;
+  }
+
   // TODO: use id for direct indexing instead of a loop
   for (unsigned int i = 0; i < list.size()-1; i++) {
     if (list[i]->spec.element == child) {
@@ -696,37 +703,37 @@ mjsElement* mjCModel::NextObject(mjsElement* object, mjtObj type) {
 
   switch (type) {
     case mjOBJ_ACTUATOR:
-      return object ? GetNext(actuators_, object) : actuators_[0];
+      return GetNext(actuators_, object);
     case mjOBJ_SENSOR:
-      return object ? GetNext(sensors_, object) : sensors_[0];
+      return GetNext(sensors_, object);
     case mjOBJ_FLEX:
-      return object ? GetNext(flexes_, object) : flexes_[0];
+      return GetNext(flexes_, object);
     case mjOBJ_PAIR:
-      return object ? GetNext(pairs_, object) : pairs_[0];
+      return GetNext(pairs_, object);
     case mjOBJ_EXCLUDE:
-      return object ? GetNext(excludes_, object) : excludes_[0];
+      return GetNext(excludes_, object);
     case mjOBJ_EQUALITY:
-      return object ? GetNext(equalities_, object) : equalities_[0];
+      return GetNext(equalities_, object);
     case mjOBJ_TENDON:
-      return object ? GetNext(tendons_, object) : tendons_[0];
+      return GetNext(tendons_, object);
     case mjOBJ_NUMERIC:
-      return object ? GetNext(numerics_, object) : numerics_[0];
+      return GetNext(numerics_, object);
     case mjOBJ_TEXT:
-      return object ? GetNext(texts_, object) : texts_[0];
+      return GetNext(texts_, object);
     case mjOBJ_TUPLE:
-      return object ? GetNext(tuples_, object) : tuples_[0];
+      return GetNext(tuples_, object);
     case mjOBJ_KEY:
-      return object ? GetNext(keys_, object) : keys_[0];
+      return GetNext(keys_, object);
     case mjOBJ_MESH:
-      return object ? GetNext(meshes_, object) : meshes_[0];
+      return GetNext(meshes_, object);
     case mjOBJ_HFIELD:
-      return object ? GetNext(hfields_, object) : hfields_[0];
+      return GetNext(hfields_, object);
     case mjOBJ_SKIN:
-      return object ? GetNext(skins_, object) : skins_[0];
+      return GetNext(skins_, object);
     case mjOBJ_TEXTURE:
-      return object ? GetNext(textures_, object) : textures_[0];
+      return GetNext(textures_, object);
     case mjOBJ_MATERIAL:
-      return object ? GetNext(materials_, object) : materials_[0];
+      return GetNext(materials_, object);
     default:
       return nullptr;
   }

src/user/user_objects.cc

@@ -1250,6 +1250,13 @@ mjCBase* mjCBody::FindObject(mjtObj type, std::string _name, bool recursive) {
 
 template <class T>
 static mjsElement* GetNext(std::vector<T*>& list, mjsElement* child) {
+  if (!child) {
+    if (list.empty()) {
+      return nullptr;
+    }
+    return list[0]->spec.element;
+  }
+
   for (unsigned int i = 0; i < list.size()-1; i++) {
     if (list[i]->spec.element == child) {
       return list[i+1]->spec.element;
@@ -1275,19 +1282,19 @@ mjsElement* mjCBody::NextChild(mjsElement* child, mjtObj type) {
   switch (type) {
     case mjOBJ_BODY:
     case mjOBJ_XBODY:
-      return child ? GetNext(bodies, child) : bodies[0];
+      return GetNext(bodies, child);
     case mjOBJ_JOINT:
-      return child ? GetNext(joints, child) : joints[0];
+      return GetNext(joints, child);
     case mjOBJ_GEOM:
-      return child ? GetNext(geoms, child) : geoms[0];
+      return GetNext(geoms, child);
     case mjOBJ_SITE:
-      return child ? GetNext(sites, child) : sites[0];
+      return GetNext(sites, child);
     case mjOBJ_CAMERA:
-      return child ? GetNext(cameras, child) : cameras[0];
+      return GetNext(cameras, child);
     case mjOBJ_LIGHT:
-      return child ? GetNext(lights, child) : lights[0];
+      return GetNext(lights, child);
     case mjOBJ_FRAME:
-      return child ? GetNext(frames, child) : frames[0];
+      return GetNext(frames, child);
     default:
       return nullptr;
   }

test/user/user_api_test.cc

@@ -75,6 +75,8 @@ TEST_F(MujocoTest, TreeTraversal) {
   mjsSite* site3 = mjs_addSite(body, 0);
   mjsGeom* geom3 = mjs_addGeom(body, 0);
 
+  mjsElement* a_el1 = mjs_firstElement(spec, mjOBJ_ACTUATOR);
+  mjsElement* c_el1 = mjs_firstChild(body, mjOBJ_CAMERA);
   mjsElement* t_el1 = mjs_firstChild(body, mjOBJ_TENDON);
   mjsElement* s_el1 = mjs_firstChild(body, mjOBJ_SITE);
   mjsElement* s_el2 = mjs_nextChild(body, s_el1);
@@ -85,6 +87,8 @@ TEST_F(MujocoTest, TreeTraversal) {
   mjsElement* g_el3 = mjs_nextChild(body, g_el2);
   mjsElement* g_el4 = mjs_nextChild(body, g_el3);
 
+  EXPECT_EQ(a_el1, nullptr);
+  EXPECT_EQ(c_el1, nullptr);
   EXPECT_EQ(t_el1, nullptr);
   EXPECT_EQ(s_el1, site1->element);
   EXPECT_EQ(s_el2, site2->element);