diff --git a/src/user/user_objects.h b/src/user/user_objects.h
index fc8cf459..b9977f7f 100644
--- a/src/user/user_objects.h
+++ b/src/user/user_objects.h
@@ -388,8 +388,8 @@ class mjCBody : public mjCBody_, private mjsBody {
   friend class mjXURDF;
 
  public:
-  mjCBody(mjCModel*);  // constructor
-  ~mjCBody();          // destructor
+  explicit mjCBody(mjCModel*);
+  ~mjCBody();
 
   // API for adding objects to body
   mjCBody*    AddBody(mjCDef* = 0);
@@ -571,7 +571,7 @@ class mjCJoint : public mjCJoint_, private mjsJoint {
   friend class mjXURDF;
 
  public:
-  mjCJoint(mjCModel* = nullptr, mjCDef* = nullptr);
+  explicit mjCJoint(mjCModel* = nullptr, mjCDef* = nullptr);
   mjCJoint(const mjCJoint& other);
   mjCJoint& operator=(const mjCJoint& other);
 
@@ -651,7 +651,7 @@ class mjCGeom : public mjCGeom_, private mjsGeom {
   friend class mjXURDF;
 
  public:
-  mjCGeom(mjCModel* = nullptr, mjCDef* = nullptr);
+  explicit mjCGeom(mjCModel* = nullptr, mjCDef* = nullptr);
   mjCGeom(const mjCGeom& other);
   mjCGeom& operator=(const mjCGeom& other);
 
@@ -716,7 +716,7 @@ class mjCSite : public mjCSite_, private mjsSite {
   friend class mjXURDF;
 
  public:
-  mjCSite(mjCModel* = nullptr, mjCDef* = nullptr);
+  explicit mjCSite(mjCModel* = nullptr, mjCDef* = nullptr);
   mjCSite(const mjCSite& other);
   mjCSite& operator=(const mjCSite& other);
 
@@ -766,7 +766,7 @@ class mjCCamera : public mjCCamera_, private mjsCamera {
   friend class mjXWriter;
 
  public:
-  mjCCamera(mjCModel* = nullptr, mjCDef* = nullptr);
+  explicit mjCCamera(mjCModel* = nullptr, mjCDef* = nullptr);
   mjCCamera(const mjCCamera& other);
   mjCCamera& operator=(const mjCCamera& other);
 
@@ -812,7 +812,7 @@ class mjCLight : public mjCLight_, private mjsLight {
   friend class mjXWriter;
 
  public:
-  mjCLight(mjCModel* = nullptr, mjCDef* = nullptr);
+  explicit mjCLight(mjCModel* = nullptr, mjCDef* = nullptr);
   mjCLight(const mjCLight& other);
   mjCLight& operator=(const mjCLight& other);
 
@@ -897,7 +897,7 @@ class mjCFlex: public mjCFlex_, private mjsFlex {
   friend class mjXWriter;
 
  public:
-  mjCFlex(mjCModel* = nullptr);
+  explicit mjCFlex(mjCModel* = nullptr);
   mjCFlex(const mjCFlex& other);
   mjCFlex& operator=(const mjCFlex& other);
 
@@ -997,7 +997,7 @@ class mjCMesh: public mjCMesh_, private mjsMesh {
   friend class mjCModel;
 
  public:
-  mjCMesh(mjCModel* = nullptr, mjCDef* = nullptr);
+  explicit mjCMesh(mjCModel* = nullptr, mjCDef* = nullptr);
   mjCMesh(const mjCMesh& other);
   mjCMesh& operator=(const mjCMesh& other);
   ~mjCMesh();
@@ -1200,7 +1200,7 @@ class mjCSkin: public mjCSkin_, private mjsSkin {
   friend class mjXWriter;
 
  public:
-  mjCSkin(mjCModel* = nullptr);
+  explicit mjCSkin(mjCModel* = nullptr);
   mjCSkin(const mjCSkin& other);
   mjCSkin& operator=(const mjCSkin& other);
   ~mjCSkin();
@@ -1258,7 +1258,7 @@ class mjCHField : public mjCHField_, private mjsHField {
   friend class mjXWriter;
 
  public:
-  mjCHField(mjCModel* model);
+  explicit mjCHField(mjCModel* model);
   mjCHField(const mjCHField& other);
   mjCHField& operator=(const mjCHField& other);
   ~mjCHField();
@@ -1310,7 +1310,7 @@ class mjCTexture : public mjCTexture_, private mjsTexture {
   friend class mjXWriter;
 
  public:
-  mjCTexture(mjCModel*);
+  explicit mjCTexture(mjCModel*);
   mjCTexture(const mjCTexture& other);
   mjCTexture& operator=(const mjCTexture& other);
   ~mjCTexture();
@@ -1371,7 +1371,7 @@ class mjCMaterial : public mjCMaterial_, private mjsMaterial {
   friend class mjXWriter;
 
  public:
-  mjCMaterial(mjCModel* = nullptr, mjCDef* = nullptr);
+  explicit mjCMaterial(mjCModel* = nullptr, mjCDef* = nullptr);
   mjCMaterial(const mjCMaterial& other);
   mjCMaterial& operator=(const mjCMaterial& other);
 
@@ -1411,7 +1411,7 @@ class mjCPair : public mjCPair_, private mjsPair {
   friend class mjXWriter;
 
  public:
-  mjCPair(mjCModel* = nullptr, mjCDef* = nullptr);
+  explicit mjCPair(mjCModel* = nullptr, mjCDef* = nullptr);
   mjCPair(const mjCPair& other);
   mjCPair& operator=(const mjCPair& other);
 
@@ -1460,7 +1460,7 @@ class mjCBodyPair : public mjCBodyPair_, private mjsExclude {
   friend class mjCModel;
 
  public:
-  mjCBodyPair(mjCModel*);
+  explicit mjCBodyPair(mjCModel*);
   mjCBodyPair(const mjCBodyPair& other);
   mjCBodyPair& operator=(const mjCBodyPair& other);
 
@@ -1506,7 +1506,7 @@ class mjCEquality : public mjCEquality_, private mjsEquality {
   friend class mjXWriter;
 
  public:
-  mjCEquality(mjCModel* = 0, mjCDef* = 0);
+  explicit mjCEquality(mjCModel* = 0, mjCDef* = 0);
   mjCEquality(const mjCEquality& other);
   mjCEquality& operator=(const mjCEquality& other);
 
@@ -1545,7 +1545,7 @@ class mjCTendon : public mjCTendon_, private mjsTendon {
   friend class mjXWriter;
 
  public:
-  mjCTendon(mjCModel* = nullptr, mjCDef* = nullptr);
+  explicit mjCTendon(mjCModel* = nullptr, mjCDef* = nullptr);
   mjCTendon(const mjCTendon& other);
   mjCTendon& operator=(const mjCTendon& other);
   ~mjCTendon();
@@ -1641,7 +1641,7 @@ class mjCPlugin : public mjCPlugin_ {
   friend class mjXWriter;
 
  public:
-  mjCPlugin(mjCModel*);
+  explicit mjCPlugin(mjCModel*);
   mjCPlugin(const mjCPlugin& other);
   mjCPlugin& operator=(const mjCPlugin& other);
 
@@ -1689,7 +1689,7 @@ class mjCActuator : public mjCActuator_, private mjsActuator {
   friend class mjXWriter;
 
  public:
-  mjCActuator(mjCModel* = nullptr, mjCDef* = nullptr);
+  explicit mjCActuator(mjCModel* = nullptr, mjCDef* = nullptr);
   mjCActuator(const mjCActuator& other);
   mjCActuator& operator=(const mjCActuator& other);
 
@@ -1750,7 +1750,7 @@ class mjCSensor : public mjCSensor_, private mjsSensor {
   friend class mjXWriter;
 
  public:
-  mjCSensor(mjCModel*);
+  explicit mjCSensor(mjCModel*);
   mjCSensor(const mjCSensor& other);
   mjCSensor& operator=(const mjCSensor& other);
 
@@ -1791,7 +1791,7 @@ class mjCNumeric : public mjCNumeric_, private mjsNumeric {
   friend class mjXWriter;
 
  public:
-  mjCNumeric(mjCModel*);
+  explicit mjCNumeric(mjCModel*);
   mjCNumeric(const mjCNumeric& other);
   mjCNumeric& operator=(const mjCNumeric& other);
   ~mjCNumeric();
@@ -1823,7 +1823,7 @@ class mjCText : public mjCText_, private mjsText {
   friend class mjXWriter;
 
  public:
-  mjCText(mjCModel*);
+  explicit mjCText(mjCModel*);
   mjCText(const mjCText& other);
   mjCText& operator=(const mjCText& other);
   ~mjCText();
@@ -1860,7 +1860,7 @@ class mjCTuple : public mjCTuple_, private mjsTuple {
   friend class mjXWriter;
 
  public:
-  mjCTuple(mjCModel*);
+  explicit mjCTuple(mjCModel*);
   mjCTuple(const mjCTuple& other);
   mjCTuple& operator=(const mjCTuple& other);
   ~mjCTuple();
@@ -1904,7 +1904,7 @@ class mjCKey : public mjCKey_, private mjsKey {
   friend class mjXWriter;
 
  public:
-  mjCKey(mjCModel*);
+  explicit mjCKey(mjCModel*);
   mjCKey(const mjCKey& other);
   mjCKey& operator=(const mjCKey& other);
   ~mjCKey();
@@ -1930,7 +1930,7 @@ class mjCDef : public mjsElement {
 
  public:
   mjCDef();
-  mjCDef(mjCModel*);
+  explicit mjCDef(mjCModel*);
   mjCDef(const mjCDef& other);
   mjCDef& operator=(const mjCDef& other);
   mjCDef& operator+=(const mjCDef& other);