add a safety check for `box2d`

3rd/box2d/include/box2dw.hpp

@@ -67,11 +67,11 @@ struct PyBody{
     PK_ALWAYS_PASS_BY_POINTER(PyBody)
 
     b2Body* body;
-    b2Fixture* fixture;
+    b2Fixture* _fixture;
     PyObject* node_like;
 
     bool _is_destroyed;
-    PyBody(): body(nullptr), fixture(nullptr), node_like(nullptr), _is_destroyed(false){}
+    PyBody(): body(nullptr), _fixture(nullptr), node_like(nullptr), _is_destroyed(false){}
 
     void _gc_mark() {
         if(node_like != nullptr){
@@ -81,7 +81,18 @@ struct PyBody{
 
     PyBody* _() { return this; }
     b2Body* _b2Body() { return body; }
-    b2Fixture* _b2Fixture() { return fixture; }
+
+    b2Fixture* _b2Fixture() {
+        if(_fixture == nullptr) throw std::runtime_error("`_fixture == nullptr` in PyBody::_b2Fixture()");
+        return _fixture;
+    }
+
+    void _set_b2Fixture(b2Fixture* fixture){
+        if(_fixture != nullptr){
+            body->DestroyFixture(_fixture);
+        }
+        _fixture = fixture;
+    }
 
     static void _register(VM* vm, PyObject* mod, PyObject* type);
 

3rd/box2d/src/box2d_Body.cpp

@@ -14,9 +14,7 @@ void PyBody::_register(VM* vm, PyObject* mod, PyObject* type){
             // a weak reference to this object
             def.userData.pointer = reinterpret_cast<uintptr_t>(obj);
             body.body = world.world.CreateBody(&def);
-            body.fixture = nullptr;
             body.node_like = node;
-            body._is_destroyed = false;
             return obj;
         });
 
@@ -49,7 +47,7 @@ void PyBody::_register(VM* vm, PyObject* mod, PyObject* type){
             float hy = CAST(float, args[2]);
             b2PolygonShape shape;
             shape.SetAsBox(hx, hy);
-            body.fixture = body.body->CreateFixture(&shape, 1.0f);
+            body._set_b2Fixture(body.body->CreateFixture(&shape, 1.0f));
             return vm->None;
         });
 
@@ -59,7 +57,7 @@ void PyBody::_register(VM* vm, PyObject* mod, PyObject* type){
             float radius = CAST(float, args[1]);
             b2CircleShape shape;
             shape.m_radius = radius;
-            body.fixture = body.body->CreateFixture(&shape, 1.0f);
+            body._set_b2Fixture(body.body->CreateFixture(&shape, 1.0f));
             return vm->None;
         });
 
@@ -77,7 +75,7 @@ void PyBody::_register(VM* vm, PyObject* mod, PyObject* type){
                 vertices.push_back(b2Vec2(vec.x, vec.y));
             }
             shape.Set(vertices.data(), vertices.size());
-            body.fixture = body.body->CreateFixture(&shape, 1.0f);
+            body._set_b2Fixture(body.body->CreateFixture(&shape, 1.0f));
             return vm->None;
         });
 
@@ -95,7 +93,7 @@ void PyBody::_register(VM* vm, PyObject* mod, PyObject* type){
                 vertices.push_back(b2Vec2(vec.x, vec.y));
             }
             shape.CreateLoop(vertices.data(), vertices.size());
-            body.fixture = body.body->CreateFixture(&shape, 1.0f);
+            body._set_b2Fixture(body.body->CreateFixture(&shape, 1.0f));
             return vm->None;
         });
 

3rd/box2d/src/box2d_World.cpp

@@ -135,7 +135,7 @@ void PyWorld::_register(VM* vm, PyObject* mod, PyObject* type){
                 if(body._is_destroyed){
                     body.body->GetWorld()->DestroyBody(body.body);
                     body.body = nullptr;
-                    body.fixture = nullptr;
+                    body._fixture = nullptr;
                     body.node_like = nullptr;
                 }
                 p = next;