Some stuff I found useful.

src/ofxBox2dBaseShape.cpp

@@ -65,7 +65,8 @@ bool ofxBox2dBaseShape::isBody() {
 }
 
 bool ofxBox2dBaseShape::isFixed() {
-	return density == 0.f ? true : false;
+    return ofIsFloatEqual(density, 0.0f);
+//	return density == 0.f ? true : false;
 }
 
 bool ofxBox2dBaseShape::isSleeping() {
@@ -105,6 +106,7 @@ void ofxBox2dBaseShape::setDensity(float val) {
         for (auto * f = body->GetFixtureList(); f; f = f->GetNext()) {
             f->SetDensity(density);
         }
+        body->ResetMassData();
     }
 }
 
@@ -233,11 +235,13 @@ float ofxBox2dBaseShape::toB2d(const float f) {
 //------------------------------------------------ 
 ofVec2f ofxBox2dBaseShape::getPosition() {
 	ofVec2f p;
-	if(body != NULL) {
+	if(body != nullptr) {
         const b2Transform& xf = body->GetTransform();
         b2Vec2 pos      = body->GetLocalCenter();
         b2Vec2 b2Center = b2Mul(xf, pos);
-		p = toOf(b2Center);
+ 		p = toOf(b2Center);
+    }else{
+        cout << "ofxBox2dBaseShape::getPosition() body is null\n";
     }
 	return p;
 }

src/ofxBox2dCircle.cpp

@@ -164,7 +164,11 @@ void ofxBox2dCircle::draw() {
 
 
 
-
+ofRectangle ofxBox2dCircle::getRect() {
+    ofRectangle r;
+    r.setFromCenter(getPosition(), radius, radius);
+    return r;
+}
 
 
 

src/ofxBox2dCircle.h

@@ -33,6 +33,8 @@ class ofxBox2dCircle : public ofxBox2dBaseShape {
 	void addRepulsionForce(float x, float y, float amt);
 	void addRepulsionForce(ofVec2f pt, float amt);
 
+    ofRectangle getRect() ;
+
 };
 
 

src/ofxBox2dRect.cpp

@@ -17,42 +17,84 @@ ofxBox2dRect::ofxBox2dRect() {
 
 //------------------------------------------------
 void ofxBox2dRect::setup(b2World * b2dworld, ofRectangle rec, float angle) {
-	setup(b2dworld, rec.x, rec.y, rec.width, rec.height, angle);
+
+        if(b2dworld == NULL) {
+            ofLog(OF_LOG_NOTICE, "- must have a valid world -");
+            return;
+        }
+
+        width = rec.width/2;
+        height = rec.height/2;
+
+
+
+        auto p = rec.getCenter();
+        b2BodyDef bodyDef;
+        if(density == 0.f) bodyDef.type    = b2_staticBody;
+        else               bodyDef.type    = b2_dynamicBody;
+        auto x = toB2d(p.x);
+        auto y = toB2d(p.y);
+
+        bodyDef.position.Set(x, y);
+        bodyDef.angle = ofDegToRad(angle);
+
+        body = b2dworld->CreateBody(&bodyDef);
+
+        setRectangle(rec);
+
+////        b2PolygonShape shape;
+//        shape = make_unique<b2PolygonShape>();
+//        shape->SetAsBox(toB2d(width), toB2d(height));
+//        fixture.shape        = shape.get();
+//        fixture.density        = density;
+//        fixture.friction    = friction;
+//        fixture.restitution = bounce;
+//
+//        fixturePtr = body->CreateFixture(&fixture);
+//        setPosition(p.x, p.y);
+//
+//        updateMesh();
+        alive = true;
+
 }
 
 //------------------------------------------------
 void ofxBox2dRect::setup(b2World * b2dworld, float x, float y, float w, float h, float angle) {
-
-
-	if(b2dworld == NULL) {
-		ofLog(OF_LOG_NOTICE, "- must have a valid world -");
-		return;
-	}
-
-    w /= 2;
-    h /= 2;
-	width = w; height = h;
-
-	b2PolygonShape shape;
-	shape.SetAsBox(toB2d(width), toB2d(height));
-	fixture.shape		= &shape;
-	fixture.density		= density;
-	fixture.friction	= friction;
-	fixture.restitution = bounce;
-
-	b2BodyDef bodyDef;
-	if(density == 0.f) bodyDef.type	= b2_staticBody;
-	else               bodyDef.type	= b2_dynamicBody;
-	bodyDef.position.Set(toB2d(x), toB2d(y));
-	bodyDef.angle = ofDegToRad(angle);
-
-	body = b2dworld->CreateBody(&bodyDef);
-	body->CreateFixture(&fixture);
-
-    updateMesh();
-    alive = true;
+    ofRectangle r;
+    r.setFromCenter(x, y, w, h);
+    setup(b2dworld, r, angle);
+}
+void ofxBox2dRect::setRectangle(const ofRectangle& rect){
+    if(body){
+        if(body->GetFixtureList()){
+            body->DestroyFixture(body->GetFixtureList());
+        }
+        width = rect.width/2;
+        height = rect.height/2;
+
+
+        if(shape == nullptr){
+            shape = make_unique<b2PolygonShape>();
+        }
+
+
+        shape->SetAsBox(toB2d(width), toB2d(height));
+        fixture.shape        = shape.get();
+        fixture.density      = density;
+        fixture.friction    = friction;
+        fixture.restitution = bounce;
+
+        fixturePtr = body->CreateFixture(&fixture);
+
+        auto p = rect.getCenter();
+        setPosition(p.x, p.y);
+
+        updateMesh();
+
+    }
 }
 
+
 // Temporary fix until OF 0.8.0
 static void rectangle(ofPath & path, const ofRectangle & r){
 	path.moveTo(r.getTopLeft());
@@ -207,6 +249,14 @@ void ofxBox2dRect::draw() {
 	*/
 }
 
+//------------------------------------------------
+ofRectangle ofxBox2dRect::getRect(){
+    ofRectangle r;
+    auto p = getPosition();
+    r.setFromCenter(p.x, p.y, getWidth(), getHeight());
+    return r;
+}
+
 
 
 

src/ofxBox2dRect.h

@@ -12,6 +12,10 @@ class ofxBox2dRect : public ofxBox2dBaseShape {
 private:
     float width, height;
 
+    b2Fixture* fixturePtr = nullptr;
+
+    unique_ptr<b2PolygonShape> shape = nullptr;
+//    b2PolygonShape shape;
 public:
 	ofVboMesh mesh;
 	//------------------------------------------------
@@ -25,6 +29,10 @@ class ofxBox2dRect : public ofxBox2dBaseShape {
     float getWidth()  { return width * 2;  }
     float getHeight() { return height * 2; }
 
+
+    void setRectangle(const ofRectangle& rect);
+    ofRectangle getRect() ;
+
     void updateMesh();
     void draw();