How to create a tank with cocos2dx - c++

Question

Me and a friend of mine are trying to make a tank with cocos2dx.
we are so far that the tank is on the screen and the barrel is attached to the tank
but now we want to try to the rotate the barrel but nothing is happening, the joint is at the center where the barrel start en de dome ends. both the tank and the barrel are dynamic bodies and we are using a friction joint (see code)
// Create sprite and add it to the layer
CCSprite *tank = CCSprite::create();
//tank->initWithFile("../Resources/tanks/001/tank.png");
tank->setPosition(pos);
tank->setTag(1);
this->addChild(tank);
// Create ball body
b2BodyDef tankBodyDef;
tankBodyDef.type = b2_dynamicBody;
tankBodyDef.position = toMeters(&pos);
tankBodyDef.userData = tank;
tankBody = _world->CreateBody(&tankBodyDef);
// Create shape definition and add body
shapeCache->addFixturesToBody(tankBody, "001/tank");
pos = CCPointMake(580, 450);
// Create sprite and add it to the layer
CCSprite *barrel = CCSprite::create();
//barrel->initWithFile("Tanks/001/barrel.png");
barrel->setPosition(pos);
barrel->setTag(2);
this->addChild(barrel);
// Create ball body
b2BodyDef barrelBodyDef;
barrelBodyDef.type = b2_dynamicBody;
barrelBodyDef.position = toMeters(&pos);
barrelBodyDef.userData = barrel;
barrelBody = _world->CreateBody(&barrelBodyDef);
tankBarrelAnchor = CreateRevoluteJoint(tankBody, barrelBody, -85.f, 180.f, 2000000.f, 0.f, true, false);
tankBarrelAnchor->localAnchorA = b2Vec2(0, 0);
tankBarrelAnchor->localAnchorB = b2Vec2(0, 0);
tankBarrelAnchor->referenceAngle = 0;
joint = (b2RevoluteJoint*)_world->CreateJoint(tankBarrelAnchor);
b2RevoluteJointDef* Level::CreateRevoluteJoint(b2Body* A, b2Body* B, float lowerAngle, float upperAngle, float maxMotorTorque, float motorSpeed, boolean enableMotor, boolean collideConnect){
b2RevoluteJointDef *revoluteJointDef = new b2RevoluteJointDef();
revoluteJointDef->bodyA = A;
revoluteJointDef->bodyB = B;
revoluteJointDef->collideConnected = collideConnect;
revoluteJointDef->lowerAngle = CC_DEGREES_TO_RADIANS(lowerAngle);
revoluteJointDef->upperAngle = CC_DEGREES_TO_RADIANS(upperAngle);
revoluteJointDef->enableLimit = true;
revoluteJointDef->enableMotor = enableMotor;
revoluteJointDef->maxMotorTorque = maxMotorTorque;
revoluteJointDef->motorSpeed = CC_DEGREES_TO_RADIANS(motorSpeed); //1 turn per second counter-clockwise
return revoluteJointDef;
}

Answer 1

I think the problem is that your anchor positions are not correct. This setting:
tankBarrelAnchor->localAnchorA = b2Vec2(0, 0);
tankBarrelAnchor->localAnchorB = b2Vec2(0, 0);
... would put the anchors at those two red and green axes you can see in your screenshot, and it will try to pull those two points together. Try turning on the joints display in the debug draw and this should be more obvious.
You need to give the anchors as local coordinates from the point of view of each body. For example from the point of view of the tank body, the swivel point looks like (1,2) and from the point of view of the barrel body, it looks like (-1,0). Of course the dimensions you are using are probably different, but the directions should be somewhat similar.
See the "Local anchors" section here: http://www.iforce2d.net/b2dtut/joints-revolute