swift optional chaining

class ca{

var num:Int =1;

}

class cb{

var a:ca?;

}

class cc{

var b:cb?;

}

var x:cc =cc();

var y:Int? =x.b?.a?.num;//或var y = x.b?.a?.num;

println("y =\(y),\r\ny type =\(y.dynamicType)");