Is the .msg in memory format the same for ROS1 and ROS2?

If I defined some some type FOO, with members X and Y, will the in memory structure generated from .msg "rosidl" be the same as the the in memory structure generated from the rosidl_dds generated .idl? That is, if I do pointer math like (void*) myfoo.X - (void*) myfoo.Y will I get the same answer in ROS1 and ROS2? I understand that there are wire format differences, but does the DDS version basically wrap the ROS1 version, such that if I get a pointer to my FOO, I can pass it to the same function regardless of which transport we use? Realize this is pretty basic, so appreciate the help and patience!