Box2d .settings

many guides used UserData.But only I don’t see the setUserData functions as in them. After understanding it, I realized that these manuals were outdated, and now there is no such function and userData should be set in a different way. After digging through the Box2d files for three days, I realized that I could not cope alone. I found out that it is possible to edit the b2fixtureDef structure to add my new data there. but I don’t understand how to do this correctly, because just adding a line to the structure of the program crashes