I don't know all that much about C++'s object system, so I can't give you a concrete example on how it breaks down.
However, you say that it is "not something people do" ... well maybe not in C++ (I highly doubt that), but it's very common in many languages.
In C, it's common to look inside structs directly and change things.
Javascript libraries do it all the time: They inspect their arguments, look at the types and change their behavior depending on it. It's a common programming practice to poke deep into the data-structures and do things.
In Java, they made it an art with reflection and monstrosity such as Spring.
Abstraction is a bit like immutability: Sure, you can try to fake it in languages that don't have it, but then you are just praying that everyone plays by your rules. :)
However, you say that it is "not something people do" ... well maybe not in C++ (I highly doubt that), but it's very common in many languages. In C, it's common to look inside structs directly and change things. Javascript libraries do it all the time: They inspect their arguments, look at the types and change their behavior depending on it. It's a common programming practice to poke deep into the data-structures and do things. In Java, they made it an art with reflection and monstrosity such as Spring.
Abstraction is a bit like immutability: Sure, you can try to fake it in languages that don't have it, but then you are just praying that everyone plays by your rules. :)