Accessing private members [closed]

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Closed 4 years ago.

Is it appropriate to access a class' private members by casting it to a void pointer and then to a struct?

I don't think I have permissions to modify the class that contains the data members that I need to access. I don't want to take a risk accessing the data members in an indirect way if it is not ethical.

EDIT: Had to edit this further... I am pretty sure the class wouldn't be modified, so it's ok to that extent... my only concern is, if the person who coded that class gets to know of this, it might not go down well with him :(.

Answer 1

Simple answer: No.

It is not ethical and it will turn into a maintenance nightmare at some point. The internal private members of a library can change and break your code. Developers of the library do not need to know (nor want to) that you are violating the encapsulation.

Classes usually have invariants over their methods that some times will not be documented, but accessing and changing the values from the outside can break those invariants. As an example, if you change the reserved space in a vector for a higher value, the vector will not allocate new space until it has filled the existing and that won't happen before hitting unallocated memory: your application will crash.

If the attribute is private, it is not for you to use it, only for the class itself or the the class' friends that know about the member, how to use it, how not to break it. If the programmer wanted you to change the field, it would be public.

Answer 2

It's ethical, but it's usually lots of dirty code neighbouring with undefined behaviour and unportability. Do it only if you absolutely have to.

Answer 3

Ah, abstraction - you can't live without it and yet it is so painful to deal with sometimes :)

Anyway, ethics aside, what if the "owner" of the class decides to change the internal implementation, or simply reverses the order of the private data members?

Answer 4

"Never say never". I'm sure somewhere in the universe, there's a situation that will force you to have to do this...

But I certainly would cringe if I had to do it. You truly need get lots of opinions on your situation before pulling the trigger. Can you describe your specific situation and maybe we could see if it makes sense or what better alternatives might exist?

In response to comment/question-- Define "permissions" -- institutional permission? That sounds like not a programming problem, but something to talk to whoever is asserting said permission. Maybe this is more of a political issue than a technical one? Again, I think we need more specifics--even if it was somewhat about the politics of the situation. That may or may not be deemed out of scope for the website, however.

Answer 5

The only "ethical" problem is what you are doing to the poor bastard who is going to have to figure out and maintain your code.

Private members are just that. He may, and most likely will, one day change that member. When that happens, you might not even notice as there most likely will still be memory there, just not the member you were expecting. Damn near any nearly impossible thing you can imagine might then start happening when your code is run. How is anybody ever going to be able to debug that?

Answer 6

I'm not sure that "ethics" really come into it. It busts the heck out of encapsulation though.

EDIT: I would almost never accept this if I were performing a code review. You'd have to work really hard to convince me there's no way of designing around the need for this. It's possible you'd succeed, but there'd have to be major warnings all over the place, and rock hard unit tests to make sure that if anything changed to break it, you'd know quickly.