private section of Qt classes is not safe

Question

I have define a QByteArray class as arr instance in Qt. normally, private members of QByteArray(and another classes) are not accessible, else use of friend members in class.

Attention bellow code :

void Widget::onBtnRun()
{
    QByteArray arr;
    arr = "this is a test";
    arr.d->size = 5;                  // stop with compile error-> QByteArray::d is private
    QMessageBox::information(this, "", arr);
}

the d member of QByteArray is private and cannot access to that!

but i have edited QByteArray header file, so adding bellow line in it's class section, then save that again.

public :
     friend Class Widget;

And now i can access to the private members of QByteArray class in Widget class, without any problem :

void Widget::onBtnRun()
{
    QByteArray arr;
    arr = "ABCDEFGHIJKLMNOPQRSTUV";
    arr.d->size = 5;
    QMessageBox::information(this, "", arr);
}

MessageBox output now is "ABCDE".

Is this a lack for classes? perhaps this cause appear very problem in future. how can save private members from thease problems?

Thanks in advance!

what problems do you mean? Maybe a cleaner way would be to write a `getter` for the private member — 463035818_is_not_an_ai, Jun 12 '16 at 12:37
I don't see how this is a problem. Since you're editing the header file, you might as well make private members public. — thuga, Jun 13 '16 at 06:55

score 4 · Answer 1 · answered Jun 12 '16 at 13:17

I think there is no way to do this. If someone can access your header file, he could do potentially anything with your class. If you are developing a simple executable application, you should not worry about this because other users cannot access your source files. In case of library you can make its dynamic and provide only .so/.dll file.

If you edit someone's header file, you will do it on your own risk.

score 3 · Accepted Answer · edited May 23 '17 at 12:10

3

The class member access modifiers in C++ have nothing to do with security. They are there to express design intent that is enforced by the compiler. They are not used to "hide"/obfuscate code or prevent access by third parties.

Quoting this excellent answer:

In general, access controls names or symbols, not the underlying entities. There are, and always have been, numerous ways of accessing private members; what you cannot do is use the name of such a member.

Why do you consider this a problem? What are you trying to "protect" yourself/your code from?

edited May 23 '17 at 12:10

Community

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answered Jun 13 '16 at 19:28

Kuba hasn't forgotten Monica

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yes , protected members cannot access, else in to the inherited class. – BattleTested_закалённый в бою Jun 14 '16 at 04:11

private section of Qt classes is not safe

2 Answers2