Data Encapsulation?

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Neo - 19 Jul 2006 11:09 GMT

I was written this code; in VS2k5

class Program

{

private: int a;

private: int b;

private: char c;

public: Program( ) : a(10), b(20), c('Y'){ }

};

int _tmain(int argc, _TCHAR* argv[])

{

A obj;

int temp = *((int *)&obj + 1);

char ch = *((char *)&obj + 8);

cout<< temp<<endl<<ch<<endl;

_gettch( );

return 0;

}

in Object Orient World we learn Data Encapsulation or Data Hiding for data
security.

In this code, what is the meaning of Data Encapsulation or Data Hiding for
data security?

Regards,

-neo

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Vinzenz Feenstra - 19 Jul 2006 11:25 GMT

Neo schrieb:

> in Object Orient World we learn Data Encapsulation or Data Hiding for data
> security.
>
> In this code, what is the meaning of Data Encapsulation or Data Hiding for
> data security?

In this code the class members are hidden and not visible to non-friends.

If you would try to access the members the correct way:

e.g. obj.a = 10;

you will get a compiler error. This would be the correct behaviour, but
you are using a bad hack. Of course your code *can* work, but it has not
to work. If you have vtable this can result in undefined behaviour.

There is no security of accessing if you access elements in this way.
And there can't be such a security.

Regards,
Vinzenz

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Arnaud Debaene - 20 Jul 2006 07:13 GMT

>I was written this code; in VS2k5

<snip>>

> A obj;
>
[quoted text clipped - 7 lines]
> In this code, what is the meaning of Data Encapsulation or Data Hiding for
> data security?

The aim of data access restriction (protected, private) in OO language is to
protect against Murphy - that is the programmer inadvertently
reading/writing a variable it shouldn't be interested in.
It is not aimed at protecting against Machiavel - that is a hacker who want
to acess data that should be forbidden to it. If you nee that kind of
protection, you probably need cryptography, authentification, access
control, etc...
Anyway, as Vinzenz has said, your "solution" is a hack that depends on
knowledge of the compiler internals working (ie, data padding and
alignement, presence of a v-ptr, data member reorganization, etc...)

Note that this is true in all OO languages, it is not specific to C++.

Arnaud
MVP - VC

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Data Encapsulation?

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